Integrated NiMH 9V battery charger

Hi

I am searching for a highly integrated solution (ideally with no external parts) IC for charging a 9 (8.4)V NiMH, 280 mAh battery from a 24V regulated power supply. I plan to use this as a backup power supply for a small circuit. Unfortunately every charger schematic I find on the internet uses too much external parts and my space are very limited.

Can someone give me an advise?

600 Ohm resistor good enought?

Thanks for your reply!
The docs for the battery says that trickle current has to be no more than 0,84 mA so this calculates down to a resistor about 20kOhm. Anyway I was hoping for something that supports quicker charging.

Trickle current isn't the same as charging current!

Trickle is the maximum current you should allow AFTER the charging is completed, in other words how much it is safe to pass through the cell when it is fully charged without causing 'over charge' problems.

The minimum component count would be one resistor and one Zener diode, but it wouldn't be the most efficient was of doing it. You place a Zener diode (say 8.2V across the cell) then use the 600 Ohm resistor suggested by Easyrider83. If the cell voltage is below the Zener voltage, all the current flows into the cell, as it approaches Zener voltage, the current starts to divert through the diode instead. The problem with this simple method is it draws current from the source all the time, regardless of the cell condition so it is wasteful of power althoug this may not be a problem in your situation. The other drawback is getting exactly the right Zener voltage to ensure the cell reaches full voltage before the current diverts. A better design would have a sharp and adjustable cut-off voltage, you may have to rely on the particular Zener voltage being close enough or building it up from a lower Zener voltage Zener diode and one or more normal diodes in series.

Brian.

Thanks for the clarification and the great suggestion!

Power wasting is not a problem in my particular case.

The voltage from the battery gets trough a voltage regulator, so not fully charging the battery would be fine for the circuit.

This opens one more question :

The other drawback is getting exactly the right Zener voltage to ensure the cell reaches full voltage before the current diverts.

Click to expand...

Is this a problem for the battery - never getting fully charged?

Thanks again for your help

Hi,

You could use special Battery Charger ICs which i think is more safe.

For Example : Check LTC4079 IC from Linear Technology. Only needs a few resistors (and a NTC - Temperature Protection) and works up to 60V input. (250ma Maximum Output current)

Great! Thanks - this looks like just what I need - 3x3 mm, few external components, controlled charging and no exploding battery

I'll give it a try