Simple and safe NiMH charger?

[antomas]

Hi everybody!
I'd like to add a battery charger for a 4xAAA NiMH pack to a portable headphone amplifier. Charging speed doesn't matter to me. After a bit of search and some thoughts (possibly wrong: I'm not a EE!) I had the idea of using a trickle charger at 0.05C with a voltage limiter. In my mind, even if voltage limit is not considered a proper charge termination strategy, my charger should:

- not overcharge the cells, if mistakenly left ON forever (yes, I'm most of the time absent-minded);
- not discharge the cells, if left OFF forever (except for the few uA leakage of the 1N4001).

Would my schematics work the way I expect?
Thanks very very much in the anticipation for every comment about it.
Massimo

[Conrad Hoffman]

A good place to read up is here. The dilemma with NiMH is they reportedly don't like to be trickle charged at low levels indefinitely. There are various battery charging chips that will sense the voltage change and terminate properly, probably for very little money.

[theAnonymous1]

Maxim makes nice battery charger IC's. Maybe one like this......

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1666

Not very expensive and they can be "sampled".

[antomas]

Hi Conrad,
Thanks for your link.
In fact mine is not a real trickle charger. If I'm right, when the battery reaches 5.5V, the trickle current should flow through the zener and do not bother the battery pack. Possibly...
Massimo

[antomas]

Anonymous,
thanks a lot. My hope was to put together a safe charger with just 4 components.
Ciao, Massimo

[Conrad Hoffman]

If you limit the voltage to a safe level, you'll never get the cells topped off and lose a lot of the energy capacity- energy capacity that's already pretty bad with AAA cells. It *will* work however, though I'd think seriously about going with AA cells and using the chip!

[theAnonymous1]

Originally posted by antomas
Anonymous,
thanks a lot. My hope was to put together a safe charger with just 4 components.
Ciao, Massimo

Four....... fourteen; what's the difference.

At least you would know the batteries are being properly taken care of.

[TheMG]

When it comes to batteries, charging can be summed up into two categories:

-standby charging, where the charger is left on and the voltage from the charger is pretty much equal to the battery's fully charged voltage, as the name suggests this is for standby use, such as battery backups, where the very long charging time and slightly reduced capacity does not matter much

-cyclic charging, usually a much higher voltage is applied to charge the battery, resulting in higher charge current, faster charge rates, and the battery gets properly "topped off". This method necessitates proper end-of-charge-cycle termination. This is the most common charging method used, especially for portable devices.

Actual charging voltages, currents, and termination method will depend on the type and size of battery being charged.

[EWorkshop1708]

What about using a comparator to measure the battery's voltage, and shut off charging when a certain voltage is reached?

[antomas]

TheMG,
Maybe I had been too concise. I'd like to use this circuit onboard on a portable headphone amplifier, so many generic or recycled wall-warts can be used as a power source. Therefore usage is cyclic and termination is voltage based. I'm aware that, for some unknown (to me) reasons, this is not the recommended charge termination method for NiMH cells.
So the question is: is current limiting at 0.05C + voltage-based termination harmful for NiMH cells?
Or more precisely: could it reduce lifespan of the NiMH cells? How much?

EWorkshop1708,
Indeed a comparator can be used at the price of additional components. The only advantage I see is the possibility of fine-tuning the voltage level used to terminate the charging process. As alternative, one could fiddle with different zener and "valve" diodes. For instance, replacing the 1N4001 plus a 1N5817 Schottky diode should give around 5.8V, A 6.8V Zener plus 1N4001 around 6.1V, etc. Another solution uses a L200 regulator. Page 9 of datasheet shows a schematics for a battery charger like mine (five components) that should be altered a bit to prevent small battery packs from being discharged through the R1-R2 resistors.

Conrad, theAnonymous1,
I understand that the Maxim chip is the perfect solution, but... sorry, the circuit doesn't seem to fit the limited space on my board.
With my circuit I just hoped to find a simple solution for charging the 4xAAA pack (4xAA don't fit my enclosure) without reducing their lifespan and using easy-to-find components.
If you have any advice about that as well as about the proper voltage termination level, please let me know. In the many battery datasheet and papers I checked I just found that charging should be absolutely stopped ad 1.8V per cell, but this value seems to me a bit too much to be used. Do you think 1.55V per cell (i.e. 6.1V per pack, using a 6.8V Zener) would be ok to "fill the bag" reasonably?

Gentlemen,
thanks for your help and patience and best regards.
Massimo