[OT] Charging UPS batteries outside the UPS

[Drew Van Zandt]

>
> On 8/7/07, Jim Kuzdrall <gnhlug at intrel.com> wrote:
> > It is safe to recharge them with a conventional automotive battery
> > charger as you suggest.  ... If anything goes wrong, they will just
> > get hot or not take a charge.
>
> On 8/7/07, VirginSnow at vfemail.net <VirginSnow at vfemail.net> wrote:
> > Gel cells require a different current/time charging profile than
> > other types of batteries. ... a "standard" 12V battery charger ...
> > CAN DESTROY THE BATTERIES!
>
>   Hmmmm.  So much for an easy answer.  :-)


OK, then I'll offer a real EE, knowledgeable about battery charging answer.
Tell me what the capacity is, and the model number, and I'll check the
proper charging profile from the datasheet or give you a SWAG on proper
procedure.  (Had to build a charger for both SLA and Ni-Cad at my undergrad,
and have looked at NiMH recently for battery backed apps at work.)  Here's
some general info:

Depending on cell type, there's a maximum charging rate that doesn't damage
the electrodes or do weird things to the electrolyte.  This rate is
generally something like C/10 or C/20, where C is the battery capacity.
Again, depending on the battery type the divisor changes.  A pretty good job
of charging (which would suffice for you) can be done just by
current-limiting the charging supply to stay under this number, and then
charging for roughly the right amount of time (one of those light timers
will do in a pinch for limiting the time.)  Once the battery's been charged
at the "high rate" for the right amount of time, most systems switch to
either a trickle charge or occasional cycles of trickle charge.  SLA
batteries are some of the toughest batteries out there as far as
charge/discharge rates.  Fancy charging systems will even have 3 or 4
charging modes they cycle through on a charge, and then a different cycle
for maintaining charge once the batteries are full.

Now, as to the proper amount of time... you'd think that if you're charging
at C/10, that 10 hours would be right, but they're not 100% efficient at
returning stored energy; it will typically take around 30-50% longer than
the above calculation suggests.  Varies wildly with battery type.

That's all fairly vague; with a battery datasheet or possibly a model # I
might be able to give you excruciatingly proper procedure.

--DTVZ
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