I've not looked at the inside volume of a M003 but have done an upgrade on my ZT-180from 2000mAh to 4700mAh about two weeks ago and with backlighting reduced to about 30 percent (still very watchable), I'm getting over 8.5 hrs of movie viewing run time. for more info go over to the ZT-180 forum, thread "Upgrading the battery? Cheap and easy?", and look at my posts. Be aware that the batteries I used are only available in the U.S. but I did post a link to the manufacturer in Taiwan for those outside the states. Please read the whole thread, Li-Po batteries are dangerous if you mishandle them, so take the time to read the info I posted. Hope this works for the owners of a M003Li-Po Lithium Polymer Battery 3.7V 3500mAh GPS SAT NAV - eBay (item 300426638084 end time Oct-08-10 03:48:35 PDT)Here is a possible option for those outside the U.S. although it's only 3500mAh and you need two with one power protection circuit for the hookup.P.S. Yes, the case does close without any mods......