The Boat Nerd: Lithium Batteries

Lithium batteries, what are they and why should we as boaters care about them? 

Disclaimer: This article is offered as information only. We strongly recommend that boat owners do NOT install lithium-ion batteries themselves, but only have authorized dealers do it. Once installed, any changes or modifications must be done by authorized dealers. These batteries are managed by specific BMS devices calibrated for each specific installation, and tampering with, or changing out parts may lead to a fire – which generally can’t be controlled.

We spend three months of the winter on our Leopard43 “Peregrine” in the Caribbean (at least we did pre-covid!) When our 630Ah AGM batteries died after seven years I decided to go lithium for the reasons outlined below. I am rolling my own 400Ah battery using 16 100Ah GBS prismatic cells in a 4P4S configuration with a Tao BMS. ( I really like the Tao simulation mode).

With Peregrine we are never at a dock or on shore power and rely on primarily on  solar, with high output engine driven alternators as a backup. The lithium battery gives us the capability to use hot electric water heating more regularly and induction unit cooking.

Closer to home in Toronto, we have a Hunter36 “Dragonfyre” which we use for cruising, mainly yacht club hopping with occasional nights out on the hook. Dragonfyre is normally on shore power. Our 440Ah of AGM batteries died last year after only 15 years of service.

I replaced these with two drop-in 128Ah lithium batteries comprised of many cylindrical cells. The case for lithium in the way we use Dragonfyre is less compelling, however I enjoy trying new technology out and I got the batteries at a reasonable cost.

You Have This Technology

If you own a smart phone, tablet, laptop or portable power tools you are already using a lithium battery. These batteries are used as they are small, light weight, and store a lot of energy (high energy density).

Lithium batteries typically have twice the energy density of other rechargeable batteries, meaning they can be half the physical size for the same amount of stored energy.

Lithium batteries are becoming more mainstream in marine applications because of these characteristics. Electric outboard motors are now available in 5 to 50HP equivalent ratings, from manufacturers such as Elco and Torqeedo. Smaller fishing trolling motors are available from Minn-Kota. These motors use lithium battery packs ranging in voltages from 24 to 96 Volts.

The lithium battery market is maturing to the point many boaters are now switching from Lead-acid (flooded, AGM, or Gel) batteries to Lithium batteries and enjoying the many benefits such as reducing weight and allowing for such amenities as electric cooktops/ranges (and getting rid of propane) or even air conditioning.

Five years ago boaters adopting lithium were on the bleeding edge, with most rolling their own systems. Now costs are falling, many more vendors are offering cells, “drop-in” replacements (the term drop-in comes with a host of caveats), or complete systems.

Some boat owners are even ditching their gas and diesel  internal combustion engines and switching to electric motors for propulsion. However, just like in electric cars, for long distance cruising at higher speeds, battery capacity is still a limiting feature. It’s hard to rival the energy contained in a Litre of Diesel fuel!

For clarity, I am using the terms battery and cell in this article in the normal sense of a battery being a collection of cells. A bank is one or more batteries typically connected in parallel or series.

In the examples listed throughout this discussion, the numbers are illustrative and are intended to give a feel for real world results. Depending on factors such battery/bank size, loads applied, age of the battery you may see or have seen slight differences from some of the values used below.

Advantages of Lithium Batteries over Lead Acid (LA) Batteries

1. Weight

Lithium batteries typically weigh 50% or less than comparable LA batteries. If you have ever lifted a 4D or 8D LA battery into your boat you know what a chore that was. A Trojan 8D AGM battery with a 230AH @20Hr rating has a listed weight of 73Kg. A 200AH Voltlithium brand battery weighs in at 24Kg. Now multiply that weight difference times the number of batteries you have on board.

2. Capacity

Two factors come into play when looking at LA battery capacities vs Lithium battery capacities.

Firstly, our Trojan 8D rated at 230Ah does not give us 230Ah per discharge cycle if we want maximum life from our battery bank. For longest life (greatest number of charge/discharge cycles) a LA battery should not be regularly discharged past 50% depth-of-discharge (DOD). If you plug into shore power regularly you can bring the battery bank back to 100%  which is great for good battery life by avoiding sulfating. If you rely on engine driven alternators for your charge source, battery current acceptance drops rapidly past 80% charged, meaning it takes forever to get the last 20% into the battery.

For example you may start off at 100A (alternator limited) but near the end you will be down to 10A or under (this number will vary depending on the size of your battery bank but you get the idea). You will burn a lot of diesel trying to get that last bit of charge which is noisy and not good for your engine as running a diesel at low loading will lead to carbon buildup and reduced engine longevity. 

If you have a solar array, the best way to charge (unless you have over one Kilowatt at which point it may satisfy all your charging needs), is to run the engine alternator in the morning when the battery is most discharged and take it up to 80% charged. Then solar can then work at bringing the battery charge up over the remainder of the day.

Thus a LA battery with a seeming 230Ah capacity is reduced to 30-50% of its rated capacity or, in this example, a usable 80-115AH. A smaller lithium battery of 100Ah or so could be a suitable replacement.

Secondly, LA batteries capacity changes depending what current you’re pulling from the battery. As the current increases the capacity falls. Peukert’s law is an attempt to account for this behaviour. For the Trojan battery above, at a 20hr discharge rate, the capacity is 230Ah.

At a four fold increase in current, (5hr discharge rate) the same battery is now rated at 179Ah. Lithium batteries do not exhibit this same behaviour, having a constant capacity. Depending on the load current you draw, especially if over 0.1C, you may get away with a smaller sized lithium battery. (“C” is a way of expressing current with respect to the rated capacity of the battery. If you have a 100Ah battery, 0.1 C would be 10A.)

3. Number of charge/discharge cycles

While not absolute and depending how the batteries are treated, lithium batteries typically have a cycle life up to 10 times that of LA batteries. The Voltlithium battery mentioned above states 6000 cycles at 80% DOD in their spec sheet.

4. Flat discharge voltage curve

Lithium batteries have a very flat discharge curve at a higher voltage than LA batteries, maintaining a terminal voltage of over 13V. Lithium batteries exhibit this flat charge/discharge curve with a fast and steep rise (charging knee) at the upper end and a similar fast and sharp drop (discharge knee) at the lower end. In normal operation you want to remain out of these knee areas resulting in a useable capacity of up to 90%.

LA batteries under load (assume a normal boat load 10A or under) will start off at about 12.8V when fully charged and drop uniformly to 12.0V at around 40% charged , falling to 11.7V or so at 20% charged. Lithium batteries in contrast start off at 13.2V and fall uniformly to 12.8V at 40%, then down to 12.5V at 20%. Most boat equipment will benefit from this higher voltage, which reduces the current it draws and the heat it generates, especially motors.

When hit with a high current load (think 100A or more) the voltage from a lithium battery will still remain just under 13V while a LA battery will be delivering  12V or under. This may prevent sensitive electronics dropping out during high current loads such as from windlass operation

5. Charge Efficiency

Charge efficiency is defined by looking at how much energy must be returned to the battery after energy has been removed from the battery,  to return the battery to the original SOC.

Basically how many columbs were removed and how many coloumbs must be returned to arrive at the same SOC. Lithium batteries are very close to 100% efficient meaning it you remove 100Ah from a fully charged lithium battery, putting 100Ah back will restore it back to a fully charged state.

In contrast LA battery efficiencies vary from 70% (flooded) to 90% (AGM/Gel), meaning an extra 10-30% energy must be returned to the battery. As LA batteries age and sulfate this efficiency  will drop even more resulting in more energy being needed to charge the battery.

6. Charge rate

As alluded to above, LA batteries will accept their maximum charging current when they are deeply discharged. As they charge the charging current is maximized until 80% State-Of-Charge (SOC) or so and then tails off exponentially so the final 20% charge takes much longer.

Flooded LA batteries can accept a charge current of about 25% of the battery/bank size. For example a 200Ah battery/bank will accept about 50A maximum. AGM and Gel versions of LA batteries do a little better accepting about 35% of their rating or about 80A maximum in this example.

Lithium batteries are a totally different animal, charging much faster, typically at what the charge source can put out.. They will continue to suck the maximum current from the available charge source until they are virtually at 100% charged. For a 200A lithium battery that can accept a 1C charge rate (lithium batteries are typically characterized by charge/discharge currents specified in terms of the battery capacity “C”. In this case C=200AH so 1C is 200A), that means it can take 200A.

For longer life, charge rates should  be limited to 0.5C or lower but that still amounts to 100A per 200Ah battery. If you have a 600Ah bank, it can accept between 300 and 600A. Not many charge sources can provide currents like this. Whereas with LA batteries they become the charge limiting entity, with lithium batteries the charge sources become the limiting element.

We will talk a little further on how this can impact your engine charging system.

7. Physical orientation, fumes

Flooded LA batteries must be installed with the water ports facing upwards to prevent the electrolyte from running out of the battery. They also release hydrogen gas when charging in the absorb phase limiting where they can be mounted in a vessel.

Sealed Lead Acid (SLA) (Gel or AGM) can be mounted on their sides, however in the event of overcharging conditions the valves will release the battery over pressure gasses and should still not be mounted in living spaces.

LA batteries can experience thermal runaway whereby they start heating up, lowering their internal resistance, drawing more charge current, heating up further -  a vicious cycle that can end with the battery exploding.

Lithium batteries have no constraints as to their physical mounting orientation and do not release gasses. They may swell under abnormal charging conditions but should not burst, explode or catch fire.

8. Storage

LA batteries should be stored at 100% SOC. They will self discharge at a rate of 4-6% per month. Left in a partially discharged state they will start to sulfate ( a build up of hard lead sulphate crystals) and this process cannot be reversed during charging resulting in a gradual loss of battery capacity. In long periods of storage they need periodic charging.

Lithium batteries do not like to be kept at 100% SOC. Before storage they should be dropped to around 70% SOC. Lithiums too will self discharge, but only at 2-3% and do not suffer from sulfation. As long as they do not get completely discharged they will be good.

9. Cost

When investigating lithium batteries for installation on a vessel, the first reaction is sticker shock. Yes lithium batteries are expensive. The good news is they are coming.