I came across this post which had me concerned about all of us using low-current (~10A, typically) chargers:
So I hooked up my 8 winston cells in parallel (100ah) with the standard copper connectors and put a 100a fuse on the positive side. Then hooked up my 3.65v 5A lithium charger and plugged in. It has
www.cruisersforum.com
Specifically: "I can't stress this enough:
you cannot determine state of charge (SOG) by measuring the charging voltage alone. At low charging current (like the current you are using) you can seriously overcharge a
LiFePO4 battery by applying any voltage over 3.4 volts per cell for too long."
I don't know if this person is correct or if any battery manufacturer even specifies these sorts of things at low charge currents. The battery manufacturers are probably assuming industrial users will have bigger chargers that can deliver hundreds of amps to a battery pack like the 4x or 8x 280Ah ones most people are assembling on this forum. That other forum's post also talks about "springback", basically saying the higher the charge current the larger the spring back, so to reach an equivalent resting voltage you need to set a higher stopping voltage if you're using a higher charge current. For a lower charge current you would not want to go all the way to 3.65V, because there is less spring back by that logic, if it risks damaging the cells.
Does anyone have any evidence on whether this is true and if we should either:
1) use higher-current chargers (well obviously we should ideally try to use 0.1C like EVE recommends)
or
2) use 3.35V resting voltage as a gauge of fullness rather than charge current diminishing at 3.65V?
Figure 1 reproduced here of the EVE documentation (
http://www.dcmax.com.tw/LF280(3.2V280Ah).pdf) suggests any resting voltage over about 3.31V is on the "vertical" part of the curve and is essentially fully charged. Granted, that plot only goes down to 0.1C discharge, not 0.0C, but we can extrapolate and guess that at 0.0C (open circuit), it's just another step higher like 0.1C to 0.2C is a step down. I'm leaning towards the "charge then rest" method as I don't have a particularly high-current charger (6A).
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Table 1 of the EVE documentation says for room temperatures (10-45degC) "charge to 3.65V cutoff with the current of 0.5C", but of course for an 8x280Ah battery, this would be 1120 Amps (right?) which is absurdly high.