# Can I charge Ni-MH battery with modem power supply?

I bought a 8.4V 3800mAh Ni-MH battery from eBay. Unfortunately, its charger is not working. I searched about charging Ni-MH batteries on the web. Can I charge this battery with TP-Link power supply which has an output of 9V and 0.6A? I think 0.6A might be too much for this battery but I am not sure. What would be the ideal voltage and current to charger this battery?

• NiMh batteries should be charged by C/10 so here 3800mAh/10 = 380 mA. 8.4 V means 8.4/1.2 = 7 cells in series. To fully charge a single cell you need about 1.5 V so 7*1.5 = 10.5 V in total. So it is actually the Voltage which is too low. You should not charge NiMh batteries directly form a power supply, you need a series resistor. For a 12 V supply you'd need 12 V - 8.4 = 3.6 V 3.6 V/380 mA = 9.5 ohm, so about 10 ohm. 3.6 V * 0.38A = 1.4 W so make that a 10 ohm, 2 Watt resistor. Feb 9 '17 at 10:38
• @FakeMoustache Thank you so much for this clear explanation. So you are saying I need to connect this battery to 12V power supply in series with 10ohm resistor. But I didn't quite get last part. If I connect 10ohm resistor in series, won't it drop the voltage to 1.2V ?And also what is your suggestion about charging time? Feb 9 '17 at 16:46
• If I connect 10ohm resistor in series, won't it drop the voltage to 1.2V ? No it won't, maybe you're calculating 12 V/10 ohms=1.2 V ? But that is not how it works. Without the 10 ohms the current would get too high. In my calculation I charge with C/10, ideally that means 10 hours charging but since not all energy ends up in the battery you need to charge 20 - 40 % longer so 12 - 14 hours will do. At such slow charging it is OK to let the batteries charge for longer (a day for example), it will not harm the batteries in any way. Feb 9 '17 at 18:43
• @FakeMoustache So after a bit more research, I want to buy a 12V 1A power supply and regulate the current to drop it to 380mA. I thought I can just connect battery in parallel with some resistor and connect resistor to ground. But how can I know the resistance of the battery to calculate the value of resistor? Maybe it is a bit stupid question. And question kind of like mine is asked here: instructables.com/answers/… .The answer is not really straightforward as I thought. Feb 12 '17 at 18:51
• I thought I can just connect battery in parallel with some resistor and connect resistor to ground That makes no sense, I'll write an answer with a schematic of what you need. It's really simple actually. Feb 12 '17 at 19:27

This is what you need to charge your battery from 12 V supply:

simulate this circuit – Schematic created using CircuitLab

This will charge the battery pack with about 380 mA. Note that Resistor R1 needs to be at least a 2 Watt resistor (3W, 5W and 10 W is also OK) as it must be able to dissipate some heat.

This is what some resistors look like:

The 3 on the left are OK

The others are too small and not OK, they would burn out.

From left to right these look to me like 5W, 3W, 3W, 1W, 0.5 W, 0.25 W and 0.125 W resistors.

• Thank you soo much for your help. I was collecting required parts to make and check if it is working or not. With the help of Kirchoff rule, it is easy to calculate the current in circuit and understand why I should choose >2W resistor. Mar 11 '17 at 16:03