Charging Nickel Metal Hydride batteries in series

Thread Starter

young707

Joined Oct 14, 2015
4
I am given this circuit have 5 Nickel Metal Hydride AA batteries in series to power LED lights.
To recharge, the 5 serially connected batteries, is plug into the output of a +12VDC 5A wall plug adapter power supply.

This circuit works for a little while in the beginning. Then, It stop working.
Can you please tell me why? Any way to fix this problem. Thank you.
 

MikeML

Joined Oct 2, 2009
5,444
NiMh batteries are charged with a constant-current; not a constant voltage. What provides the current-limiting?

If the batteries have been charged using constant-voltage, they are likely toast and need to be replaced.
 

Thread Starter

young707

Joined Oct 14, 2015
4
Thank you.
What happens when NiMH batteries is charged with typical constant voltage wall plug +12VDC power adapter?
 

Dodgydave

Joined Jun 22, 2012
8,477
They will get hot due to the over voltage and explode... Not recommended, use a correct charger for that battery.
 
Last edited:

wayneh

Joined Sep 9, 2010
16,114
The nominal voltage of your 5 cells in series is 6V. Your power adaptor supplies at least 12V, possibly a lot more if it is an old (unregulated, transformer) wall wart.

As Mike noted, this will have dramatically overcharged your cells and likely destroyed them. You need current limiting and a way to turn off (or reduce to a trickle) charging once the cells reach full charge.
 

Thread Starter

young707

Joined Oct 14, 2015
4
where can I get a current charger? Or, do I need to build a circuit to convert a typical wall plug type adapter (regulated voltage adapter) to charge this as constant current and cut off when fully charged?

since I am given this circuit, I probably have to proof current regulated voltage wall plug adapter set up does not work? How best should I demonstrate this?

Thank you.
 
You can make a simple current-limited charger with a resistor by looking at the output voltage of your power supply, at the low voltage of your battery pack, and calculate a resistor based on the current to charge the cells at 0.5-1C (slower charging, less heat, longer life). If the cells are 5.5V and the power supply is 15V output (measured with voltmeter), the resistor needs to drop 9.5V. If the NiMH cells are 2000mAh, you could limit the charge current to 1000mA, and that's at least a 9.5 ohm resistor. Resistor power dissipation would be E**2/R, or 9.5W. So I would overdo the power handling of the resistors and dissipate at least 20W (it'll get hot).

So for 2000mAh cells, you could wire five 50 ohm, 5W resistors in parallel.

If you did this you'd have to carefully limit the time on the charger or you'd burn them out from overcharging.

--- OR ---

Build a current-regulated charger circuit out of, like, an LM317. Here's a circuit and article. You may need to modify it slightly for the number of cells -- the article has a handy table for that purpose.

http://www.learningelectronics.net/circuits/float-charger-for-nimh-cells.html

Or buy one from a battery-supply house.
 
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