Lithium-ion battery myths
Duncan Kent looks into the latest developments, regulations and myths that have arisen since lithium-ion batteries were introduced
Although there remains a large number of lead-acid battery aficionados in the more traditional marine electrical businesses, battery technology has recently progressed in leaps and bounds. Over the past couple of decades, the world’s top battery experts have been concentrating all their efforts on the development of more efficient energy storage, both on land and at sea. But despite this, there remains a great deal of confusion among boat owners, marine businesses and chandlers as to which batteries are safe to install into sea-going vessels.
Traditional batteries
Most interested parties have their own theories and experiences of which type of battery is best for use in the marine environment, particularly boat owners who live aboard for extended periods. In the past, the choice was inevitably big and heavy, open flooded lead-acid batteries if you wanted them to last and handle a certain degree of mistreatment. Many still swear by this simple, flooded lead-acid technology, where you can top them up with distilled water every month or so and regularly test the capacity of each cell using a hydrometer.
There’s a certain amount of truth in the old saying ‘heavy is best’, referring to the fact that the heavier the battery was the thicker the plates were likely to be and the longer they would last. Indeed, they’re still easy to source and less expensive initially than newer tech batteries. However, their main drawbacks: their large size, extreme weight, resistance to rapid charging and tendency to lose their capacity over time are what is gradually condemning lead-acid batteries to the recycle bin in favour of more modern battery designs.
It is now generally accepted by most of the marine industry’s regulatory groups that the safest chemical combination in the lithium-ion (Li-ion) group of batteries for use on board a sea-going vessel is lithium iron phosphate (LiFePO4 or LFP for short).
While rumours about ‘lithium’ batteries causing fires are rife, most of these arise in the electric vehicle (EV) arena, where there have indeed been some quite frightening cases of the more volatile types of Li-ion batteries bursting into flames and the fire services being unable to extinguish them quickly.
A safer alternative
Although part of the Li-ion group of battery chemistries, LFP batteries have been proven to be as safe, if not safer than the more traditional lead-acid variety when installed and managed correctly.
Experiments have been carried out by numerous regulation bodies, including the particularly stringent American Boat & Yacht Council (ABYC), and all have (in some cases reluctantly) agreed that LFP batteries are the safest of the group and the only Li-ion type they would approve for use on board. Admittedly, this is
supplemented by their insistence that any LFP battery bank must always be professionally installed and/or certified.
Benefits and limitations of LFP batteries
Like all Li-ion batteries, LFPs have a much lower internal resistance than their lead-acid equivalents, enabling much higher charge currents to be used. This drastically reduces the time to fully recharge, which is ideal for use in boats where charging sources and time can be limited. In addition, they offer a cycle life way beyond that of any lead-acid battery, including top-quality absorbent glass mat (AGM) batteries.
The maximum discharge rate of an LFP battery will be limited, however, so you’ll need to know what this is for any particular battery when you’re planning your new system. This is especially important if you want to run heavy current draw items such as an electric cooker, microwave, kettle, water heater etc, via a direct current-alternating current (DC-AC) inverter, as you may need to install several batteries in parallel to supply the total current required.
Battery management
All Li-ion batteries require an electronic battery management system (BMS) to ensure they achieve their optimum performance and condition, while remaining safe at all times.
A good quality BMS will…
■ protect the battery from a dead short
■ provide reverse polarity protection
■ monitor and balance the voltage level of each cell in the battery
■ warn and prevent the cells from being over-or under-charged
■ warn and prevent the battery from overheating
■ prevent the battery being charged if its temperature is below freezing
■ monitor the battery status and transmit the relevant data to the user via a display or to a smart device via Bluetooth
■ communicate with the charging system to ensure optimum performance
Most LFP batteries come with a built-in BMS and are often sold as supposed ‘drop-in’ replacements for lead-acid batteries. However, there really is no such thing as a ‘drop-in’ LFP battery. You could, in theory, simply add an LFP battery in parallel to an existing lead-acid battery bank, but not without really knowing what you’re doing and only if you’re prepared to risk alienating your insurer. You would also find it very tricky to get anyone else to work on your system should it fail as few qualified engineers would go near such a hybrid of differing chemistries!
Often poor quality, the more basic integral BMSs usually provide far less protection. Some only offer under/over voltage, overload and over-temperature monitoring, with the triggering of any of these events resulting in the battery simply shutting down–not ideal when you’re navigating at night, especially as many of them then require a spike from another, non-smart 12V source to reactivate them.
In many ways, it is safer to buy LFP batteries with no integral BMS and add a top-quality one yourself.
Installation
Apart from any mistakenly perceived fire risk, LFP batteries do have a few drawbacks. To start with they’re still a relatively expensive investment when upgrading from an existing lead-acid system, although prices of the LFP batteries have more than halved over the past five years. The extra expense is because much of the ancillary equipment also has to be upgraded to make it properly compatible with the LFPs. This frequently involves the purchase of a new AC-DC charger, alternator charging system and split charger. Plus you’ll probably need to beef up the cables due to the higher charge currents and upgrade the main fuse to T-class.
Charging LFP batteries
When considering a change from lead-acid to LFP it’s important to understand their limitations and quirks, especially their charging parameters, in order to create a safe and efficient energy bank that will last a very long time.
Most importantly, LFP cells must never be overcharged. Unlike lead-acids, they actually prefer to remain between 40%-80% charged most of the time. If regularly cycled then charging them closer to 100% state of charge (SoC) makes sense and does no harm, provided you stop charging them as soon as they are full (usually 3.65V per cell or 14.6V for a nominal 12V battery).
Unfortunately, the SoC cannot be accurately determined by monitoring the battery voltage alone, which is likely to peak when the battery is only actually
half-charged. Fully charged, a 12.8V LFP battery has a rested voltage of between 13.3V-13.4V, notably higher than the 12.6-12.7V of a regular lead-acid battery. At 20% SoC it could still be registering 13.0V, so it is almost mandatory to install a good quality, shunt-based battery monitor with current measuring capabilities.
LFP cells should be charged at constant current until the charge current drops to between 0.03C-0.05C (depending on the maker’s specifications), at which point charging must cease immediately so as not to overcharge the cells. Most will accept at least half of their own capacity (0.5C) in charge current, ie a 100Ah battery will accept a constant 50A charge. Depending on the make, some even allow up to a 1C (100A) charge rate, although this results in very rapid recharging, which can damage the batteries if done repeatedly.
No LFP battery likes an absorption charge as it stresses them and can cause overheating. Ideally, you need to skip the absorption stage entirely and set the float level to between 13.3V-13.5V.
Depending on the BMS, most LFP batteries do need to be charged between 3.5V-3.65V per cell at least once a month in order to allow the BMS to rebalance the cells.
AC-DC chargers
Li-ion compatible AC-DC chargers don’t actually require a multiphase regime, just a bulk charge to nearly full and then off. So, you could, in theory, use a simple single-stage charger, provided you’re there to switch it off immediately when it reaches full charge. Older, transformerstyle battery chargers are not suitable for Li-ion, though, as their output is often too crudely formed. They can sometimes be used to ‘spike’ a sleeping BMS into action again if it has shut down for any reason, although this action is not recommended unless you know what you’re doing.
If you already have a ‘smart’, multi-stage shore power charger for lead-acid batteries you may still be able to use it providing it offers an LFP setting or a customisable output that offers sufficient control over the charging regime. You will need to be able to limit the bulk charge to 14.4V (or a little less if preferred), plus disable the float and de-sulphation (aka: equalisation) functions. The increased voltage of an equalisation charge, often up to 16V, can quickly destroy unprotected LFP cells and risk system meltdown.
If in any doubt ask the manufacturer of the charger for advice as this is very important for the ongoing health of the batteries and the safety of the installation.
Alternator charging
A standard marine engine alternator with its own internal regulator is only really designed for charging a thin-plated, starter type lead acid battery. It is expected to supply a high current initially, before rapidly dropping to a level well below 50% of its peak output capacity.
Because of its very low internal resistance, however, an LFP battery will suck everything it can from any charging source until it is nearly full, which is a quick way to overheat and destroy a regular automotive-type alternator.
Not upgrading your charging sources and management systems not only risks damaging your new LFPs, but could also negate their principal benefit, their capacity for rapid charge acceptance.
If your engine alternator is your principal source of charge for your batteries then I’d recommend you replace it with a proper marinised, high-output version, along with a high-tension drive belt and pulley
arrangement. Furthermore, unless your new alternator has a smart regulator, it is highly advisable to fit an external regulator designed specifically for use with Li-ion cells, such as the Wakespeed WS500 or Balmar MC-614-H, which will ensure you get the optimum charge without the risk of damaging either the alternator or batteries.
Apart from the risk of overheating your alternator, another concern is what happens when your batteries reach full charge. Without a smart regulator to take control of the situation before problems arise, the BMS is likely to abruptly disconnect the charge source when it sees the battery is fully charged. Disconnecting the load on a standard alternator when the engine is running will instantly blow its internal diodes.
Solar charging
Solar is an ideal method of charging LFP batteries, particularly for long-term cruisers who spend a lot of time at anchor. You can feed as much power into the bank as the panels will produce during their most productive part of the day, only topping up via other methods in the evening if more power is needed.
Because of the LFP’s ability to accept a high current bulk charge, some owners choose to charge their batteries exclusively from solar energy. This works particularly well as solar charge controllers aren’t vulnerable to damage should the BMS abruptly switch off the charge when the batteries are full.
As LFPs don’t mind being put to bed at a low state of charge, they will happily sit idle all night ready for the sun to come up in the morning.
Temperature compensation
One disadvantage of the use of LFPs in the northern hemisphere is their behaviour in low temperatures. Although many will still supply power down to-15°C, they won’t actually accept any charge if the ambient temperature is below freezing. If you’re planning an extended cruise in extreme latitudes then you’ll need to find a way of keeping them warm, ideally above +5°C. This is common in electric vehicles, where the Li-ion battery modules are selfheating. The heating element, of course, consumes some of the stored power.
Li-ion cells don’t like being too hot either, 60°C being the point at which most battery management systems (BMSs) will choose to disconnect for safety purposes, which rules out their installation in most engine compartments.
Status monitoring
I’d also suggest you incorporate a shunt-driven battery monitor with programmable parameters and a switch to disconnect the charging source once the battery reaches the manufacturer’s recommended maximum charge threshold.
Furthermore, when choosing your smart batteries or buying an external BMS, I strongly advise you purchase the type that can transmit the BMS data via Bluetooth to your phone or tablet, as this will give you the real-time status of each individual battery cell. Communication is the key to a safe and efficient energy system, so try to buy equipment that will network together and share vital data with each other.
Conclusion
If you are thinking of installing a lithiumion battery bank on your own boat then please read everything you can find on the subject first and speak to as many suppliers as you can. Even then I’d recommend you seek the advice of a professional marine electrician, at least during the planning stage, unless you’re a competent DC electrician with extensive Li-ion battery knowledge.
It’s also worth checking with your boat insurer before you shell out on an expensive new system as not all are happy to accept any type of lithium battery bank, even LFP. The situation is changing as they become more knowledgeable about the subject, but even then they might well insist on professional certification of a DIY installation.