how to charge a 3.6v NI-MH battery from 5v

[danrogers]

I have a couple of these... **broken link removed** that I want to use in a couple of led projects. I thought it would be nice to be able to charge these cells from the 5v of a usb socket.

is there an IC charge controller available? or would I have to build something up?

thanks

 

[audioguru]

The tiny battery will not power LEDs for very long. It has 3 cells so it will be 4.2V to 4.5V when fully charged.
Its charging current should not be more than 75mA (but confirm it on its datasheet) and when it is fully charged its trickle charge current should not be more than 4mA.

Why is everything in the UK over-priced?

 

[Blueteeth]

Depends on the LED, a red LED has a typical voltage of 1.2V Green 2.2V, its only the blues and whites whic require > 3.0V. But at 'reasonable' brightness, the current would have to be 15mA, so thats a max of 150mAH/15 = 10 hours.

Charging NiMH is a bit tricky as they prefer to be charged quickly at high current, with charge termination ( a way to stop pumping current into it so it doesn't die). Trickle charging isn't always a good idea. I guess the easiest method would be to use am LM317 as a constant current source, and a comparator to turn off the current once the battery reaches full charge. But you would have to set this threshold a bit below the 'max charge voltage' to make sure you don't' over charge it. And this threshold depends on the charging current and the battery itself So, without experimenting, its hard to give a figure. Perhaps 1.3 - 1.4V per cell is a cautious number, that's 3.9V for the little batpack.

Its not the best way to charge a battery, but its relatively simple. Adding hysteresis to the comparator would mean it doesn't constantly flutter between charging and termination. The circuit should only turn on when its say < 3.4V, and only turn off when its 3.9V.

 

[Gayan Soyza]

is there an IC charge controller available? or would I have to build something up?

MAX712/713

Nobody cares on building complex battery chargers these days.A trickle charge will be more than enough.

If I were you I just build a constant voltage current limited charger using LM317.