I have some headway cells in the original boxes, charge them every few
months, if I wait too long some will be 2V and others 3.65, from being
equal the time before. I have jigs to measure self discharge directly, a
4 wire very accurate charger with a current measurement, I calculate the
self discharge from that. some have zero, i can't remember the worse
number, but it corresponded to 5% per month, as per manufacturer's spec.
I wrote to them, asking if I can get them in tighter groups, but no answer.
The reason it this started was that a pack Verne got for his bike was
very low capacity, found no cells were actually faulty, just at the
limits of self discharge.
On 15-May-18 1:18 AM, paul dove via EV wrote:
Hey thanks for your input. I’m sure a lot of people have varied experiences. I
must’ve met there probably differences between manufacturers. However that was
not my experience. I bought Bestgo sales hundred amp hour. I did extensive
testing before installing them in a vehicle. I put 40 for 100 amp our cells in
a 1986 Toyota Celica. I charge them 3.65 V per cell or 160.6 V. After sitting
for a while the voltage dropped 148.7 v or 3.38 volts per cell. I drove the
vehicle every day for two years. There was never a variance at the end of
charge. All the cells measured 3.38 V several hours after charging. I took the
cells out of the vehicle And they said on the shelf for year and a half or so.
I measure the voltage and they were all 3.38 V. However they do experience
reversible capacity fade. I discharged all the cells and Got 45amp hours the
first time. The second cycle I got around 65 amp hours and it continued to
increase for five cycles. On the last cycle all the cells measured Close to 100
amp hours.
Sent from my iPhone
On May 12, 2018, at 12:23 PM, Lee Hart via EV <[email protected]> wrote:
From: Cor van de Water via EV <[email protected]>
I also did tests on LiFePO4 cells and while self-discharge was low, I was able
to prove from my measurements over many weeks, that there is was about a factor
2 difference in self-discharge current between the best and worst cell. Sample
size was over 40 cells.
This is what I have found as well. Brand new cells, all bought at the same time
from a quality source are very similar. Their amphour capacity, internal
resistance, and self-discharge rates are very close.
But cells from cheap or low-quality sources have a much broader spread in
characteristics. Cells also get worse as they get old, or get cycled, or as the
temperature changes. Differences between cells accumulate over time, getting
worse and worse. You may get by without a BMS initially, but it gets needed
more as the differences between cells grows Not having a BMS means shorter life.
This is exactly what the author of the quoted article found, that the capacity
of the cells had not degraded, but the cells had gotten out of balance.
Yes. The BMS was too primitive to do its job of balancing the cells. Only
having upper and lower voltage limits prevents against catastrophic failures;
but does not compensate for differences between cells.
--
Excellence does not require perfection. -- Henry James
--
Lee A. Hart http://www.sunrise-ev.com
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