Im thinking of getting 4 RC 6cell packs (7.2V 3300mAh each)

I assume there is some reason you want to go all high-tech and expen$ive.

It is really very easy, very cheap, and very reliable to get a 12V lead-acid gel-cell battery, about 3.5" by 2.5" by 4" that will run your player for 8--10 hours, and cost you about $15--$20. You can spend another $25 or so for a "smart" battery charger that won't overcharge it, or if you have the discipline you can use the battery charger you already have for your car and keep an eye on things while you're charging it, don't let it charge for more than about 45 minutes at a time, otherwise it will overheat and shorten battery life.

Avoid deep-discharging, and you'll get hundreds of cycles out of it. Abuse the hell out of it (overcharge, deep-discharge) and you'll still get dozens of cycles out of it, and it's so cheap that if you wreck it you won't care very much.

I've been using this setup for the last three or four years when I use my player on my bicycle and am still using the original battery.

tanstaafl.