I'm looking for a (hopefully) simple circuit to limit voltage while charging some batteries.
At the moment I'm charging a 6s pack of A123 2300mAh LiFe batteries using two 12v leisure bateries jumped together to give 24v nominal.
This is connected to the batteries using a long length (15 ft or so) of 0.75mm2 wire as a current limiting resistor and running through a Watt meter, which shows voltage, current, power & mAh put into the battery.
As soon as the batteries are connected, the charging starts, at about 15A & 19v initially, dropping to 11A & 20-21v for the bulk of the charge.
As the battery is nearly fully charged, the current drops & the voltage rises. The cells have a terminal voltage of 3.6v, so the charge stops (by me pulling the plug) when the voltage rises to 21.6v.
At the point I stop the charge, 5a or so is still flowing, so what i would like to do is hold the voltage at 21.6v & allow the current to drop to zero as the cells fully charge.
I know the immediate answer will be to 'buy a battery charger', but I already have a charger that I use to balance charge these cells in 'slow time', at 1c, taking an hour or so to charge, it's the rapid field charging I'm wanting to address.
So, does anybody have any ideas for a circuit that would not affect the charging at 20v & 15A with a 24-25v supply, until the limit voltage of 21.6v was reached & hold there, while still allowing a good 5A to pass?
I've considered a LM317 type regulator with power transistors to handle the current, but that seems very wasteful of power & I'm not sure it would work at all with the voltage differential between 21.6v & 24V.
Thanks for listening!