I understand that the battery meter is somewhat of an estimation and can never be 100% correct but its barely functional at all on my 3G.
It will 'discharge' from 100 to 0 in probably the space of about 5-6 hours depending on use and then it will stay at 0 for another 1-2 hours so obviously it thinks the battery is dead well before it is.
If I then plug it in to charge I can watch the percentage go up, probably takes all of 20minutes to go from 0 to 100 in rockbox, of course that isn't correct and if I unplug it when it gets to 100 it'll fall very quickly down to around 20 (which I what I would expect after a 25min charge).
If I boot up into the standard apple firmware when rockbox reads the battery as 0, for example, the ipod firmware will display around 1/3rd of the bar, if I recharge in rockbox until it say 100 and then boot into apple firmware it displays about 1/4 on the bar, then it will keep charging for about another 2 hours before it reports it is full.
I don't think there is anything wrong with my battery, its less than 6 months old and will last about 7-8 hours on average, I thought after a while rockbox would figure out where the 'low' and 'high' points of the battery were and adjust the meter accordingly but apparently not.