At least 1amp (1000mA) charger I think the stock 1.2amp(1200mA) is about all that the N900 will do internally. I have been using double-Scud mod battery packs with a Mugen cover I about 3200mA/h capacity new and drops to about 3000mA/h after a few years, even though double battery the charging still maxes out at the N900's charge controller maximum speed around 1.2A(1200mA).