Electronics > Projects, Designs, and Technical Stuff
Automatic Ni-MH battery charger woes
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VanMarco:
I did use the schematic of the circuit posted by markF
dave_j_fan:
i would differ here as overdesigning nimh circuit is notneeded

simple resi and capacitor design works well


camelion chargers have very simple design
IanB:

--- Quote from: MarkF on February 02, 2020, 07:03:29 am ---The NiMH battery chargers are constant current sources that will (if the charging isn't terminated) will charge
the batteries to a voltage higher than their rating. (i.e. You don't set the voltage.) 
That is why they need to check the temperature and/or voltage drop to know when to stop charging.

This charger NEEDS to have it's charging voltage set at the batteries full voltage.  This will prevent over charging.
It will charge at the set constant current value until it nears completion. 
As it reaches the set voltage, the charging current will decease and stop when the set voltage is reached. 
(The LED goes out when charged.)

I believe the charging current is somewhat exponential.

Whether this is good or bad for a NiMH battery, I don't know.

--- End quote ---

The characteristics of NiMH cells mean that they cannot reasonably be charged using a CC/CV algorithm. An NiMH cell does not have a concept of a "full voltage" that you can program into a charger. Every NiMH battery is different, and for a given battery the voltage can vary a lot with age, temperature and charging current. It this respect NiMH batteries are quite different from lithium ion batteries and cannot be charged that way in any useful sense.
BravoV:
Curious if the op aware that he is building a Li-Ion chargers, for his NiMh battery ?  ::)
Zero999:
I'll add my name to the list of those who agree the circuit posted by markF is not suitable for NiMH cells and is probably originally designed for lead acid.

The circuit originally posted is much better. U1 should have a resistor to Q1 because relying on the op-amps current limit to restrict the base current is bad design. Yes, a proper comparator IC such as the LM393 or LM311, would be a better idea.
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