The Nokia N900’s BL-5J 1320 mAh battery might not be enough for some, for this reason Mugen Power is creating a 2300-2400mAh extended battery. Good news is that the battery should be available some time in February. The bad news? The battery is reportedly going to be a bit larger than the original one, requiring a custom cover, which will be included in the package.
These custom batteries most often don’t do justice to the original design, but considering that this battery is a lot smaller than the N97’s 3600mAh version, there’s a good chance that it the N900’s version won’t be that big in size.
Is the BL-5J enough for the N900 or are you looking for a larger battery?
Mugen Power: Nokia N900 Extended Battery on the Way

Yea I would like a larger battery, if they put in a 1800mah battery in there and mayb an OLED screen for battery life improvements the phone would have been perfect to me. Easily would have forked out an extra 50 quid for it and if your gonna buy this phone compared to most other touch smartphones out on the market your not going to be to shallow about the design.
The N900 is definetly the best phone i have owne but the battery life makes it almost impractical. i would go for a bigger battery if i can locate in the UK.
You would think nokia would have sorted these battery life problems out by now
It’s not just nokia, all modern powerful smartphones have a similar issue.
if you don’t touch them (except for receiving a call) the battery lasts for ages, but their capabilities just mean you use them a lot lot more.
3G in particular is worse for battery than wifi.
I have a N900 and on wifi it seems to last the day with some reasonable usage. That’s not at all bad & no worse (probably better) than my N96.
In future I agree OLED would help, and personally I’d take a minor 1-2mm hit on thickness (even to the N900s already large size) to accomodate a larger battery.
I’ll certainly be looking at this mugen option & then deciding whether to get a spare regular battery or this one.
One nice feature is the micro USB charging — these cables etc are becoming more popular making charging at home or in the office easier.
if n900′s battery power can b extended it would awesome…..i will run for it…..but is it out in the market……let me know.