[RE-wrenches] Magnum System - Discharged Battery Bank

[Larry]

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.
>
>