Here's a drawing to illustrate. When the 70AH is drained, the string
voltage will drop because that cell will go to 0 volts. Continued
discharge will reverse the cell voltage. When charging, the 70AH will
get full first and be overcharged.
On 12/19/14 9:44 AM, Larry wrote:
Corey,
The reason a cell or a battery in series will reverse polarity is due
to that cell having a lower capacity than other cells. For example,
you have a string of 100AH cells. One cell only has a capacity 70AH.
If you discharge the string more than 70Ah, in order for that cell to
continue passing current from the string, the voltage will reverse and
begin climbing in a negative direction.
If it were me, I would pull that cell (cells) from your pack and test
individually. Fully charge and do a controlled discharge rate. If it
comes up less capacity than the others it should not be returned to
the bank. For one reason, the entire string is limited to the lowest
capacity battery. That battery will always be first to reach full
charge and first to totally discharge, reversing polarity again.
Another reason is while charging, it will be severely overcharged
while you try to reach 100% SoC on the other batteries. If the battery
is AGM or GEL, it can go into thermal runaway and may cause a fire.
At the very least, charge the bank to 100% SoC and measure the
temperature of the cell(s) that had reversed. Measure before
transition to Float charge. If they are hotter than the others, you
know you still have a problem.
Larry Crutcher
Starlight Solar Power Systems
On 12/19/14 7:48 AM, Corey Shalanski wrote:
Update on my investigation:
I visited the customer's house yesterday to set up a pulse charger.
Much to my surprise the three batteries that originally registered
negative voltage are now reading positive. Does this make sense - are
batteries able to shift between negative and positive voltages at
such a low charge level?
I do not see any evidence of distorted cases.
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