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8 hours ago, Kbeck said:I have major issues with lifepo4 cells. The discharge curve is so flat that you only catch the imbalance of cells at the extremes of the discharge and charge. I've had lifepo4 cells catch fire because the balancing circuits couldn't handle how big the imbalance was as it was charging. Many people built EV's using lifepo4 and ran into destroyed packs in as short as 6 months. Just my 2 cents on it.
I know someone on a somewhat personal basis who burned through several sets of LFP batteries (actually from the same seller as the batteries I have) in about a year with daily 80-100% DOD cycles...and some wimpy 30mA Chinese "balancing BMS" units. However, with a proper BMS setup (disconnecting charge/load if cells go out of range), the batteries should last a very long time.
Personally, I have a DIY 12kwh LFP bank...it'll have been in full-time service for 4 years here in a few months, and I've had absolutely no issues whatsoever with it. Worth noting that they were NOS (new old-stock) cells, and I have some 3A balancers of my own design on them. I've probably only ever ran to 80% DOD on 2 occasions (gloomy cloudy weather for a week). But my loads never come close to 1C, either. TBH the balancers hardly do anything. They got warm/hot the first few days after I put 'em on (in May 2017)...but ever after that, they barely activate at full "absorption voltage" of 3.53vpc. ("Float" is at 3.50vpc.)
After "tasting" LFP, I'm never going back to lead-acid batteries ever again. True, balancing and cell protection is critical for any lithium-based battery system...and true again, 90% of the "BMS" setups out there have a balance current of well under 1 amp. Fine for a little 5A battery...but not for a 300 amp-hour off-grid battery.
I know someone on a somewhat personal basis who burned through several sets of LFP batteries (actually from the same seller as the batteries I have) in about a year with daily 80-100% DOD cycles...and some wimpy 30mA Chinese "balancing BMS" units. However, with a proper BMS setup (disconnecting charge/load if cells go out of range), the batteries should last a very long time.
Yea. That was my problem. Most bms systems are super small balancing current compared to the capacity of the cells and charging rates of 1/3-1c. The other issue is a lot of inconsistent manufacturing from most of the lifepo4 producers I know of causing wildly varying cell capacities of batteries even in the same batch. You could buy 100ah cells that are +/-10% capacity. It is super hard to balance large format cells like that within a pack unless you combine them perfectly in parallel assuming you are making a pack large enough. It can still work, just super time consuming. That's why I like automotive ev packs. Volt cells, bmw cells, Nissan leaf, tesla cells. They are perfectly balanced when they come out of those packs and have very little variance. Lately I have been buying model 3 cells for around $120/kwh. Crazy good wh/kg and wh/liter with great cycle life and almost linear in discharge and charge so it is easy to spot the voltage and know what capacity it is sitting at without any coulomb counting.