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

The 10yr lifespan might be a bit skeptical---I'm still driving my 2011 THINK City car (85kmiles) on its 1^st battery pack with date codes of 2010 --- ie. 14 years+ old cells.  I've never had an issue with any of the cells, BMS or modules in the 12 years I've driven the car (knock on wood).  But then I have been pretty lucky.  The furthest I've driven on a charge was about 81 miles maybe 11 years ago.  Then the other day I drove 66 miles and still had about 7 miles range left before the car would shut me down…

I have also been powering my 26' boat with 10 of the same module assemblies for about 10 years now with no issues or degradation that I've seen.  Early on, with the 1^st module to power my boat I came out to the marina after being away for a month and found the module had dropped from near 48v to 3vDC.  My heart sank (I had paid about $1000 for it at the time).  I had forgotten to physically disconnect it from the powered-off charger, which drained the module over time.  Not panicking, I pulled it out, took it home and found it was already back at 13v.  Slowly I charged it up to the 30v minimum and then charged at a good charge rate to 48v.  It absorbed rated capacity and all cells tracked.  I was pretty lucky.

A Nasa study on lithium chemistries found that contrary to popular opinion, the cells don't just die when you take them below, say 2.5vpc (e.g. lithium ion).  Rather, they found no irreversible damage until the cell voltage dropped below something like minus 0.4v (guessing).  Essentially this meant that any single cell that under load can only drop to zero volts won't get permanently damaged.  The problem comes when we string these little buggers in series.  In that typical case, even a single module of, say, 12 cells in series runs a significant risk that at least 1 of those cells will reverse voltage if a load is connected up that drains the module (or pack).  Less risk if the cells are all balanced and have nearly identical capacity.

There are ways to eliminate the risk, but all add parts, volume, connections and cost.

 

-MT

 

From: electricboats@groups.io [mailto:electricboats@groups.io] On Behalf Of gsxbearman via groups.io
Sent: Monday, November 11, 2024 3:40 PM
To: electricboats@groups.io
Subject: Re: [electricboats] Cells (modules) are dying.

 

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.

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.