Re: lead-acid battery open circuit voltage

dmartin@xxxxxxxxxxx wrote:
> I've looked and looked for real data about how fully charged Open
> Circuit Voltage changes with age or cycles for a lead acid battery.
>
> The open circuit voltage I'm refering to is the voltage that you
> measure at 77-78F for a fully charged lead-acid battery that has
> rested for a while (actually the voltage you get if you extrapolate
> to zero current.)
>
> Differing OCVs can cause trouble when batteries are placed in
> parallel. Say that two fully charged 12V batteries have OCVs that
> differ by 0.1 volt. Maybe one has lead-antimony plates and the other
> lead-calcium plates.
>
> Now we hook these two batteries in parallel. Current will flow from
> the higher voltage battery to the lower voltage one until both
> voltages are equal; ie. 0.1 volt will be lost from the higher OCV
> battery.
>
> This is bad because that 0.1V probably represents more than 10% of
> that battery's total charge.
>
> I'm guessing that sulphation effects increase a battery's internal
> resistance but should have little direct effect on its OCV at full
> charge.
>
> It is hard to see how OCV would change with a battery's age unless the
> plate composition changes with age. This is entirely possible. I just
> don't know what happens in real life.
>
> Dave

Toooo many variables.... Battery charge and discharge is almost a subject
in it's own right. Obviously best to use identical batteries when in
parallel.
This is a useful link www.batteryfaq.org
Remember every discharge of a battery shortens it's life! Plus discharge
more than about 15-25% (car battery); 40-50% (leisure battery); 70-80% (deep
cycle) and you are doing irreversible damage. Questions on charging /
discharging batteries come up at regular intervals on boat / caravan groups
(I know at least 1-2 times a year on uk.rec.waterways)

--
--
Ron Jones

Don't repeat history, see unreported near misses in chemical lab/plant
at http://www.crhf.org.uk
"The most exciting phrase to hear in science, the one that heralds new
discoveries, is not 'Eureka!' (I found it!) but 'That's funny ...'" :
Isaac Asimov:


.

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