How Do NIMH Chargers Detect End of Charge?

[borisgazza]

i am intersted in the end of charge detection(-deltaV) on some NIMH chargers, i would like to test that effect on some chargers.
I got some chargers(like GP Smart power bank, GP H500,etc...),all the chargers i have use the drop of voltage as end of charge detection, so what i did is, I connected in serie to a battery a low ohmic resistance, i initiate the charge and after i while a short the resistor ,so drop of voltage but none of the charger did react to that drop,i manage to get a drop of 0.1V but still none of the charger i worked with reacted to that drop, so i would like to know if there are more criteria that i have to take in consideration to generate that drop?

 

[vk6kro]

You would have to know how the charger worked.

Microprocessors are so cheap now that the charger may well have one in it.
If it did, the programmer may have got it to do a series of voltage readings over several minutes and tried to detect a gradual decline in voltage which would indicate that the battery was charged.

An instantaneous voltage drop like you produced would not fool it. If it was only taking readings every minute or so, it would not even know you did anything unless you happened to do it when it was taking a reading. Even if it did register, it would look for a further decline in the next minute and so on over several minutes.

I have two chargers that work by detecting a rise in temperature of the battery when it is charged. Each battery sits in a metallic frame that gets hot if the battery gets hot. There would be a thermistor on each one that detects if the temperature varies above ambient.