Two years after he built it, engineer Andy Pag shares some surprising lessons about the performance of his lithium domestic battery setup…

Andy Pag's Lagoon 410 Cushla Battery And Bms

All our energy needs, for two of us, living full-time aboard Cushla for the past two years have been met by the lithium battery bank I assembled from individual lithium cells.

I used 16 LiFePo4 cells, each rated 3.2V and 120Ah. Connecting four cells in parallel I created what I call a ‘supercell’ of 3.2V 480Ah, and put four supercells in series to create the 480Ah 12V battery. I have a Daly Battery Monitoring System (BMS) rated for 250A. This protects the lithium battery in a few ways.

Firstly, it acts as a 250A trip. Technically lithium batteries will drain and charge at a current equivalent to 1C – their amp-hour rating – so in my case 480A, but there’s nothing on the boat that would draw that much so 250A is a good safety margin. Secondly it protects the individual cells by keeping them balanced.

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The key to a lithium battery having good capacity is that all the cells are in balance. When building the battery, the first step is to “top balance” the cells; put all the cells in parallel and charge them up to 3.65V.

This is the voltage where they’re pretty much all 100% charged. Then as the current flows in and out of the battery, all the cells will rise and fall together. Over time though one or two cells might drift and that means when you put in your 14.6V charge some cells will be lower but some will be over 3.65V.

Overcharging can damage them or worse cause them to overheat. I’ve found that the balancing function on the BMS doesn’t really have the power to keep on top of the large 480Ah capacity supercells and every 12 months I’ve had to disassemble the battery and top balance it again. The whole operation takes about a day if you plan it right.

Andy Pag’s 480Ah DIY domestic lithium battery bank uses individual lithium cells wired in series and parallel. He can power everything from his kettle, toaster and washing machine with his lithium battery – but not all at the same time. The limiting factor is the boat’s inverter, not the lithium cells

LiFePo4 also doesn’t like to be charged in temperatures below freezing. The battery management system protects against this by cutting off the battery. If your boat is in a cold climate, think about how to create a heated space around the battery so you don’t get cut off just as conditions get icy.

I chose the largest capacity cells available at the time. By using cells, I reckoned, if any one went bad I could swap it out without having to buy a whole new battery. But just a year later the 120Ah cells were pretty much obsolete and certainly hard to find.

The closest I could find were 240Ah. The pace of development and of price dropping has been astounding. Now if one of my cells dies I’ll have to upgrade the whole system.

Tools of the trade: a cheap power supply for top balancing cells, and good crimp tools for tight, corrosion free connections

LiFePo4 cells like to be charged no higher than 3.65V each or 14.6V. I set my chargers to the slightly lower 14.4V. It supposedly puts less stress on the chemistry and the cells will last longer. It also gives some headroom – if one of the cells is out of balance, it won’t overcharge.

The battery monitor connects to phone and tablet via bluetooth. The software is clumsy and glitchy but does what it needs to do. Checking ‘Diff Volt’ is a weekly maintenance task. If the cells are more than 100mV out of balance, it’s time to pull the cells out and top balance them

Make sure all your battery charging sources are tuned to keep below 14.6V; solar charge controller, shore power, generator and engine alternators.

Much is said about alternators overheating if asked to charge lithium batteries. The trope is that the alternators aren’t designed to output their max current for long without getting hot.

I haven’t tested this extensively as my solar panel array gives me plenty of juice, but when I have tested it I’ve found that keeping the engine revs high means the alternator fan is more than a match for the heat generated, so test your system before you splurge on a current limiter or upgraded alternator.

According to my reading of my policy, as long as the systems are all installed to code there’s no wiggle room for boat insurers to get out of covering a boat with lithium batteries.

As a qualified engineer I’m happy to sign off on the system. But it’s a worthwhile and sobering exercise to think through the consequences resulting from the failure of your battery.

When I boil the (2kW) kettle it pulls almost 150A from the battery. That’s enough to weld metal together so it’s more than enough to get the resistance of a corroded connection to glow red hot.

I had this on the cut-off switch for the inverter which I thankfully noticed due to the voltage drop between the inverter readout and the BMS readout.

Fig 1: Charge curve for LiFePo4. The voltage of a lithium cell shows if it’s full or empty. Nearing full charge, this jumps to 3.65V. The BMS uses this feature to disconnect the battery, preventing overcharging. Range per cell is 3V – 3.65V or 12V – 14.6V for the battery bank.

I’m constantly comparing voltage readings and checking connection temperatures with a laser thermometer. This is the best way to find corroded connections and it’s a maintenance job that needs to be done regularly. Now I polish all ring connectors with 400-grit on a flat block and slather everything in dielectric grease.

I’ve invested in good crimping tools for small and large gauge wires. After years of badly crimping connectors with pliers these things are a pleasure to use.

If I was starting today, I might look at off the shelf lithium batteries, which are cheaper than DIY cells these days and better at balancing themselves, but I’d definitely still get rid of the lead acid.

Fig 2: Discharge curve for LiFePo4. Lithium batteries have a lower internal resistance than lead acid, so high currents can flow through for longer and they charge faster. They don’t suffer from voltage drops as they discharge, even if they’re delivering high currents.

There’s a difference between lithium ion and lithium iron (also known as Lithium Iron Phosphate or LiFePo4) batteries. Phones and electronic devices use lithium ion.

If they aren’t constructed correctly crystals can grow inside the battery, and eventually stretch between the positive and negative terminals causing a short circuit and generating enough heat to start a fire.

Larger capacity applications like house batteries are better suited to lithium iron chemistry, and while they aren’t immune to catching fire, the internal chemistry means crystal growth isn’t such a problem.

Putting too many volts across them can cause them to heat up and deform, creating a risk. Piercing a cell could also cause an internal short and a fire so protect the cells from moving engine or gearbox parts that might fly off.

This feature appeared in the November 2022 edition of Practical Boat Owner. For more articles like this, including DIY, money-saving advice, great boat projects, expert tips and ways to improve your boat’s performance, take out a magazine subscription to Britain’s best-selling boating magazine.

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