I just realized it’s been two years since I installed the LiFePO4. No regrets.
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Note that fully discharging the battery does no harm, it is the charging process due to the high voltages that does the mayor aging. Together with prolonged high voltages due to any other reason (storing at 100 % capacity) as well as high temperatures.
high voltages… prolonged high voltages… high temperatures… that does the mayor aging
agreed
fully discharging the battery does no harm
There is academic research suggesting deeper discharge is associated with accelerated capacity degradation in LiFePO4 cells.
digression
It’s interesting to see how Dragonfly (owner of Battle Born) dances around the topic:
Dragonfly Energy lithium-ion batteries have expected life cycle ratings between 3,000-5,000 cycles for a heavily used battery. Light use can well exceed this rating. (emphasis added)
Why would light use lead to exceeding the cycle rating? TBF, they could be talking about C-rates or other factors in addition to DoD. But DoD is the focus of the paragraph.
In most cases, lithium battery manufacturers limit the depth of discharge to 80%.
Why would they do that if going to 100% DoD would not affect cycle life?
However, some manufacturers, like Dragonfly Energy and our consumer brand Battle Born Batteries, rate their batteries with a 100% depth of discharge. This means that you can use 100% of the capacity without excessively damaging the battery. (emphasis added)
IMO this is a tacit admission that there is a negative effect associated with discharging to 100% DoD.
I also suspect a bit of wiggle room here in the wording. “100% of the [marketed] capacity” is not necessarily 0% SoC when the manufacturer underrates battery capacity. A 105Ah batt with 100Ah removed still has 5Ah remaining (actual 4.8% SoC).
conclusion
I suspect discharging to 0% SoC (4.8% actual) is harmful but
- BB has sufficient padding in the price to cover support and (non-transferable) warranty service
- BB bets that LFP cycle life is long enough that the original (warrantable) owners will not still have the battery by the time any degradation appears
- and that the owners would not be willing/interested in actual capacity testing even if degradation were apparent
I have limited funds so I treat my LFP as gently as practical.
DoD does not say where it happens. 0…80 and 20…100 are both 80 % DoD, but with vastly accelerated degradation in the later case.
Low voltages are only associated with degradation when they are far below the cut off voltage, so <2.5 V here as well as for Li-ion and Li-Po. For Li-Po, which has arguably the least tolerance:
You can go down to 2 V and it will only damage the cell slightly(!) faster over the next 100 cycles of always going down to 2 V. All the batteries I ever had accidentally over-discharged recovered just fine. No increased leakage, no capacity loss.
Here an actual source, look at figure 7. Even down to 1.2 V the cells still reached 50 cycles until they were at 80 % remaining capacity.
I don’t know where the misinformation comes from regarding “instand damage” below somewhere in-between 2.5 and 3 V. But I assume that, since the voltage drops very rapidly at that point, people actually ran the battery down into the ground, even reverse biasing (negative voltage) the most depleted cell(s), thus doing real damage. This should not be possible with a BMS.
