Oct 292015
 

This article is part of a series dealing with building best-in-class lithium battery systems from bare cells, primarily for marine use, but a lot of this material finds relevance for low-voltage off-grid systems as well.

Here, we detail the hands-on process of building a lithium battery bank from individual single prismatic cells. There is more to it than just arranging and connecting the cells, because those can only be assembled into a battery after they share a common state of charge. They also need to be protected before anything can be done with the battery, which is the object of another article.

Before that, preliminary decisions also need to be made: how much capacity to install and what cells to source? What is the most suitable interconnection scheme to adopt?

200Ah LFP battery pack for a marine application

A 200Ah DIY lithium battery back for a yacht, balanced and instrumented for cell voltages and temperature. A standard 12-pin plug connector provides the interface to the battery protection module.
Cell clamping arrangements can be very simple and effective.

Buying cells and assembling the bank is not the beginning. Learning about lithium cells and understanding their properties and their risks is, before committing to building anything.

As it is an extensive topic in itself, the integration of a lithium battery on board is also dealt with separately.

Disclaimer

A good understanding of DC electrical systems is needed to build and commission a lithium battery installation. This article is aimed at guiding the process, but it is not a simple blind recipe for anyone to follow.

The information provided here is hopefully thorough and extensive. It reflects the knowledge I have accumulated building some of these systems. There is no guarantee that it will not change or grow over time. It is certainly not sufficient or intended to turn a novice into an electrical engineer either. You are welcome to use it to build a system, but at your own risk and responsibility.

How Much Capacity?

Generally speaking, a LiFePO4 bank will offer about twice the usable capacity of equivalent deep-cycle lead-acid cells in good condition, and much more when such lead-acid cells have deteriorated. This can provide a rough guideline when considering the purchase of lithium cells. In practice, it only suggests the maximum capacity that should be considered as a starting point: no more than 50% of the lead-acid capacity.

In the traditional lead-acid way of thinking, more capacity meant smaller cycles and longer life and a justification was found there: the situation is almost the exact opposite with Li-ion batteries

Many lithium banks installed on yachts nowadays are in fact not only much larger than they need to be, but also much larger than they should be.

The oversize bank approach can in fact deliver less value: there is nothing suggesting that a bank twice as large will last twice as long: it will more than likely just result in twice as many old buggered cells at the same point down the track if not earlier. The first consequence of installing an oversize battery bank, especially when sustained charging is involved as with solar panels, is that the bank remains at a higher state of charge much longer, if not most of the time. This is very detrimental to its ageing for reasons that were developed earlier. Lithium cells like cycling because it means they don’t spend any amount of time near full; alternatively, they can sit happily half-discharged, or even lower, for years.

Invest in energy efficiency or charging capacity, not in unnecessary storage

The bank needs to be large enough to provide the capacity needed between recharges, but beyond that, all what comes out needs to go back in and the size of the battery makes no difference there. Money is best invested in energy efficiency on board and charging capacity than storage.

The question therefore revolves around the cycle duration that must be accommodated. A yacht spending all its time in the tropics with considerable solar supply available on a daily basis doesn’t technically need to store much more than its overnight consumption, strictly speaking. The ability to accommodate a 2-day or 3-day cycle may be valuable however, but this calls for adapting the management of the battery to suit. Consumption can also be reduced in adverse conditions, extending cycle duration and this is a sensible way of looking at the matter, compared to calculating everything on maxima and worst-cases.

In practice, lithium banks of about 200Ah are easily capable of supporting yachts with an electric refrigeration system and auxiliary loads in the mid-latitudes and it is very difficult to present a valid case for installing more than 300-400Ah on a sensibly outfitted pleasure craft. Some, however, are fitted out and operated as if they were permanently tied to the power grid.

Some of the installations I built and commissioned included a provision for expansion by adding an extra set of cells later if needed, in order to alleviate the owner’s concerns. None of them were expanded afterwards

While a lithium battery bank can easily be expanded by adding more cells later if needed, unneeded capacity cannot be returned for a refund. Best long-term value is achieved when both the installed capacity and the management of the installation are correct and adequate.

Sourcing Cells

Manufacturers

Those are all common cells on the market today: the CALB SE-series in blue and CALB CA-series in grey. Sinopoly cells are black and Winston cells are yellow.

Those are all common cells on the market today: the CALB SE-series in blue and CALB CA-series in grey (now identical other than for the casing). Sinopoly cells are black and Winston cells are yellow.

Just a few years ago, there were many manufacturers of LiFePO4 prismatic cells. Many of those names have disappeared: Hipower, GBS, Thundersky and others are all gone. The oldest names still in the game today are Sinopoly, CALB (China Aviation Lithium Battery) and Winston, the latter having had a troubled history in recent years.

Sinopoly and CALB operate their own research and development labs. CALB in particular has also established a very strong reputation for product quality control with each cell being measured and labelled with its actual capacity before being shipped.

While it is often possible to source cheaper cells of questionable origin with obscure brand names, such bargains are most unlikely to turn into long-term value because the ageing behaviour of a cell is extremely dependent on the quality of its manufacture and trade secrets associated with electrolyte composition.

Cell Sizes

Single 3.2V prismatic LiFePO4 cells can nowadays be obtained in huge capacity, as high as 10000Ah. Commonly available cells range between 40Ah and maybe 1000Ah. It should be pointed out that the larger sizes are intended for stationary applications where no accelerations, vibrations or shocks are ever experienced.

A sales manager at Sinopoly I was talking to was adamant about using 100Ah or 200Ah cells only for assembling marine battery banks, with 100Ah being preferred and 200Ah acceptable. Large cells simply don’t have the structural strength-to-weight ratio required to be taken to sea on board small crafts and would exhibit shortened life due to internal mechanical damage arising from on-going vessel motion. It is common sense: as a cell becomes larger, its internal weight increases much faster than the rigidity and surface area of the casing and the casing is all what holds the plates together in a prismatic cell.

Failures have been reported on vessels equipped with 700Ah cells following ocean passages: some cells were suddenly found to be losing charge inexplicably, rendering the battery bank completely unmanageable and the matter ended in a complete write-off. All big-brand commercial marine lithium battery packs on the market today are built from cells no larger than 200Ah.

While there certainly are examples of marine DIY systems that were built with large cells in series without issues, closer inspection usually also reveals a houseboat usage or infrequent good weather, sheltered waters sailing. In other words, the data point is null and void if the intent is sailing and designing upon the assumption that the boat won’t be going anywhere would be questionable.

Physical cell dimensions, space availability on board and interconnection topology are the other factors that influence the final choice of cell model. 200Ah cells are usually taller and require more “headroom”.

Condition Check

As much as possible, when sourcing cells from a local agent, I try to physically go there and check the cells as they come out of the crate. I normally decline buying cells that are no longer factory-packaged and may have been tampered with.

A set of brand new Sinopoly cells just out of the factory crate. All are reading within less than 1mV and their state of charge is just over 40%.

A set of brand new Sinopoly cells just out of the factory crate. All are reading within less than 1mV and their state of charge is just over 40%.

  • I ask for cells from the same production batch, with consecutive serial numbers. Those should hopefully exhibit more consistent characteristics than randomly chosen cells.
  • All cell voltages must read below 3.300V. Pay attention to multimeter calibration there, there is a vast difference in terms of state of charge between 3.31V (over 75% SOC) and 3.29V (less than 45% SOC). This is to ensure that I am not getting cells that have been sitting around at a high SOC. This is not normally a problem with factory-shipped cells, but more caution applies with cells on the retail market.
  • All cell voltages must be very closely matched. I like to see differences of 1mV or less, but sometimes accepted up to 2-3mV. At the SOC cells ship at, there is no justification for deviations in voltage, which could indicate a defective cell or prior tampering.
  • Obviously, no cell must show unusual signs of physical use or prior connection. All cells are connected, charged and discharged at the factory following manufacturing, so there is no reason for any of them to appear any different, unless the cell is in fact second-hand.

If I place an order and cannot physically check the cells myself prior to purchase, I explicitly state all these conditions in writing with my order, so they become contractually binding if the order is filled. It can go a long way with eliminating the temptation to slip a “perfectly good” second-hand cell in a batch to get rid of it, knowing that returning it would be a major hassle for the buyer.

Warranty Conditions

Prior to purchase, I also get a warranty statement from the supplier. While warranty is usually limited to one year, this should cover any problems arising from major manufacturing issues.

Warranties on lithium battery cells are tricky, because the cells can easily be damaged through misuse and suppliers know that only too well. Chances of making a successful claim for a ruined bank or on an installation where cell-level protection didn’t exist would be near-zero (and rightly so), but it would be very difficult for a supplier to push back in the case of a single-cell failure on a properly engineered system.

In some countries, warranty clauses offered by vendors in general deliberately conflict with applicable consumer protection laws, so a one-year warranty doesn’t automatically mean that all bets are off after 12 months.

Pre-Balanced Packs

Some resellers sometimes offer “ready-to-go” balanced cell packs. While this could appear simpler than having to carry out cell balancing, the large amount of uncertainty existing around how the cells were balanced and treated prompts for extreme caution. These packs may have been exposed to excessive voltage and then left fully charged for extended periods of time, which makes them rather undesirable to own: the cells are already damaged.

The considerations about cell balancing further below contain all useful information required to validate the process used by the vendor, should one ever be tempted to go this way.

Cell Links

Consider sourcing cell links and stainless steel bolts in the same time as the cells. Cell manufacturers nearly always offer those. Use solid copper links in marine installations. Braided straps, such as earthing straps, even tinned, are not a good idea. They have a lesser cross-section than a solid conductor and will not age as well in the marine environment. They are bound to corrode and heat up severely one day.

Copper cell interconnection link. Those are readily available from battery manufacturers and resellers.

Copper cell interconnection link. Those are readily available from battery manufacturers and resellers.

Alternatively, source 40 x 6mm (1 ½ x ¼’’) aluminium flat bar, cut it and drill it to suit. Sand the contact areas bright to remove the thin oxide skin. If using DIY links, consider insulating the sections between cell terminals using heat shrink tubing; it will greatly reduce the risks of causing an accidental short while working around the cells afterwards. Such home-made long links are particularly effective when dealing with blocks of cells in parallel and work out much cheaper than copper single links.

In all cases, the bolts must be long enough to thread deep into the cell terminals and be fitted with locking washers.

Transportation Considerations

Shipping crate for lithium cells

Shipping crate for lithium cells

Shipping of lithium-ion batteries currently falls under very restrictive rules as they are classified as Dangerous Goods UN3480 Class 9. This determination can significantly increase freight costs and makes air freight essentially impossible today (2016).

Sea freight costs, on the other hand, are usually calculated on a minimum quantity of 1 cubic metre or 1 metric tonne (break bulk or LCL) and obtaining a good honest quotation for a small one-off shipment can be more than problematic at the best of times. Unless the order is large enough to approach 1 cubic metre, it is typically uneconomical to consider international sea freight, from China typically, because of the multitude of fixed processing fees and charges associated with landing and clearing the cargo.

The most practical pathway for sourcing small numbers of cells is often going through a company already importing such batteries for a purpose or another, such as electric vehicle conversions.

Battery Bank Topology

Once the system voltage and intended capacity have been established and a source/manufacturer has been identified for the cells, the topology of the bank can be determined according to cell size.

Electrical Interconnection

The principle is always the same: a 12V nominal system requires four identical blocks of 3.2V nominal cells, and a 24V installation requires eight. Each one of these blocks must offer the capacity sought after. Cells in the 100Ah to 200Ah range are relatively small building blocks and assembling larger banks requires creating parallel configurations.

Cell terminals and link plates must be sanded clean and bright prior to assembly: high resistance connections immediately result in hot spots at high current with the heat flowing straight into the cells. Connections should always be very tight for the same reasons.

In its simplest expression, a 12-volt lithium bank is built out of 4 cells connected in series; this is also the safest configuration. If more capacity is required, two main options are available in terms of architecture and interconnection schemes.

Parallel First, Then Series

The most common and simplest scheme is creating parallel blocks of cells of the required capacity, and then linking them in series to reach the voltage sought.

A 200Ah LiFePO4 bank can be assembled using four 200Ah cells connected in series, or four groups in series of two 100Ah cells in parallel. The first topology would be referred to as 4S (four in series, figure A below) and the second as 2P4S (two parallel, four times in series, figure B below).

Cell interconnection schemes for 12V systems. "A" represents a 4S configuration, "B" a 2P4S arrangement and "C" is the same, but fused.

Cell interconnection schemes for 12V systems. “A” represents a 4S configuration, “B” a 2P4S arrangement and “C” is the same, but fused.

The main advantage of these configurations is that they minimise the complexity of the protection required. It is also very easy to physically interconnect cells this way. The drawback of configuration B is that, should one cell fail by shorting internally, the others connected in parallel will discharge into it, potentially aggravating the situation.

This introduces a low, but additional, risk into the system that doesn’t exist with a pure series interconnection scheme as in figure A. Connecting cells in parallel to achieve large capacities is very commonly done however, even at industrial scale in stationary installations.

Fusing Individual Cells, or Block of Cells

A variant on the parallel blocks scheme of figure B is fusing some or all of the parallel cell links (figure C above). The challenge resides in sizing the fuses as small as possible, while still large enough to carry the normally expected currents without undue voltage drop and risk of blowing. We will note that in the case of configuration C, the fuses should never see much more than half of the bank total current. Fusing can’t prevent good cells from discharging into a faulty cell, it can only prevent them from heavily discharging into it, so the outcome and effectiveness of such schemes is uncertain.

On board marine vessels where loads such as inverter and windlass commonly draw in excess of 100A, the fusing requirements can be placed so high that they undermine the value of such arrangements for small banks.

Fused 3P4S LFP bank

In larger parallel interconnection schemes, such as this 3P4S configuration, individual fusing of the cells becomes increasingly important and provides a degree of fault tolerance.

The more cells connected in parallel, the higher the amount of energy available for heating a failed cell uncontrollably, but the smaller the individual cell currents. This can make individual cell fusing schemes more effective for larger installations. In the 3P4S configuration in figure D, each cell only contributes to one third of the total current and failure of a cell fuse doesn’t immediately compromise electrical supply to the vessel.

Large vessels can use banks comprising 8 to 10 cells in parallel in each block and then individual fusing can become very effective.

Parallel Banks

Deviating from parallel group topologies leads to building completely separate banks then connected in parallel. This requires a complete duplication of the protection/management system, but can be justified.

An active protection scheme with two independently managed 4S banks provides both redundancy and the highest degree of protection, but at the cost of duplicating the management system.

An active protection scheme with two independently managed 4S banks provides both redundancy and the highest degree of protection, but at the cost of duplicating the management system.

The approximate 200Ah physical cell size limit determines the capacity of each individually protected pack if no parallel discharge risk is the goal. This is the way commercial marine lithium offerings are usually constructed, as it minimises associated liabilities. A cell failure causes disconnection of the associated pack and the only energy involved is the one contained within the failing cell.

Summary

A simple 4S configuration (diagram A) offers both simplicity and maximum safety for a 12-volt nominal system. It allows building systems up to 200Ah.

The majority of the DIY lithium battery banks built to date have used the parallel+series configuration (figure B), occasionally with partial fusing as shown in diagram C. These topologies are not uncommon in large stationary installations either. At the time writing, I am not aware of any incidents arising from isolated cell failure within a bank on a marine DIY system. This doesn’t mean it couldn’t possibly happen.

Prospective owners of very large lithium battery banks should seriously consider using individual cell fusing, as shown in figure D, or going to multiple parallel banks as depicted in diagram E. The large number of cells increases the chances of seeing an isolated failure and the small size of the cell in relation with the bank suggests greater potential effects.

Production automotive battery packs are commonly made of very large numbers of small cells and typically fused as per figure D, and also broken up in separately managed and protected blocks. Those are usually connected in series afterwards to obtain high DC voltages, which is out of scope here.

Configuration E is arguably the best when it comes to minimising risks and maximising reliability while achieving a larger target capacity. It is more costly due to the duplication of the battery protection equipment and high-current disconnectors, but in the context of building a large lithium battery bank, the cost of protection should be seen as small.

Using larger cells in order to remain with a simple 4S configuration while achieving higher capacity would probably constitute a very dubious choice. Cells suffering internally from physical stresses and damage are much more likely to fail and short out than smaller, more robust cells simply connected in parallel.

Mechanical Installation

It was once thought prismatic cells could be operated in more or less any position as they do not really contain free liquid. Nowadays manufacturers are a lot more prescriptive with installation position. In most instances, the only acceptable position is upright, vent cap and terminals on top (Sinopoly, Winston). Sometimes it may be acceptable to mount them on edge, with the terminals on the side (CALB). This may vary not only between manufacturers, but also between cell models, so seeking specific guidance is a sensible step if an odd installation position is being considered.

When questioned, Sinopoly indicated that installing the cell in any other position than upright would cause some of the plates to run dry after a while, damaging it. Installing them flat on their side is out of the question in all cases.

The cells must be installed securely in such a way that no movement is possible in relation with each other, or it will stress the terminals and link plates. Prismatic cells should also be clamped together between compression plates as the application of a modest amount of pressure helps with preventing electrode delamination, even more so in the presence of shocks and vibrations as found on marine vessels. It also helps with preventing the internals of the cells from shifting in case of violent shock, which can lead to internal cell short-circuits. Clamping is a common warranty condition from manufacturers. Strapping the cells together is simply not good enough for that matter.

The bank must also be installed in such a way that it can’t shift and nothing can come and short-circuit the cell terminals. This can involve fitting a cover over the cells.

Location

Ambient Temperature Considerations

Lithium batteries age at an accelerated rate and degrade very quickly at high temperatures. For this reason, installing a bank in an engine compartment is completely out of the question. Ambient temperatures in the battery compartment should not exceed 30°C.

Conversely, exceedingly low temperatures can lead to temporarily reduced performance and capacity on discharge and cell degradation during charging. Marine house batteries are not normally operated at very high currents, but charging below 0°C is an issue that can arise for some vessels in some areas and needs to be prevented, especially at high currents.

Volume within the accommodation space and below the waterline is often the most suitable in terms of ambient temperature conditions for housing a lithium battery bank. Vessels operating in polar waters or facing harsh winters may require special dispositions ranging from heating the battery compartment to disabling charging.

Shocks and Accelerations

Prismatic cells are made of thin plates stacked together within a semi-rigid plastic housing. The stack itself hardly has any structural strength other than in compression. The edges of the plates are weak and can be prone to damage if the cells are exposed to violent impacts. Installing prismatic lithium cells into the bow section of a marine vessel is out of the question, no matter how tempting it may be to power a windlass. Wound cylindrical cells would be far for robust for that matter, but the battery assembly containing a large number of such small cells may not be. Shaking it loose over time could turn the battery into a fire risk.

A lithium battery bank should be installed aft of midships typically, in the most comfortable part of the vessel and the cells must be firmly clamped as discussed earlier. In the case of offshore vessels, the prospect of falling off a wave in heavy weather cannot be entirely excluded, hence the importance of selecting cells of modest size and weight for building marine battery banks.

Measuring Cell Voltages

4000 counts multimeter displayBefore moving ahead with building a lithium battery bank and balancing cells, make sure you have access to a good quality, calibrated digital multimeter: cheap, junk-grade instruments are little else than voltage-inspired random number generators. It should read at least within 10mV of the true voltage in the 3 to 4 volts range with perfect repeatability and regardless of changes in ambient temperature.

Many of the “marinised” multimeters I have come across over the years were out by 0.1V or worse. If you happen to own one of those, complete with the proverbial bent probes or broken leads, do yourself a favour and place it carefully in a rubbish bin if you can’t give it away.

While most multimeters can be adjusted internally, the cheap and nasty ones resist calibration attempts by drifting all over the place afterwards. The internal voltage reference they measure against is worthless and the circuits are not temperature-compensated.

An instrument with a range of 4000 counts, rather than the more common 2000 counts found on low-end devices, also means that it is capable of displaying differences down to a single millivolt between 3 and 4 volts.

Always measure cell voltages directly from the cell metallic terminals themselves, rather than the cell links or bolts. The readings are much more reliable. And keep your instrument in a sealed plastic freezer box, with the leads neatly folded and a spare battery: this also makes for more reliable readings on the long run!

Safe Handling

New cells come out of their crates fitted with insulator caps over their terminals to prevent accidental short-circuits. The extraordinary current capacity of lithium battery cells has been discussed already.

Accident risks are very high while repeatedly connecting, disconnecting and re-arranging cells for balancing and building a battery bank.

Keep the insulator caps on the cells terminals for as long as nothing is connected and insulate the tools used for making the connections. Most of the cells in the sizes suitable for building marine banks use M8 bolts and require a 13mm spanner or socket drive.

Wrap this tool with insulating tape, or better, heat-shrink tubing, if it is going to be used in a sustained way, only leaving exposed metal at the working end. On the same token, cover the top of the cells you are not working on.

Shorting cells while working on their connections with a spanner would near-invariably result in intense burns and the offending tool might weld itself to the terminals before failing “fuse-style”, sending molten metal flying all around.

Watching people working on cell connections always makes me extremely nervous.

Cell Balancing

Before a bank is physically assembled into place, the cells must be balanced. This step is absolutely critical, because if a cell becomes fully charged ahead of the others, its voltage and resistance increase very rapidly, the charging current collapses and the other cells can’t be charged any further.

When cells are manufactured, their actual capacity always deviates more or less from their intended nominal capacity; next the cells are cycled, tested and then left partly charged by the factory before shipping. There is every chance that cells even belonging to the same production batch won’t all share the exact same capacity and will not land in a state that would allow simply connecting them in series to obtain a balanced battery bank.

Unbalanced battery pack

Trying to operate an unbalanced battery bank, such as when assembling new cells without precautions, results in problems both with charging and discharging. Here, cell 4 gets over-charged while the others are not yet full and cell 2 reaches an over-discharge condition while the others still have some capacity left.

At best, on a well-designed system, cell imbalance causes a reduction in available capacity and potentially some kind of alarming or even disconnect; on an unprotected, unmanaged system, it leads to cell destruction and can result in dangerous developments.

A bank can be top-balanced or bottom-balanced, but never both, because the cells never share the exact same capacity. The choice depends on the application and type of service.

Bottom Balancing

Bottom balancing is normally very undesirable for marine house banks as they hardly ever, if ever at all, get fully discharged and it creates most unwelcome difficulties with charging. Charging and managing bottom-balanced banks will not be developed here for these reasons.

For the sake of completeness and understanding only, some information is provided here about bottom balancing.

All cells are first discharged to a common low level prior to being assembled together. Charging results in one cell reaching full charge before the others, but deep discharge is no issue on the other hand.

All cells are first discharged to a common low level prior to being assembled together. Charging results in one cell reaching full charge before the others, but deep discharge is no issue on the other hand.

The goal of bottom balancing is ensuring that all cells get to their low charge limit evenly together. Bottom balancing makes most sense in applications where deep discharge happens routinely, like in the case of electric vehicles that are driven almost to the point of running out of energy. For this reason, bottom balancing was introduced (and rather successfully at that) by people building DIY electric cars, such as Jack Rickard at EVTV; until then, not only they lost a lot of cells, but some also managed to incinerate a few vehicles.

In order to bottom-balance a set of cells, each cell must be discharged down to a voltage that is at or below what the low voltage cut-off setting will be. Typically, this would mean a value of about 2.5V. The best and quickest way to achieve this is wiring all the cells in parallel and discharging them through an automatic low voltage disconnect device. Power resistors or light bulbs are all usable loads for discharging.

One should remember that if the cells are accidentally over-discharged in this process, they will be destroyed. Over-discharge means reaching below 2.0V for LiFePO4 chemistry.

Once all cells are down to the same low stabilised voltage, the bank can be assembled and charged.

A bank that has been bottom-balanced will invariably go out of balance at the end of the charge. This is unavoidable. The voltage of the smallest cell will peak up ahead of the others and throttle the charging current. If, at this point, charging is not immediately discontinued, this cell will quickly get damaged through over-charging.

Top Balancing

The goal of top balancing is ensuring all cells get full together at the end of the charge instead. Top balancing is almost the rule for all applications where very deep discharge essentially never happens, and this precisely includes marine house banks. Top balancing makes the task of recharging the bank more straightforward, because the total battery voltage is distributed quite evenly across the cells near the top end.

At the bottom end, one cell will invariably drop out first and if the bank is discharged beyond this point and the voltage of the weakest cell falls below 2.0V, it will be destroyed by over-discharge.

All cells are fully charged before being assembled together. A deep discharge can cause the smallest cell to "hit the bottom", but charging is normally no issue and all cells can easily be stopped short of over-charging.

All cells are fully charged before being assembled together. A deep discharge can cause the smallest cell to “hit the bottom”, but charging is normally no issue and all cells can easily be stopped short of over-charging.

Top balancing is by far the most common process used for building a lithium battery bank, because cell imbalance issues at the low end normally never become apparent, on the basis that cycling that deep doesn’t normally happen; at this point, the bank hardly has any stored energy left and cutting it out becomes a simple and logical response.

In order to top balance the cells, they need to be charged in parallel until well into the upper “knee” region of the voltage curve, where small differences in state of charge become very visible in terms of cell voltage.

How the cells actually get charged is irrelevant as long as they are kept within their voltage limits throughout. Unlike often stated, there is no point pushing the cells to voltages far exceeding 3.6V to balance them. It is just a good way of starting with electrochemical damage and achieve absolutely nothing else.

We are going to present two options for top-balancing a set of cells.

Method 1: Charging and Balancing Cells Using a Regulated Power Supply Unit

There are a few options available for first charging and balancing the cells. Using a regulated bench top power supply unit (PSU) is the commonly promoted approach and also the least practical and accessible for a one-off job on board – which is often the context in place when building a DIY system on an ocean cruising yacht. This process is very slow, inefficient and requires a regulated power supply unit and mains power for several days.

In some cases these constraints don’t apply or this method can be combined with the second method to “finish off” the cells, so the process is explained below, but you should prefer the second method described.

Never use a common battery charger: its output is unregulated and, even if it is able to hold without overloading and tripping, it cannot limit the voltage as the cells charge up. The guaranteed outcome will be a totally destroyed set of cells at best, or a fire. Don’t imagine for a second that you will be able to “see it coming” and prevent it. The voltage seems to remain constant forever and then rapidly rises without any warning.

You need an adjustable, regulated power supply unit to follow this process.

Top balancing lithium cells

Parallel charging and top-balancing cells using a regulated power supply unit (PSU).
Voltage regulation is essential to ensure the target voltage cannot be exceeded.

First of all, power the PSU before connecting anything to it and never interrupt the mains for as long as there are batteries connected to it. Some PSUs are not well protected against reverse current flow and not intended for use with large capacitive loads!

If possible at all, use a PSU that is explicitly suitable to charge a battery; in doubt, use great caution as a mishap can easily damage it. If smoke escapes from it, you will never get it back in.

  1. With the output disconnected, set the voltage regulation limit at 3.40-3.45V and preset the current limit (if any) to a value that won’t overload the PSU. Refer to the manual as required. In doubt, always start with a low current limit and never exceed 80% of the rated output.
  2. With all the cells wired in parallel, connect the PSU, bulk charge and absorb until no current flows any more. The voltage will stay around 3.3V for a very long time before starting to rise. Charging this way can take several days. This will near-fully charge the cells without stressing them unduly, but don’t hold them at that voltage indefinitely. Keep checking up on them at least a couple of times each day. Briefly disconnect the cells and recheck the voltage limit setting on the PSU: better safe than sorry. Avoid charging the cells individually, or in batches; the whole process would take just as long, but would also result in some fully charged cells lying around for several days.
  3. Once the voltage has reached the PSU output regulation limit and there is no apparent charging current any more, disconnect the cells from the PSU and increase the output voltage regulation limit to 3.60V.
  4. Then, while standing by only, reconnect the cells and allow the voltage to rise up to 3.60V and stabilise for a few minutes; this normally takes little time and additional current, provided the cells were fully absorbed at the lower voltage. Whether you target 3.60V, 3.65V or even 3.70V is of no consequence or interest if you are actively monitoring the process, because these values are often reached seconds apart only.
  5. Disconnect the PSU from the cells again and wait. The cells should hold above 3.50V for at least 30 minutes. If not, bring them up again and hold them for a little longer until they do. At 3.60V, you may need to insist a little more than if using 3.65 or 3.70 volts; that’s all.

As with all unattended charging of lithium batteries, some very careful thoughts must be given to the potential consequences of a failure somewhere

Using a regulated PSU, a failure – no matter how unlikely – of the unit cannot be entirely excluded and there is no other line of defence in place. Hopefully, it would just trip, but if it didn’t, it could lead to a battery fire. Someone could also come past and interfere with the equipment during charging with the same outcome.

Method 2: Charging and Balancing Cells on Board

After going about charging and balancing cells in a few different ways, I devised this method. It has since become the solution of choice for one-off lithium battery projects, because it is much more efficient and doesn’t require equipment that is not already available on board.

The idea is rapidly bulk-charging the cells using the boat’s engine and alternator and then addressing the balancing part separately.

I also consider it as potentially safer, because it is short enough to be fully supervised.

While the process usually takes a couple of hours only, it requires unfailing vigilance. This is only feasible because the timeframe is short. If you have access to a regulated PSU, proceed up to step 6 and then consider finishing using the first method.

  1. Assemble the lithium cells in the final topology the bank will be using and bolt the on-board battery cables to it, as if performing a direct replacement. In some instances, this requires shifting the old lead-acid cells out of the way first. It is important that a heavy-duty connection is made between the lithium battery and the alternator.
  2. Start the engine normally, run at idle for a couple of minutes and then rev it up. This will immediately result in a high alternator output. Check that the B+ (output) post of the alternator doesn’t heat up; this would indicate a bad or dirty connection and easily cause the alternator to fail. Also check all the cell connections for any temperature rise. All electrical connections should remain cold. Next, be mindful of alternator temperature. It is advisable to keep the charging current no higher than 80% of the alternator rating. Keep the engine compartment open if necessary and reduce engine revs if required. Twin-engine vessels like catamarans can (and usually should) charge with both engines. Keep a voltmeter connected directly to the bank.
  3. Make a cup of tea and watch the voltage. New cells normally ship at 40-50% SOC, so a simple initial calculation can provide an idea of charging time. It is normally a matter of 1-2 hours. After remaining stagnant around 13.40V for a long time, the voltage will eventually start to rise. Periodically measure the individual cell voltages to ensure they don’t diverge abnormally and all remain below 3.60V. If this becomes tedious or distraction sets in, shut the engine down, disconnect the bank and carry on later. Should any cell reach 3.60V prematurely or, conversely, clearly lag behind the others, it is an indication that the cells weren’t in a consistent state of charge at all when sourced. This should be seen as a warning flag about a potential quality issue, like a significant difference in self-discharge rate or internal resistance.
  4. The voltage will eventually reach the alternator regulation limit, normally 14.20-14.40V. If this was set higher (through the use of an external regulator typically), don’t allow it to exceed 14.40V. At this point, the individual cell voltages should still appear very even, because the cells were charged at a fairly high rate and are not full yet; only the bulk charge has completed.
  5. From this point onwards, differences in cell voltages are going to start appearing. Only individual cell voltages matter. Identify the highest cell(s) and gradually reduce engine revs, so none exceed 3.60V. Keep reducing revs until down to idle, then shut the engine down. On a twin engine vessel, cut back and shut down one engine first. After about 30 minutes, most of the absorption phase is complete and the unbalanced bank cannot be charged any further without experiencing excessive cell voltage issues.
  6. Disconnect and break up the bank, and now connect all the cells in parallel. If a cell is reading more than 0.1V higher or lower than the “pack”, parallel it with a small jumper cable at first to prevent any large current inrush, and connect it with the heavy link plate once the difference has subsided.
  7. Once all the cells are wired in parallel, they need to be properly topped up and balanced.
    1. If significant solar capacity is available, take the solar feed from the panels (before any charge controller!) and connect it directly to the lithium bank. Solar panels are current sources and don’t care about their output voltage. They will contribute about the same current at any voltage.
    2. Alternatively, bridge from the old lead-acid batteries (or a basic battery charger) using a few metres of electrical wire (not cable!). The wire acts as a resistor, dropping the voltage and limiting the current. Depending on the length available, 2.5mm2 (12AWG) or 4.0mm2 (10AWG) are normally suitable choices. If there are 6V batteries available, bridge from 6V, otherwise bridge from 12V. The wire will heat up as a result of the voltage drop. If it gets too hot, stop and use a longer or smaller wire. Use caution and common sense.
    3. Bring all the cells up to 3.60-3.65V, disconnect and keep recharging this way until they all hold above 3.50V for at least 30 minutes. They are then full and balanced. Don’t leave the circuit closed and unattended under any circumstances; it would very quickly destroy all the cells.

This method is many times faster than parallel charging throughout, but more labour intensive and requires continued attention. It is only feasible because maintaining complete focus for a period of 2 to 3 hours is not unreasonable. If you are negligent or over-confident and leave the process unattended for any amount of time, you will likely damage or even lose the cells completely.

Additional Considerations

A few additional notes regarding cell balancing:

  1. Refrain from pushing cell voltages above 3.60-3.65V. You can trade a little more time for less voltage and achieve the same without stressing the cells. Balancing cells is not a hazardous process involving excessive voltages, infrared temperature guns and a fire extinguisher in standby: it is just a one-off, full parallel charge within normal voltage limits.
  2. Use heavy link plates or substantial cabling in relation with the charging current to connect the cells in parallel, and preferably feed “in the middle”. The objective is keeping all the cells at the exact same voltage while they are charging. If the cell interconnections are dropping voltage, the cells away from the feed point will see a reduced charging voltage, at least until near the end of the charge.
  3. Don’t waste time leaving cells sitting around connected in parallel. They don’t balance or equalise unless they are being charged in parallel at the end. I have specifically tested that. There is not enough voltage difference to keep driving current between cells until they balance out.

Discharging Afterwards

Once the cells have been charged and balanced as described above, they should be assembled into their final topology immediately and the bank must be discharged at least down to the equivalent of 3.325V/cell: that is 13.3V for a 12V nominal system, or 26.6V for a 24V nominal system. Don’t allow the cells to sit around at 100% SOC after balancing! If the balanced pack is going to remain out of service for some time, it must be further discharged until it doesn’t read more than 13.15V (or 26.3V) after balancing.

There are dealers who offer – for a premium – “pre-balanced” lithium battery packs that were fully charged in parallel, interconnected and then stored on a shelf… buying those often equates to paying more money for deteriorated cells, because the packs are typically never discharged again once charged and balanced. They may also have been exposed to excessive voltages for long periods while being charged.

This why I only ever source factory-packaged cells.

Next Steps

The tasks described in this article lead to building a top-balanced battery bank of a given capacity. Before anything can be done with it, the cells must be protected from voltage excursions outside their safe operating range and automated measures must be put into place to take action if any abnormal conditions, whether it is voltage or heat, are detected.

The electrical system on board must also be altered in order to separate charging sources from loads, so both can be isolated independently at any time to protect the battery if necessary.

Last but not least, charging sources must be made compatible for charging lithium batteries in terms of voltage and operation. This is not always possible and some devices may need to be replaced.

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