well battery light indicates theres either to little voltage or to much voltage normally. unless you didnt connect the cable up to the new alternator? which i think will cause it not to charge, and throw this light on?

either way mate, check what the voltage is doing with the engine running, can trace the problem from there then.

it wouldnt suprise me if the new alt is dodgey though!

does the light get brighter or dimmer with more revs? do the headlights get brighter or dimmer at all?