NiMH Charging

[diff_lock2]

I made a 12 cell (14.4v) 3400mAh batt pack. My question is; should i charge the cells separately or the whole pack as a whole...

Im not sure what specs i should charge the batt pack at, i calculated 16.92v (1.41 v per cell), and for a c/10 charge 350mAh (right!??!?) so thats like a 13hour charge.

I wasn't sure where to post this (we need a battery section lol)

 

[SilverFox]

Hello Diff Lock2,

The battery section and discussion is here.

To answer your question, you can charge them as a pack, if you have a charger capable of this, or you can charge the cells individually, if the pack comes apart.

If you want to charge the whole pack, you should start with discharged cells, and do a 350 mA charge for 16 hours. This will form and balance the cells in the pack. Now you can use it normally and charge in the range of 2000 - 3400 mA. However, if you are using a cell holder with coiled spring contacts, you may be limited to 1000 mA charging. Above that the added resistance will cause things to heat up and you will melt your cell holder. If the cells are spot welded or soldered together, this is not a problem.

If you decide to charge the cells individually, you should do the same 350 mA charge for 16 hours on each cell. Then you can charge each cell in the 2000 - 3400 mA range and reassemble them together for your pack.

When charging the cells in a battery pack, at the end of the charge each cell should be the same warmth, and the pack ending voltage should be around 17.4 volts. If you find a cell that is not the same temperature as the others, your cells are out of balance and you need to do a 350 mA for 16 hour charge again to balance things.

Tom

 

[diff_lock2]

Thanks for the info.

Lets say i charge the batts with 17v (as a pack), will the end charge volt reading be around 17.4v?

And i am using a cradle with springs. Do you think the cradle could handle a 2amp discharge? I am running a 12v 20w mr16 light, and i estimated around 1.6 amps at 12v so at 14.4 it would be more, i would think.

If this cradle doesn't work i will have to use some abs/pvc tube method.

 

[SilverFox]

Hello Diff lock2,

NiMh cells are usually charged with constant current. The voltage end up where ever it ends up. I believe there is usually a maximum voltage to cut off the charge should things get out of hand, but you don't use constant voltage to charge.

With a cradle with springs, I would play it safe and go with 1000 mA charging. If you are drawing 1600 mA, you should check for any signs of high resistance through heating after a run. If everything checks out, you can then step your charge rate up to 1600 mA.

Tom

 

[MikeSalt]

I tried charging a pack assembled from loose cells with a delta-peak sensing fast charger. Due to unbalance, it kept triggering false peak detection. Eventually, I charged the cells seperately in a trickle charger. Only had to do this once (and then discharge) before the fast charger took to the cells nicely.

 

[SilverFox]

Hello MikeSalt,

That is exactly the reason for the first charge to be at 0.1C for 16 hours. This brings all the cells in the pack into balance, so you can do normal charging the next time.

This slow charge needs to be done every 10 - 20 cycles to keep the cells within the pack in balance.

Tom

 

[diff_lock2]

Well i got my cells in the mail. And im using 2 different 16hour chargers (4 cells per charger). Once one of them finishes ill put the rest of the batts.

I think its time to get a 12 cell charger.