The "battery board" provides control systems for "intelligent batteries". The charging controls monitor voltage, current and temperature when the cells are charging; the controls will be very sensitive to the characteristics of the cells. A different control will monitor cell discharge, informing the OS when the cells are losing power and performing the optimisation that maximises battery life and endurance.
Since the Blackbird 5xx series, Mac OS has included software that communicates with the battery control systems. External battery chargers also include intelligent systems, hence the occasional firmware update for early rechargers.
Re-celling an intelligent battery is no doubt possible but non-trivial. You need to obtain cells with identical characteristics to the originals and be able to reset the battery board to recognise the new cells. You also need to be able to solder cleanly (dodgy joints will be a safety hazard) and to disassemble a battery pack containing nasty chemicals without breaking the cells.
As a first test of skill, I suggest learning how to re-cell the battery for a Portable or PB 100, which use lead acid cells. If you want to learn about re-celling NiMH batteries, start with an eMate 300 or similar. LiIons are scary and you really need to understand the safety considerations before you even try to take one apart.