Help with simple NiMH battery charger

Thread Starter

Jargon

Joined Mar 22, 2012
2
Hi, I'm wondering if someone who knows about NiMH batteries and charging them could help me out here. I don't know much about these charging batteries, but I'm trying to charge 5 of them in series at a constant current (the C/10 rate). I tried to use an LM317T to do this, however, whenever I connect more than one battery the current starts dropping exponentially and as a result the batteries don't charge. My multimeter says the correct value of current flows through when I charge one battery, so I think the circuit is operating correctly but I may be going about it incorrectly. Thanks in advance. NB: The LM317 is being powered by a DC adapter, 6V 2.5A.
 

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#12

Joined Nov 30, 2010
18,210
You're running out of voltage. The 317 chip eats about 3 volts, the diode in the battery string eats about .7 volts, and you have....2.3 volts left for batteries.

ps, be careful about how you charge these. They are a lot pickier than lead acid or ni-cads. www.batteryuniversity is your friend.
 

Thread Starter

Jargon

Joined Mar 22, 2012
2
Yeah, I think I'm going to use a 6v SLA battery instead, they seam easier to manage and charge. Thanks for the help though.
 

#12

Joined Nov 30, 2010
18,210
Same limits apply to an SLA charger. You need about 11 volts to feed all the components and still have enough voltage to top up the battery. Good job for a 12VDC wall wart. (DC adapter as you called it)
 
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