Charging batteries - what should I know
Charging NIMH (and most modern battery chemistries) isn't done at constant voltage, but at constant current.

You'll need some fairly involved electronics to provide a constant current source and monitor the battery state.

Battery University has a pretty good description of the charging cycle of NIMH batteries.

The initial charge is at 1C (in your case, 175 mA). There are 3 different stop-checks (whichever is reached first):


Temperature rise rate of the cell being charged.
Voltage across the cell.
NDV (negative delta voltage). Basically check for when the voltage rise of the battery stops rising at a certain rate.


At this point, the charger will charge the battery at significantly less current, repeating similar 3 checks and iteratively stepping down the current charge rate.

The linked page suggests that slow-charging a NIMH battery is difficult/impossible because it's very difficult to get reasonable readings for many of the time-dependent stop checks. You're basically relying on a time charge, which could excessively charge the NIMH cell and damage the cell.

The primary safety concern is excessively heating up the battery. You might risk rupturing the battery, though I don't think NIMH are susceptible to catching fire as some other cell chemistries are. Most likely the battery casing is designed to rupture in a safe manner, but no guarantees.

It's unlikely you'll do anything which would be considered a significant safety concern, just use your common sense. There aren't any high voltages involved, and the power outputs are relatively low.