Charger Circuit

To charge Nicads you need a constant current that has the headroom required. Nominal open circuit voltage for a NiCad can go as high as

1.5V during charging, per cell (trust me, I've measured it) or sometimes higher if the temperatures are right. Your packs are 19.2/1.2 = 16 cell packs in series. The output for these could be as high as 16 x 1.5 = 22.5V, to which you need to add headroom for the current source so it can remain compliant.

For 20 units in parallel, you need 3.2A for 14 hours (standard charge). Keep in mind that if some cells / packs are defective, that extra current will flow through the remaining packs. if that happens, you'll need to change the charging time (apart from the fact some NiCads can NOT be charged above the C/10 rate).

There are issues in there that would need to be addressed.

You could charge from a lower voltage source if you used a boost circuit (you don't care too much about regulation - any output from about 25V to perhaps 35V would do, fed to a constant current source. Of course, the input to the boost would have to supply about 6 to 7A (for the step up conversion and efficiency losses).

One thing I'll stress is that NiCads should not be charged with a voltage source unless you want interesting leakage and perhaps fumes all over the place.

Cheers

PeteS

Do NOT charge 20 packs in parallel!

You need to limit the current to the packs individually. Make or buy a 24V regulated supply capable of 4 amps or more. Then do this, 20 times.

D1 LP-65 5 ohm 1/2w

+24 --->|---BULB---+----/\\/\\/----+ 1N4003 | | | D2 | +----->|------+---> To + on NiCd pack 1N4003

LP-65 is a 12 volt, 50 watt bulb from Allelectronics (2 for $1.00)

The bulb limits the maximum current (when the pack is discharged) to roughly 2 amps. Current to the pack starts high, and diminishes as the pack charges and its voltage increases. D2 shunts most of the current around the 5 ohm resistor, until the pack voltage rises to about 22.4 volts, when D2 shuts off. Charge continues through the 5 ohm resistor, limited to ~180 mA at 22.4 volts, and less than

120 mA when the pack is fully charged at 22.8 volts. D1 prevents the pack from discharging to the other packs or into the supply, if the supply voltage is turned off.

Answering your questions: It may be possible to make a DC-DC converter capable of

24 volts at 3.2 amps (or more) with input V of 11 to 14 volts, but it is irrelevant. It is not safe to charge multiple packs in parallel without individual control of the charge to each pack, so the question is moot, given your objective.

The second question stipulates if price is no object ... Sure - eliminate price as a consideration and you can do *lots* of things that are not practical. And yes, price is no object is just a pipe dream, sad to say. But to bring it back to reality: given 20 NiCd packs to supply an input of 19.2 Vdc at a 32 amp hour capacity, a SMPS (switch mode power supply) could produce 12.5 amps at 12-14 volts, both regulated and adjustable.

Ed

By your voltage input, I'm guessing you want to use 12V batteries to do the charge. If you can put two in series, that will give you something above 24V, so you can use a circuit posted by another in this thread. If not, you will need a fairly hefty DC-DC converter, which you can will want to buy "off the shelf" for your application. It might cost more than $30, though, given the current requirement.

--
Regards,
  Bob Monsen

He was not in a hurry, "hurry" being one human concept he had failed
to grok at all. He was sensitively aware of the key importance of
correct timing in all acts but with the Martian approach: correct
timing was accomplished by waiting. He had noticed, of course, that
his human brothers lacked his own fine discrimination of time and
often were forced to wait a little faster than a Martian would but he
did not hold their innocent awkwardness against them; he simply
learned to wait faster himself to cover their lack.