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I want to uprate my bots voltage to 16.8v by adding a 4.8v pack which is the same type as the original 12v battery (4300mah and nimh). Is the best way to go about this to make sure they are both dead then wire them in series and charge as a 16.8v pack?
You would be best to charge them both seperately first before wiring them in series.
If you mean having a 12v and a 4.8v batteries wired up in series, don't, they would try and discharge the larger amount into the smaller to try and even it out, I've seen it happen before
If they are the same capacity then they won't. All the cells in a battery pack are in series so by the same theory then all the cells would dump their charge into the last cell. If the cells are the same then it will be fine. I would recommend shortening the wire between the two packs as much as possible to keep the resistance similar across the entire pack.
There was even one event where i ran A123s in series with nicads for a bit just to jack up the voltage. Wouldn't recommend it though, nicads got bloody hot! :?
Yep definately not a problem in series. Only, if the capacities are vastly different you'll end up over discharging the smaller pack. Likewise, if you charge two packs in series, if they have different discharge levels you'll end up over charging one of the packs.
Keeping your wires short will reduce the resistance thus increasing power.
Connecting NiCd's and NiMH in parallel is a different matter and definately a bad thing to do when charging them.
Parallel, not series, my bad :P I knew if you connected them up some way was bad, forgot which one :P
So just to double check, charge both then wire them up or discharge then wire them up?
Charge both, then wire them up.
Good, that's what I've done so I must be doing something slightly wrong in the charging process
Thank you!