Batteries and Boats (Part Two)

So, another word (or thousand) on batteries, following on from Part One. This time the focus is on LFP batteries. We have pretty much exhausted the conversation on lead acids! I mean, my previous post on this subject was over 4,000 words long. This one will be shorter because we’ve covered the basics already.

At the risk of repeating myself and labouring the point, this is about Lithium Iron Phosphate batteries (LFP or LiFePO4), not Lithium Ion. If you don’t know the difference, read Part One again. If you still don’t understand the difference, feel free to leave a comment, below. Apart from your phone, tablet or laptop, do not put Lithium Ion batteries on your boat, ever! Re-read that previous sentence again, and make sure it is firmly implanted in your brain. An LFP cell has a nominal voltage of 3.2 volts. A Lithium Ion cell has a nominal voltage of 3.7 volts. That is all I’m going to say about those things.

So LFP batteries have a great Joule/kg energy rating. A lead acid battery has around 80,000 Joules per kilogram. In other words, how much energy can you store, per kilogram on your boat. LFP by contrast can store 580,000 J/kg. Over seven times the energy density. That might be enough to swing your purchasing power towards the newer technology, but the price comparison is painful. These batteries are coming down in price, but they’re still pretty expensive. They could cost anywhere from 2-4 times more than an equivalent lead acid. But wait! There’s more!

Remember when I said that running a lead acid battery to 0% charge was a really bad idea? Well, the same cannot be said for LFPs. You might not get to 0%, but draining the battery to 10% isn’t a big deal. Looking at this a bit closer, an LFP battery “lifespan” is measured in charge/discharge cycles. Imagine that you drain your house battery overnight, every night. You then use the engine or the solar panels to recharge it during the day. The typical life of an LFP battery says you can do this between 3,000 and 4,000 times. You’re doing the maths in your head, aren’t you? At that rate of going, assuming you are on the boat every day to discharge the house battery, it will last ten years. But it’s not that the battery will just roll over, point its toes to the sky, and stop working after 3,000 cycles. What happens is the capacity drops by perhaps 80%. So, today you buy a 280Ah house battery and in 2035, it’s only good for around 225Ah. But it’s still good! It’s just not as good.

So, these things sound wonderful, right? There are some things you need to worry about. First off, in my previous article I differentiated between cells and batteries. If it’s OK with you, I won’t repeat myself. An LFP cell has a nominal voltage of 3.2V. Not much use on a boat with 12 volt electrics. So you put four LFP cells in series and now you have a nominal voltage of 12.6 volts. That should be fine. But the charging voltage is 3.65V per cell which means your battery now has a charging voltage of 14.6 volts. The maximum voltage your lead acid charging circuit might show is 14.4 volts. Is the extra 0.2 volts going to make a difference? Probably not. But…

Whereas the Lithium Ion cell will catch fire if overcharged, LFP has no such terminal state. If you overcharge it, it will die. Silently. It will convert itself into a very heavy doorstop. Likewise, if you let the cell voltage drop too low, it will also die. You might think this isn’t an issue, and you can just make sure that you never let your boat battery voltage go below 10 volts, but you are making an assumption about balance. Let’s try a hypothetical example. Four cells. Three are fully charged and showing a 3.2 volt nominal voltage. One has been discharged and is showing 2.5 volts. The battery voltage is 12.1 volts. Fine, you say. Keep pulling amps. But the cell with the low voltage will now roll over and die. As they’re in series, your entire battery will stop working. The same thing happens when you’re charging the battery. You never want the cell voltage above 3.65 volts. But what if one cell is already fully charged? You will overcharge it and it will die.

There is a solution. It’s called a Battery Management System (or BMS) and it is essential. It has a connection to each individual cell and measures the differential voltage so that it can determine the state of charge of each cell. In the case of undervoltage in one or more cells, it will disconnect the battery from the load. Likewise if one or more cells is in danger of over-voltage. Without a BMS, you will ruin one or more cells very quickly. Fancy BMS units can also rebalance cells so that the charge voltage is directed mostly to the cell with the lowest charge. Most, however, will just passively monitor individual cell voltages to make sure no single cell is under- or over-charged.

As I mentioned last time, you should always use a lead acid battery to start the engine. Engine starter motors and lead acids were made for each other. Don’t mess with that relationship. But this raises an interesting issue. You might, for example, have a smart relay which connects the engine and house battery together, when the engine is running. Remembering our lead acid voltages, anything over 13 volts would indicate that there is a charging voltage present. Generally all batteries on the boat will have a common ground. When a charging voltage is detected, the relay will close, connecting the battery positives together. The charging voltage is then shared between both batteries, and the current is shared on an as-needed basis. Do not do this with LFP batteries. Firstly, there is no real float voltage with LFP batteries. Secondly, they can take considerable current when discharged. They will easily burn out your alternator (or the fuse, if there is one). Victron manufacture a DC/DC converter system which can be used to bring the charging voltage from the alternator to the LFP battery, without undue stress on the alternator and even though the charging voltages between lead acid and LFP are different. As they say, don’t leave home without one.

The same could be said for solar charging. Very fancy smart relays can also detect that there is a charging voltage on the house battery side as well as the engine side. The gain here isn’t enormous because the engine battery should always be at or close to 100% so the ability to charge the engine battery from the solar system probably isn’t worth the investment. Mind you, some sort of “in case of emergency” mechanism for diverting solar charge to the engine could be worth consideration. Something as simple as a cheap solar/battery unit and a A/B selector switch to send the solar power to the main MPPT and LFP charging system, or to the cheap lead acid charger. It might get you out of trouble.

One thing to consider, if you’re switching from lead acid to LFP for your house batteries, is the “front of house.” In this sense, I mean those big motors up near the bow; namely the windlass and the bow thruster (if you have one). In an ideal world, these items would have their own lead acid up at the bow and would charge from the engine or solar at the stern. When you think about it, a windlass isn’t all that different from a starter motor. It takes a large current draw for a relatively short period of time. Likewise, a bow thruster. It is quite possible to run these off a LFP system as they are capable of large current loads. But the use-case fits that of a lead acid battery more than an LFP. You’re never in danger of fully discharging the battery, unless for example your anchor has hooked some massive underwater chain and you’re actively trying to haul up half the sea bed. The issue with a lead acid at the bow is that it is a lot of weight in the wrong place. If you’re lucky enough to have a boat big enough to carry a lead acid at the bow, then separate your anchor windless and bow thruster from your house electrics. This also has the added advantage of reducing the I2R (I-squared-R) losses in the cable from the stern where your battery bank probably resides, all the way to the bow, where the windlass lives. On some boats, I’ve noticed that the anchor windlass is connected to the engine battery on the basis that you would normally run the engine when using the windlass. The disadvantage to this approach is if you need to adjust your anchor at 4AM in a gale.

Another consideration with LFP batteries is the discharge curve. Unlike lead acids, it is quite flat. What this means is that the nominal cell voltage of 3.2 volts is maintained from 90% state of charge right down to around 25% SoC. So how do you tell if your battery is at 90% SoC or 25% if you can’t just measure the voltage and apply a formula? Do you just keep pulling power until you hit the 25% SoC “shoulder” and the voltage quickly drops off? Ideally here you would measure current to and from the battery. You might find a BMS which will do this for you, or it might be part of the solar charging system. The system knows the rated amp-hours for the battery and tots up the current consumed/replaced. It needs to know that the battery is either fully charged or fully drained, before it can accurately predict the state of charge. It is nothing more than a system which says “well, they’ve been pulling ten amps for the last fifteen hours and it’s a 280Ah battery and it was fully charged to begin with. I’m going to say that the battery is at 46% of rated capacity.” Remember that this is an estimate. It assumes the battery is indeed 280Ah. It also assumes that the battery was charged and discharged through the shunt resistor and that no extraneous loads (or indeed charging current) has been provided out of band.

Overall, while LFP batteries aren’t as rugged as lead acid, and require specialist management and charging systems, they offer a new lease of life on the worn out house battery. Even deep cycle lead acid batteries weren’t really designed for the 100% to 10% cycling of your average house battery. Before switching from your current (pun intended) setup, make sure you have a proper BMS in place, you have a way of charging the house batteries from the alternator in such a way as to not burn it out, and you have a mechanism for connecting your solar (or wind) power to the battery. Also make sure that any sensitive electronics on board can handle the different voltage ranges.

On a final note, I have heard through the grapevine that insurance companies will request proof that any LFP battery bank has been installed by a “qualified professional.” This is quite the hurdle to overcome if you’re thinking of installing your own system. For starters, it is not for the casual DIY-er because you need to make sure you have essential components such as a BMS. You also need to make sure the batteries are kept in a dry area and free from seawater as much as possible. Bear in mind that when an insurance company says “qualified professional,” what they really mean is someone with a professional liability insurance policy. They want to be able to transfer any claim to the installer. Even if you’re smarter than the average marine electrician and have completed ABYC certification, you may run into issues because you don’t have any indemnity insurance.