# [EVDL] how does internal resistance affect battery voltage?



## EVDL List (Jul 27, 2007)

I thought I had a fair handle on how internal resistance affects
battery voltage, both standing and while under charge. But I
knew I didn't understand it all. Example: when I was doing my
monthly battery maintenance a couple of weeks ago, I found a
battery that was significantly down on voltage (standing). I do
the battery voltages first, then walk through with the
hydrometer. I couldn't lift the float on my big hydrometer,
implying the sg was down below 1170 or so. It was 1258 or
thereabouts a month ago on that cell (I remember it since it had
been acting a little lower recently). So my question is, because
of the greater internal resistance than the other batteries,
shouldn't that battery have a higher voltage at, say, a charging
current of about 9A? It was significantly lower voltage. What
that means to me is that battery, at least to the extent that
cell affects the overall voltage, was at a lower state of charge,
and had more lead sulfate to be converted back to lead dioxide
and lead. My guess is that the battery had a lower internal
resistance than the other batteries. It has somehow fallen
behind in charging. Shorting through sludge at the bottom or
some other phenomena. Does it seem like I understood it and made
a proper diagnosis?

I have had some success recovering batteries by putting them on a
1-2A eternal charge at my battery nursing home, nursing them back
to health, and putting them back in the pack. (It's interesting
the recent comment that Neon John made about sulfate nuclei and
pulsing.) The four spares I buy with every pack (I've done that
for the last pack and this pack, my 3rd in my EV career) give me
some wiggle room. But now I am having to juggle more cells that
are dropping out. I sense this pack is closing out, although I
should have a fair shot to getting to next spring before I buy a
new pack - pack four. Pack 3 is just over 10K miles and over
three years. It was the pack that I learned a lot about
batteries with from Roland Wiench and Roger Stockton in the fall
of 2006, thread something like "flooded battery murder mystery".
I got the sg average up from 1230-ish back to about 1270 by
gassing off over several months the excess water I had added in
since the electrolyte levels were so low from the factory. I had
planned on purchasing 1265 battery acid to bring the electrolyte
levels back up closer to the meniscus (and I would've been real
interested in seeing just how much that would affect capacity),
but I think it's not worth it now. Just stretch it out till
spring, get my 12K miles and 10cents/mile battery cost.

I'd really like to get Roland's 2-4cents/mile battery cost
(wouldn't we all). But I doubt I'll ever get that. Especially
now with battery cost so high I may have to settle for
20cents/mile for a goal, given the hilly terrain and high amps my
batteries see every day. But maybe with pack 4 I'll understand
batteries a bit better and make fewer mistakes. Don't add lots
of water if your batteries are low from the factory - lesson 1.
Every one is a learning experience...

Thanks,
Chuck

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## EVDL List (Jul 27, 2007)

Chuck Hursch wrote about "how internal resistance affects battery voltage,
both standing and while under charge."

Chuck wrote about one cell that "was 1258 or thereabouts a month ago on that
cell (I remember it since it had been acting a little lower recently). So my
question is, because of the greater internal resistance than the other
batteries, shouldn't that battery have a higher voltage at, say, a charging
current of about 9A?"

Mick says: Chuck's mention of the "greater internal resistance than the other
cells" may reflect an incorrect assumption. Conductance checkers are so costly
that I assume there was no measurement of higher internal resistance. The
electrolyte at lower specific gravity may be "less conductive" than a more
highly concentrated electrolyte, but the plates are part of that circuit too
so the total internal resistance of that slumpy cell may not be "greater"
after all. 

When the end of charge voltage rises on a lead acid cell, that's related to an
increase of internal resistance. From that, if one cell does have a higher
internal resistance than others, it should indeed "have a higher voltage at,
say, a charging current of about 9A". 

This is true of a sulfated battery for example: the sulfate crystals increase
the resistance of the plates and the voltage ramps high under charge...
because there's not as much storage capacity for the energy being pumped in. 

Mick Abraham, Proprietor
www.abrahamsolar.com
Voice: 970-731-4675


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