Well, if you have the complete system you can split it in two, and the interface is only two wires :
Part 1 : current-limited voltage source, outputs a very precise 4.15V/cell (LiIon) or other voltage for other chemistries like LiPo, A123 etc : inside the power brick
Part 2 : protection (overcurrent, overcharge, overdischarge) and balancing : inside the battery
> the charger is nothing but a transformer and the intelligent part of the charging circuit is integral to the laptop
Yes, the power brick is a simple switchmode power supply, whose purpose is :
- saving weight and space in the laptop (always advertise the weight without charger !...)
- saving lots of certification fees (only the brick has to be certified for mains voltage safety, not the entire system)
Usually laptop batteries have a connector with lots of pins so I guess the balancer must be in the laptop itself. Here is a pack I hacked :
LiIon Helmet Light | Peufeu's Electronic Stuff
I have a stand-alone charger for it, which has a little connector which uses all the pins, so it must have a cell balancer too.