Re: [electricboats] Cells (modules) are dying.

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I think that the charging to 100% is only detrimental to NMC batteries, not LFP. But you are correct that all the batteries have about a 10 year lifespan (max for NMC, min for LFP).

The thing to remember about the cycles is that the cycle rating is for degradation down to 80% of original capacity. This is still a very usable battery if this happens before your 10 years ends. Most of the NMC batteries are rated around 2000 to 3000 cycles and LFP around 4000 to 8000 cycles.

The 80% to 20% use/charging is to prolong the cycle life of NMC batteries because the NMC’s get most of the charging/discharging degradation in this first and last 20%. The cycle rating is so large on LFP’s that there is no need to prolong the cycle life because they will age out before they reach 4000 cycles. And they don’t seem to suffer near as bad degradation in these First and last 20% ranges.

From everything I have read the main detriment to life expectancy is rate of charge/discharge, but with a huge caveat that this rate of discharge/charge is relative to the discharge/charge rating of the battery. So discharging a battery at 2C or 3C that is rated for only a 1C discharge would lead to premature degradation. But discharging that same battery at 0.5C would actually make it last longer.

Float voltage is indeed different on any Lithium batteries, and there shouldn’t be constant voltage applied. My Victron solar charger disconnects the charging when the batteries are full (programmable) and then periodically checks the voltage and if it drops reconnects the charger. I can also program what voltage the charger converts from “bulk” charge (fast) to “absorption” charge (slower). I have mine set to do up to about 85% on bulk mode, then on up to 100% on absorption mode (3.6V per cell for me).

On a boat you should always follow the “rule of 3rds”. 1/3 out, 1/3 back, 1/3 in reserve. I’m going to repeat the most important part KEEEP 1/3 IN RESERVE.

So while it’s not advisable to regularly go below 33% on your boat batteries, it will not hurt your LFP’s if you do.

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