Voltage to charge NiMH batteries?

[Fred Furia]

I recently noticed that my walkie talkies have a recharge port for an
AC adapter. In each unit, I currently use 4 AAA batteries
(rechargeable NiMH), which I currently place in my battery charger by
hand. I am interested in using a universal AC adapter if I can, to
avoid buying a costly adapter from the manufacturer.

Here are two questions for the local experts:

(1) If a universal AC adapter fits, what voltage should I use to
properly charge the batteries? (Is it just 4 x 1.5 = 6?)

(2) Since my universal adapter does not know what I am using it for,
it will not stop powering when the batteries are charged. How will my
batteries be affected if I forget to stop the charge at the right
time?

Thanks.

Fred (fred AT phillyzone.com)

 

[AC/DCdude17]

X-No-Archive: Yes


Fred Furia wrote:

I recently noticed that my walkie talkies have a recharge port for an
AC adapter. In each unit, I currently use 4 AAA batteries
(rechargeable NiMH), which I currently place in my battery charger by
hand. I am interested in using a universal AC adapter if I can, to
avoid buying a costly adapter from the manufacturer.

Here are two questions for the local experts:

(1) If a universal AC adapter fits, what voltage should I use to
properly charge the batteries? (Is it just 4 x 1.5 = 6?)

(2) Since my universal adapter does not know what I am using it for,
it will not stop powering when the batteries are charged. How will my
batteries be affected if I forget to stop the charge at the right
time?

Thanks.

Fred (fred AT phillyzone.com)

They need to be current regulated, not voltage. If you're building a
dumb charger, you'd want to use a configuration of resistor that gives
approximately C/5 (if battery is 750mAh, charge current should be
150mAh). Go for 6 hours of charge when fully discharged. If you
overcharge too much, you'll cook the cells. Use a simple appliance
timer with your universal adapter and problem is solved.