Hi Folks,
My GM 48 V LiFePO4 drops 3 Volts very quickly after a full charge. I assume that means I have a bad cell. Any testing suggestions, alternative possibilities, sources of replacement parts, etc? For background, I have kept it charged during the winter by charging it when it dropped to 52-54 volts indicated. My guess would be that it has been 100% discharged a very few times, most times I go by output voltage to determine when to charge it. Probably charged it less than 50 times total. Never out in the rain, never dropped, I use a remote disconnect switch....
TTFN,
Dennis
Hey.
It does not matter if you fully discharge a LiFePO4, I mean it does reduce the number of cycles you'll get, but you will still get at least 1200-1500 100% depth of discharge (DoD) cycles. If you only discharge to 80% you can get about 1600-2000 cycles out of it, depending on the cells make and type.
LiFePO4 only use at most about 3% of their charge over a month so if you store them over winter at 50% state of charge (SoC), after 4 months it will still have about 38% so you DO NOT NEED to charge it once a month. That is a typical misconception inherited from the lead-acid type batteries. Also LiFePO4 have no memory effect ( as all lithium based cells) so you can charge and discharge them as it suits you best without consequences.
It is important to store the Lithium based cells (LiFePO4 included) at ~ 40% SoC when not used for more than 1 month, that is to prolong it's life because at the 40% SoC level the internal oxidation processes are at their lowest.
For a typical 48v LiFePO4 battery nominal voltage is 3.3v so that means that a 16s pack voltage (nominal) will be 52.8v, but can reach as high as 56.8v when fully charged. Do not measure the voltage while charging because LiFePO4 chargers output between 57.7v and 58.4v, depending on make and type. Right after charge the 48v LiFePO4 battery voltage can be as high as 56.8v but it will drop over time (around 3 hours) to about 53.6v. That is probably why you say that your pack's voltage drops about 3v. It is a normal behavior of LiFePO4 cells. Although you can put about 3.6v into each of them, over time they tend to stabilize at around 3.35v... At least that is what I can confirm based on my 2 years and about 100 cycles limited LiFePO4 experience.
I never use
BMS on my pack. Both type of
BMS modules largely available will over time kill your lowest capacity cells, one by one. There is no way in avoiding that as all cell are different in capacity.
What I've done is I bottom balanced my cells to 2.75v each before making the pack. I've set my charger to output only 57.6v (instead of 58.4v). I try to use the cells in the 10%-90% state of charge interval, that is to prolong their life and avoid any unbalance. When the top set voltage is reached my lowest capacity cell reach about 93% SoC while The highest capacity one is about 89% charged.
When my pack is fully discharged, the cells will always reach 2.75v at the same time, thus keeping them properly balanced. There is no need to discharge them to 2.75v each time as the 2.95v is reached the same time and that is under 5% state of charge.
Initially I planed to bottom re-balance the pack every 6 months or so but after 2 years and over 100 cycles the cells are still balanced within 0.01v, probably more but that is the precision of my volt meter, and my pack also has had it's fair share of fully drained experiences... at least 10 cycles to 0% SoC.
I initially learned about the lithium based bottom balance technique from EVTV guys (EVTV.me), hope it's not considered advertising because it's not intended that way)
Hope this helps someone out there. Cheers!
