<html>
<head>
<meta content="text/html; charset=windows-1252"
http-equiv="Content-Type">
</head>
<body bgcolor="#FFFFFF" text="#000000">
<div class="moz-cite-prefix">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.<br>
<img src="cid:part1.02000200.04020002@starlightsolar.com" alt=""
height="504" width="210"><br>
On 12/19/14 9:44 AM, Larry wrote:<br>
</div>
<blockquote cite="mid:549455E0.3030700@starlightsolar.com"
type="cite">Corey,
<br>
<br>
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.
<br>
<br>
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.
<br>
<br>
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.
<br>
<br>
<br>
Larry Crutcher
<br>
Starlight Solar Power Systems
<br>
<br>
On 12/19/14 7:48 AM, Corey Shalanski wrote:
<br>
<blockquote type="cite">Update on my investigation:
<br>
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?
<br>
<br>
I do not see any evidence of distorted cases.
<br>
<br>
<br>
</blockquote>
</blockquote>
</body>
</html>