# Charging NiMH with a bench top regulated power supply



## anotherbrian (Mar 18, 2005)

I've read through most of the threads on charging NiMH, and they point to using smart chargers to get the best charging performance.

I have a bench top regulated power supply that can be set for constant voltage or constant current. Can I use this to more efficiently (than the basic NiteRider trickle charger) charge my NiMH battery?

I used to use the power supply as a trickle charger for my motorcycle during the winter when it wouldn't be ridden frequently. For that I set it at CV for ~13v and just let it do its thing. I wasn't hooking it up to a dead battery though, so the current was always very very low.

Can I dial the power supply in for the 6V of the NiMH and let it do its thing unattended, or will the initial current be an issue (in some threads I've read NiMH likes constant current)? Otherwise, if constant current is important, what is an advisable maximum current to use (6V 4Ah), and is that current used all the way up to 6V, or does the current ramp down at some point?


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## airman (Jan 13, 2004)

anotherbrian said:


> I've read through most of the threads on charging NiMH, and they point to using smart chargers to get the best charging performance.
> 
> I have a bench top regulated power supply that can be set for constant voltage or constant current. Can I use this to more efficiently (than the basic NiteRider trickle charger) charge my NiMH battery?
> 
> ...


With the charger you have you will only be able to "trickle charge" your battery at fixed current for a set period of time. It MUST be constant current, I'd go approximately the capacity/10, but not less than capacity/20 - for the 4ahr, that would be between 200 and 400ma. Charge for 12 to 14 hours. The battery charge won't be optomized, but it will most likely get a full charge. The small trickle charge is a very crude way to get the charge into the cells. Figure it out this way...

- rated is 4 amps for 1 hour = 4ahr
- replacing the energy discharged would be 400ma times 10 hours

The recharging isn't 100% efficient, so you add 2 to 4 hours on the charge time and you will top the battery up. The drawback to charging this way is that the battery will only take so much energy, beyond which, you just dissipate heat. Heat kills batteries. So you want to watch how warm the battery gets during this process. If the battery was only half discharged, then it will top up in half the time, and then just get warm. At C/10 charge current - you should't damage the battery  . A fully charged pack can probably stay trickle charging at C/20 indefinitely because you are pretty close to supplying the juice you lose in self-discharging.

Now, the fine print... I use this guideline myself, have done for years and it works for me... Free advice however is sometimes only worth what you pay for it  .

Good luck


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## jeffj (Jan 13, 2004)

It's pretty easy to tell when a nimh is charged. It will get warm. Don't let it get hot. It will stay cool or room temp when charging until it is charged. Luke warm is perfect. Don't let them get hot.

I would be plenty comfortable charging a nimh at 4 amps (done it hundreds of times). At that rate, however, it will go from room temp to warm (or hot if left on the charger) in a matter of five to ten minutes so you do have to monitor fairly closely when fast charging.


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## firstrax (Mar 14, 2005)

anotherbrian said:


> I've read through most of the threads on charging NiMH, and they point to using smart chargers to get the best charging performance.
> 
> I have a bench top regulated power supply that can be set for constant voltage or constant current. Can I use this to more efficiently (than the basic NiteRider trickle charger) charge my NiMH battery?
> 
> ...


For a cheap(er) intelligent rapid charger buy Texas Instruments evaluation modules. They even have thermisters you can tape to the battery to monitor temperature. 
DV2002L2 will charge 4-10 cell packs.
edit: Looks like Ti raised their prices on eval modules. Still, cruising semiconductor sites for charger eval modules could turn up some bargains.


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