Adapting charger for NXT 9798 battery I have an old style  9798 NXT rechargeable battery pack 7.2V 1400MnAh, but no charger.
Can I adapt a  spare non lego 9v 200mAh charger for this? Or is voltage too high? can anyone read the output voltage from their old charger?
As I will need  to add a new connector plug to the charger, can anyone tell me if the battery charging socket centre pin is positive or negative? 
Also what size of plug do I need as it slightly larger than current version for current battery? 
Thanks

NB I have looked at my newer charger lego element 94165 10v 700mA for the current battery and it is a positive centre hole (for centre pin in battery case) and a negative outside (measured with red lead of multimeter in the centre hole of charger plug and black lead on outer tube showing +10.5v)
The two batteries and chargers are very different, in that the new one is 10V DC whereas the old one is AC (while it's supposed to be 9V, I think the transformer was actually 10V too).

As it's AC, the polarity doesn't matter, but I don't think using DC current is recommended (even if in theory the rectifier circuitry in the old battery should be able to accommodate it).

You should be able to buy a spare fairly easily though, whether at a local LEGO event or directly on BrickLink. Note that it's the exact same that came with 9V trains, so you may want to look for the complete transfomer + regulator as well, which may sometimes be cheaper to find.