Voltage Limiter

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!

Use the Selection Guide at the Linear, National, TI, or Maxim web sites

I'd definitely look for a proper fast charger. Either an IC that you can add the correct resistors to for your desired charge, or a commercial one. As mneary said all of the major manufacturers make them.

I've been led to believe that charging lithium batteries is much more complex and potentially dangerous than NiCad or NiMH. And therefore I'd be a bit more careful!

Lithium polymer batteries are indeed different beasts to charge & potentially explosive if charged from a simple power source. A123 LiFe batteries are much more like Nixx batteries in that respect, and can be charged using a NiMh charger if the cutoff voltage is set correctly.

350w chargers are available, but cost a big load of money...and where's the fun in that??

If you wanted to simulate what you do manually, you could add a voltage comparator and a relay in series with the battery. When the voltage reaches 21.6V you would open the relay. You would need to add some hysteresis to the comparator so the relay wouldn't turn back on when the battery voltage drops after the charge current is stopped. You can add a resistor across the relay contacts to maintain a trickle charge, if you like.