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Significant information’s in favor of a professional Workshop => All about NiMH and Ni-CD chargers in European Union => Topic started by: Kiriakos GR on November 20, 2025, 05:45:33 PM
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As reference I will mention one option that I am exploring as 9V NiMH DIY charger circuit.
The theorem this is adding a current limiting resistor at a stable power supply.
Resistance value calculation follows this math: I = (V Power supply minus V Battery) / R
https://www.rapidtables.com/calc/electric/ohms-law-calculator.html? (https://www.rapidtables.com/calc/electric/ohms-law-calculator.html?)
The problem with this method, even if a quality resistor this is used (2W 1% tolerance), this is that only termination current this can be calculated.
Starting current this is not controlled, an totally discharged 8.4V battery at 7V Cut-Off, this will shortly obtain 8V float voltage.
Our goal this is 30mA constant current at 10.300V.
At my experiment I did use my Kenwood PD35-10
Output 10.532V
4X Royal OHM resistor in series, 12.3 Ohm 2W 1%.
Available Ohm scaling: 12, 24 , 36 , 48 Ohm.
At my current experiments, I ended in the conclusion, that if I am not activate current limiting by my PSU with a combined limiting current resistor, there is no chance controlling both start up and termination current.
Kenwood PD35-10 and it CC potentiometer, this is not helping for mA adjustment with much of precision, its not designed for such work.
With the assistance of current limiting resistor in series, I did succeed to add a fine-tune adjustment.
From the other hand, its impractical using you master bench-top power supply, for several hours, this acting as 9V charger. :P
There is a second idea that I have not explore in person.
And this is using LM317L in constant current mode.
LM317L this is a different component than the commonly known as LM317.
LM317L = Output current: Up to 100mA
An LM7812 this can possibly used as voltage source, and LM317L this will act as DC current control.
The only problem that I can predict, if you think charging sever NiMH simultaneously, lets say four 8.4V NiMH 35mA its, the circuit it can have single LM7812 and one LM317L per battery.
Food for thought :)
Note: Low cost DC/DC buck converter (with CC adjustment) at such low currents, they are unstable, they drop their PWM frequency too low, and output stabilization this is slow.
DC/DC buck they are stably working at 150mA and above.
There is a chance to add a Current limiting resistor in series, but this is not tested, its just another idea.