ferincr
05-10-2007, 07:48 PM
Hi, I have two identical NiMh 4300 Sub C's packs and I was wondering how daring would be to make a Y harness to charge then together on my Triton.
I guess that would be like connecting them in parallel so I can set up the charger for twice the amps. Like if I'm charging a 8600Mah 4.8V pack?
Any reason not to do that?
Thanks for any comments.
SilverFoxCPF
05-24-2007, 07:08 PM
Hello Fernando,
Charging NiMh packs in parallel is not recommended. Can you hook them up in series and charge them as one big pack?
Tom
ferincr
05-24-2007, 10:44 PM
Tom,
Thanks for the reply
I guess I could connect them in parallel, but if you don't mind me asking what would it be the major difference?
Is it the fact that if they are not both with the same amount of charge to start with one of them would get over charged (or something like that?)
SilverFoxCPF
05-25-2007, 12:20 AM
Hello Fernando,
It has to do with balancing the cells in the packs. When you hook up NiCd or NiMh cells in parallel, they do not balance with each other. The voltages will come close, but there is very little capacity that is changed from he fuller cells to the emptier ones.
When parallel charging, the charger is looking for an end of charge signal. When a cell reaches full charge, its voltage is reduced. If, at the same time, a paralleled cell is just reaching the end of charge, the net effect tells the charger that more charge is needed. The fuller cell gets damaged through overcharge.
In series, the end of charge signal is amplified by the number of cells in series. This gives a strong end of charge signal. At the end of the charge, there still may be some cells that are under charged, so you add a trickle or topping charge. The pack is brought into balance by a slow overcharge that is slow enough that it does not damaged the fuller cells. 0.1C is a good rate for this.
When charging Li-Ion or Li-Ion Polymer cells, you can parallel them and they will balance with each other, but this does not work with NiMh cells.
If you have the proper equipment and time to play, an interesting experiment is to take one pack and fully charge it, then parallel it with a pack that is fully discharged. Let it sit overnight, then do a discharge test of both packs. You will find that very little charge has been added to the empty pack.
Tom