[OT] Charging UPS batteries outside the UPS

[mike ledoux]

On Wed, Aug 08, 2007 at 09:24:40PM -0400, Ben Scott wrote:
> On 8/8/07, mike ledoux <mwl+gnhlug at alumni.unh.edu> wrote:
> >> I've checked the voltage on the battery, and it's less than 2 volts DC.
> >
> > In that case, there is almost certainly at least one internal short
> > in the battery, and no amount of charging is likely to fix it.  It
> > takes some serious work to get a 12V battery down to 2V.
> 
>   Even better: That was for each of the 24V packs.  Your comment made
> me curious, so I pulled all the wiring off the individual units and
> measured each one.  Each unit is giving between 0.5 and 0.7 volts DC.
> So either each unit has the same "internal short", or they really are
> drained (or something else).

There is almost no chance that you'll get those batteries to take a
charge at all, if they were drained that low.  As you're probably
aware, each of those 12V batteries is made of 6 2V cells.  Fully
charged each of those 2V cells is good for about 2.15V, fully
discharged is about 1.75V (for good, working cells).

Generally speaking, if a 12V battery with no load is reading
below ~10V, something is pretty badly wrong with the battery,
usually a short across one or more of the cells.

>From what you're saying, you're reading less from the entire battery
than a single "discharged" cell should provide, so I'm going to go
out on a limb and guess that these batteries are pretty old, and
heavily sulphated.  You may be able to salvage the battery well
enough to start the UPS by running an equalization cycle on it, but
that is somewhat tricky/dangerous with a sealed battery as you're
*trying* to overcharge in order to boil the electrolyte to break up
the sulphation on the plates.

>   Te battery was sitting, hooked-up, inside the UPS for I dunno how
> long.  You can cold-start this UPS model (turn it on without AC
> input), so I expect it is always drawing at least a little power from
> the battery (since the UPS front panel is microprocessor controlled).
> Maybe that would do it?

Yeah, PbA batteries do not store well when not kept fully charged.

>   According to a doc on Panasonic website, these are, indeed,
> "absorbed glass mat with calcium grids".  Another doc does have dire
> warnings about charging, though.  The short version is that without
> "constant voltage control" (whatever that means), the electrolyte
> breaks down and the battery performance is shot.  Maybe they were
> overcharged and that's why their voltage is so low.

It is perfectly safe to use any *quality* 12V PbA charger on an
AGM battery.  The only thing to "worry" about is that you don't
try to charge too quickly, as that can generate excessive heat in
the battery, causing the electrolyte to boil, which is a bit of a
problem in a sealed battery.  For batteries as small as typically
used in a UPS I wouldn't charge at anything over 2A.

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