Having that many cells in series is a bit of a challenge. By my math, it's
about 2 dozen cells to get to 28 volts, so if you're looking for a tad more
capacity then maybe 48 cells, 2 sets of 24. (NiMH cells are about 1.2 volts
fully charged.)

In principle, it's pretty simple business to charge NiMH cells, just put a
little more voltage on them than they normally produce, and limit the
current. If you want to charge them fast, you need to monitor the
temperature of the cells and manage that temperature. If you are patient,
though, just charge them slowly and no need to watch the temperature.

However, when you get cells in series, slight differences between the cells
will cause one cell to take more of the charge than others. Eventually,
this leads to the failure of the battery, but for small numbers of cells,
it's typically not a problem. With your 24 cells in a line that could be a
problem, but again., if you charge them really slowly there's less need to
be concerned.

In use, you need to have the 24 cells in series, but for charging, you might
consider some sort of a scheme where there are fewer cells in series and
more in parallel. When charging the cells in parallel keep in mind current
limiting on each parallel branch individually, but that could allow you to
use something a little more common as a voltage source, say the station
supply or a 15 volt wall-wart. Older cell phone batteries used to use this
sort of scheme, where the connector was arranged so that the batteries were
connected to the charging connector differently than to the discharging
connector.

...

"Quiet Voice" wrote in message
om...
Greetings:

I am interested in finding a source for schematics and/or a kit for
constructing a battery recharing unit.

I'm new to "homebrew". So kits and/or schematics with explicit
directions is appreciated.

The battery pack I am looking to recharge is a collection of approx 40
1.5v NiMH cells for about 28V overall. In fact, any
kits/plans/suggestion on making the battery pack itself would also be
appreciated.


Thanks!!