On Fri, 17 Nov 2017 19:24:05 +0000, newshound
wrote:

snip

To reply to the two posts together, I recharge the batteries with a
traditional transformer/rectifier type car battery charger, so that the
current drops off as it becomes charged (because the cell voltage
rises).

Ok.

Once a cell becomes fully charged, the current it is passing
causes electrolysis, so it "fizzes".

'As it becomes near to being fully charged ...' ;-)

In old batteries, the capacity may
vary from cell to cell,

I think that happens in all / new cells within the same battery but by
varying amounts. We used to minimise this (for racing) buy buying
batteries made from 'matched cells'.

so you may find one cell fizzing while another
is not, that's a good reason to keep charging until they all fizz,

And the last one 'to fizz' also needs to fizz for a while to minimise
stratification.

but
of course you do not want to concentrate the electrolyte by doing this
for too long.

I think the risk is more the water loss from the electrolyte in
general and exposing the plates than any change in 'concentration (as
that will be redressed when you top up with water again).

I realise that a charged battery may still have some trapped hydrogen
and oxygen on the plates. So once it is charged, I will jiggle it and
leave it open for a day or two in the hope that the gas all escapes.

And the water. ;-( Are you sure the cell caps aren't the
recombination type (designed to allow the cell to vent pressure but to
reduce the water loss)?

After that, I check the level and I don't normally find I need to add
any more demin water.

Assuming the electrolyte is above or at least level with the tops of
the plates, I think you are supposed to top up (after it's been fully
charged)?

IIRC this was originally an 80 amp-hour battery and it certainly still
provides more than 10 AH at 12 volts, based on the lights that I run off
it.

Ok (the C20Ah rate would be only tested at that rate etc). So, for
your 'calibration' of the battery as being 10Ah I believe you would
need 48W of 12V lamp and would need to time it down to 10.5V or
(ideally) higher (if not a deep cycle battery). So, for it to be 10Ah
you would be able to run your (48W) lamp for 2 hours or a 100W for
something less than 1hr?

And it is usually powering a couple of PIR lights that also get
triggered through the night by cats, foxes, etc, so I suspect it's doing
better than that.

Ok.

I'm certainly not charging it for ten days. Might leave it connected for
two days, and while the charger starts at 8 amps or so it has dropped to
an amp or so after a day.

That sounds pretty reasonable for a largish / old LA battery. ;-)

So, I might be tempted to not remove the cell plugs when (or after)
charging and see if that makes any difference to your electrolyte
loss.

Cheers, T i m