At 08:29 AM 5/05/2013, John wrote:
So, my old Electric Forklift has a weak battery.
Even after a full charge, the cells are reading a SG of 1.15.
Is there any way to restore some capacity to this? I really can't
afford a new battery for it.
G'day John, All
We bought two machines from a closed sawmill, each with 48V packs, of
unknown Ah, as they were made especially for these machines and not
labled. The batteries were divvied out amongst three people for RE
use, (1 x 24V dual string, 1 x 24V single string, 1 x 12V dual
string, but that is irrelevant to the discussion).
We discovered that all of the cells had low S.G. and low terminal
volts, but still had good capacity (since we don't know how much they
were supposed to have originally, we guess at least 70% and up to 90%
of capacity). How this came to be we don't know.
Well, the person with most available time (retired engineer) with the
single 24V pack has most time spent on bringing them up, and
discovered that just putting in new acid didn't work very well,
within a week the acid strength and terminal volts were low again. So
over the last 6 to 8 weeks he has been extracting all the 'top space'
acid from all the cells at full charge, and boiling it in a pyrex
bowl to increase the strength, topping up the bowl with
standard-dilution acid until he has added about 3 x the original
amount (making it about 4x stronger). Once cooled, adding it back to
the cells, and then doing a gas up charge.
Keep in mind that the guy doing this is a capable retired engineer,
used to dealing with hazards, so he is doing it in the open, keeping
away from the fumes, using eye and body protection, etc., and plenty
of Bicarb on hand in case the glass bowl decides it doesn't want to
be. The two other users are gassing up and topping up with acid (not
distilled water) so doing the same effective thing at a much slower rate.
The result of this is an incremental increase in terminal voltage and
S.G. so much longer run time before his equipment starts to complain
about low voltage. We don't know why it needs so much stronger acid
to have an effect, but that is the empirical data. These batteries
were replaced at the mill 18 months and 30 months ago, so were not
old, and we have no idea why the voltage was low.
In your case, strengthening the acid in the one cell may help, but
you are dealing with an old cell, which we were not. Improving acid
strength will not replace lost active material, if that is the
underlying reason.
I would suggest a 2-step approach: 1) attempt to improve the acid
strength, and add a voltmeter across that cell only to watch it
whilst you are using it, and see what it is doing. If the cell
voltage is reversing, then you are left with bypassing the cell and
running at a lower voltage. One of my forks had a cell bypassed
before I got it, and has just had the bypass link fail (the link was
screwed to the cell interconnects, not soldered, so they corroded and
failed. I have pulled the pack out, and left it sit for a couple of
weeks (longer than needed, but to let the H2 get away). I'll then
flame-solder a new shorting bar. Removing a cell may upset your
charger, if it is a smart charger it will not see the profile it is
looking for, if your charger is stupid it won't care, but you will be
gassing up the remaining cells more than they should.
If you are going to bypass a cell, hook a headlight globe or similar
across it for a few days to be sure that it is fully discharged
before you short it permanently :^)
Hope this helps.
Regards
[Technik] James
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