Re: Is this the right way to charge a battery?


ngdbud@xxxxxxxxxxx wrote:
> My brother got a Vectron ultralite made by Air Hogs. the battery is a
> 6v 100mah nimh, the cherger used is a 13v 600ma transformer. This
> voltage and power rating sound way to high for a baattery so small.
> Will this transformer kill the battery? would a lower power
> transformer, like 7v 200ma be a good investment? Thanks in advance!


You are right. The transformer is too hot for the battery.
The crude solution, which is used in cheap chargers, is a series
resistor.
First: does this thing put out dc or ac? Obviously you need dc. If
it's ac, you need to rectify it or get a wall wart that does put out
dc.
Then you need to limit the current with a resistor. You should go for
a low charging current to avoid damage to the battery. That's because
you will have no way of knowing exactly when the battery has finished
charging, so you need a charging rate low enough that you don't have to
worry about how long you leave the battery charging. Nicad and nimh
batteries don't like overcharging. Find out the slow charging rate for
the size nimh you have (google is your friend). If they suggest C/20
for example, you need 5 ma (wow sounds small). Anyway whatever it is,
you can then figure out the resistor value. It's just 7 volts divided
by the charging current (7 volts across the resistor because you have a
13 volt source going into a 6 volt battery). (Of course the actual
voltage from your power source varies with the current, so it might not
be exactly 13 volts.) Then the power the resistor dissipates is going
to be current squared times resistance. Mske sure your resistor has a
higher power rating by at least a factor of three.

.

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