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QUESTION:I recently picked up an old walk behind electric forklift. It has a 24 VDC
battery supply made up of 12 very ;arge 2 volt cells. (1300 pounds worth).
Since it did not come with a charger, I am wondering if any of the
electrical gurus here could suggest a schematic and spec for building one.
ANSWER: If you live within driving distance of Sacramento,
I know a surplus dealer who has one. He'd probably
sell it for less than scrapmetal prices.
I went through this myself, except for a 48 volt bank on a Hyster
electric forklift.
I wound up using a Lambda regulated power supply and ammeter, nominally
48 volts, but with adjustment, I could turn it up to about 53 volts,
where the current was what the power supply could deliver. Mind you,
this was a 600 watt power supply, and it took days to charge that much
battery. I imagine you'll need something like 27 to 30 volts. You need
a lot of current, and you won't be able to build that yourself cheaply.
Definitely you should get a specific gravity meter from the auto parts
store, and distilled water, to top up the cells and check their
condition.
Here are some more notes I made based on some research:
Lead-acid battery open-circuit voltage 2.15 V (full charge) to 1.9 V
(full discharge) (12.9 V to 11.4 V in a 12V battery). Thus a 12 V
battery has a 1.5 V open-circuit range which can be taken roughly as a
linear state of charge. Charging amperage typically 1/10 to 1/5 of
amp-hour rating. This might require 13 V at the start, 14.5 V
mid-charge, 16.2 or 16.4 V to finish, and 13.2 V to float.
Equalizing-charge voltage (controlled overcharging of 5 percent excess
voltage, applied after full charge, observe gassing, remixes electrolyte
and breaks down sulfation, read SG every hour, useful when the SG
between cells differs by 0.030 or more, finished when SG no longer is
rising) is 15.8 V. A simple "automatic" charger merely applies a
regulated voltage of about 14.4 V or 14.5 V and shuts off when the
current drops, in about 10 hours from full discharge. Charging voltage
may be lowered for higher electrolyte temps by a compensation factor of
3 mV per deg F above/below 70 deg F. Specific gravity is typically from
1.130 (full discharge) to 1.280 (full charge). Another authority gives a
lower range of 1.120 to 1.265. Cells in a fully charged, idle battery
should be within +/- 0.05 volts of each other, and specific gravity
within 0.030; if not, an equalizing-charge process may be needed.
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