I can't really be sure what is causing your problem, but battery charging on the N900 is software controlled so it is possible it will be fixed by reflashing, before reflashing you can try just reinstalling the packages related to battery managementI doubt reinstalling those packages will fix it but shouldn't hurt, so is worth a shot. There is also the possibility that it is hardware related, maybe a dodgy charger, it is worth trying different charger if you can.