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Super simple Ni-MH charger, how it works?

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neazoi

neazoi

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I found on the internet a guy who has reverse engineered one of these USB Ni-MH cheap chargers. It's circuit is shown below.
I have 4 questions:

1. How does it work?
2. Will it stop the battery from charging when it is fully charged?
3. What is the charging current for the values shown?
4. I need to charge a 750mAh AAA Ni-MH battery, how many hours should I wait to get charged with these values?

I won't expect any kind of performance and the battery may be mischarged, but will it really work without exploding the battery?
 

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1. I doubt it does, at least properly.
2. It doesn't.
3. Depends on the battery but as much as about 0.25 A.
4. If you assume a C/10 charge rate, about 15 minutes but much longer might make it explode.

I think the schematic may be wrong or it could be using a constant current LED. It certainly will not work safely as it is. As far as I can tell, D1 and R2 are there to divert some of the LED current so it dims as the voltage across the LED reduces but there is nothing to control the charge rate or cut it when the battery is fully charged.

Brian.
 

betwixt said:
1. I doubt it does, at least properly.
2. It doesn't.
3. Depends on the battery but as much as about 0.25 A.
4. If you assume a C/10 charge rate, about 15 minutes but much longer might make it explode.

I think the schematic may be wrong or it could be using a constant current LED. It certainly will not work safely as it is. As far as I can tell, D1 and R2 are there to divert some of the LED current so it dims as the voltage across the LED reduces but there is nothing to control the charge rate or cut it when the battery is fully charged.

Brian.
Click to expand...

Hm, 0.25A seems too high, it is a usb charger anyway.
I will use it in my portable active antenna attached. I have already include it in the schematic, as I did not know that it wouldn't work.
I have found the DS2711 but this is a much more complex circuit for the low space available in the antenna enclosure.

What if I just include a diode and a series resistor and let the battery charge in very low current (say 20mA or so)? There will be no indication of the charging state, not any cut off, but I can instruct the user that he can manually remove the charger after 6 hours (or more) and the battery won't get hot (safer).
I do not mind to have the battery fully charge.
Any ideas?
 

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neazoi said:
Hm, 0.25A seems too high
Click to expand...
This chrager will provide 0.220 A current for most commonly available NiMH batteries if led resistance is 13 ohms. Many USB chargers operate between 0.5-0.9 A as well. Rest all quotes from betwixt.
 
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    neazoi

    neazoi

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... but bear in mind that most 5mm LEDs won't last for long at 0.22A, it is about 10 x their normal rating!. Big high power ones may drop too much voltage to work anyway.

Removing the 1N4001 and 1K resistor will make no difference to the battery charging and increasing the 10 Ohm resistor to 150 Ohms will limit the charging current to about 25mA which should make it safe to leave charging all the time without overheating. It will of course increase the time to charge fully to several hours.

Brian.
 
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    neazoi

    neazoi

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