why cant AC charge a DC battery? surely the positive peak has got to cause a chemical reaction of some sort. all i can think of is it will charge on the postive peak and discharge on the negative peak so the charge will cancel out, but if the supply remains postive there will be a constant charge, is this correct?

 

You pretty much explained it yourself there.

 

thanks bill marsden, i came to the conclusion that a rectified AC source would charge a DC source

 

It would, but it could still damage the battery. Some batteries are sensitive to how they are charged.

The Battery University