On 06/19/2015 10:58 AM, Lee Hart via EV wrote:
Cor van de Water via EV wrote:
I have Lithium cells (CALB 180Ah) sitting in my garage since 1.5 years
and have measured their self-discharge and plotted it in an Excel
spreadsheet.
I can *see* their self-discharge.
I can *see* effect from ambient temperature on the change in
self-discharge rate...
I can *see* differences up to a factor 2 in self-discharge between
cells...
I've done the same thing with Thundersky. The results were the same,
except that the self-discharge rates differed by more than 4:1.
Additionally, the self-discharge rate is dramatically faster at higher
states of charge. The closer it gets to 0 SOC, the slower the
self-discharge rate.
Since the voltage-vs-state of charge curve is so flat from about
20-80% SOC, voltage alone is a poor indicator of state of charge. This
will fool you into thinking there is "no self-discharge", because the
voltage won't change enough to measure between these SOCs.
To determine how much charge was lost over time, I fully charge the
cells. Then let them sit in parallel for a day. Then remove the
connections, and wait X days. Then measure the amphour capacity of
each cell. Recharge the cells, and repeat the process, but with
successively larger values of X. So for example, I might find that a
60ah cell yields:
- 60ah for X = 1 day after charging
- 58ah for X = 30 days after charging
- 56ah for X = 6 months after charging
- 54ah for X = 1 year after charging
Now, 10% a year is a pretty low self-discharge rate. 6ah per year
corresponds to an 8ma self-discharge current. The problem is that it
varies so much between cells. It means the cells can drift 10% apart
per year. This won't matter in the short term, but it adds up over time.
I'm also testing a set of A123 cells. The self-discharge rate is
similar, but the differences between cells is much smaller; less than
2:1. I'm coming up on the 2-year point for them next month.
Thanks for the methodology for REALLY measuring "self discharge". I was
quite taken aback when Paul claimed there was none! All cell
manufacturers make claims on low self discharge but, as far as I know,
no manufacturer claims no self discharge. I am curious as to how Paul
came to his belief.
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