Originally Posted by Jd1
The cells that Balqon delivered are Thundersky cells which may or may not be the same as Winston (lost track of who stole who's design). If I take the weight of the 700 Ah cells and adjust to 400 Ah I get 26.8 lbs. The 400 Ah cells are spec'd at 30 lb so the difference is about 10%. I am not sure if this alone explains the difference but it sure is an interesting theory.
The 700 Ah cells also have 4 power posts compared to two posts on the 400 Ah cells so that might bring the difference in active cell compound up a bit - possibly sufficient to explain the capacity shortfall.
Interesting though, if you look at the 1000Ah cells they are way heavier per Ah then even the 400 Ah cells .... I guess I am saying that I am not sure if this weight theory holds true.
What were the cells charged to voltage wise?
Was this voltage measured at the battery terminals or at the charge source?
What was the tail current
at that voltage when you stopped charging?
What was the voltage of the cells that were not at 2.81V?
What were the voltages of the cells when you DC'd charging?
What were the temps of the cells during discharge?
How was the Ah counter set up?
When capacity testing it is important that you set it up in a scientific manner and that you can repeat this in the same manner the next time. In other words tests done in an A to B manner will yield different results test to test, while tests conducted in an A to A manner can be directly comparable.
These cells are impacted by cell temperature and folks tend to forget that. The lower the cell temp the less capacity to LV you get out of them. While not impacted as badly as lead acid by discharge temps, they are impacted by cell temp changes.
I try to do all my capacity tests at 75-80F cell temps. Terminal voltages and tail current are always cut off at the same point so that each discharge capacity test is done as close to an A to A manner as possible.
Also your cells are old-stock, may have been used, and may need to be cycled a few times to come up to capacity. You bough used old stock cells, I did too, and I feel I got lucky with mine. You seem to be on the opposite end of where I ended up, but I would not consider it bad
, especially for what you paid.......
Even if you are 60Ah's shy, which I would want to confirm more than once, you still have a 640Ah LFP bank for not a lot of dosh....
If you really want to confirm your usable capacity
do a discharge test at your vessels average load
... While capacity can
vary slightly between high current discharge and low current it is nowhere near the difference you see in lead. These have a low Peukerts constant but not zero Peukert effect....
As I have said many times I would not again buy cells from Winston/Balquon. I think they are horrible to deal with and the variability of the Winston cells seems to be
worse than with CALB. Lucky for me mine are doing fine. If I had to buy again, for my own boat, it would be CALB CA's.....
640 Ah's, for what you paid, seems pretty good... Run some more tests and see if it is repeatable or if it changes..