Ok, I'm not going in-depth, just a brief description for those who are interested. The two custom rechargeable battery packs I use consist of three batteries connected (soldered) in series, wrapped in electrical tape. The batteries used look like this:
and my battery packs look like this:
To charge them you also need a 'power supply'-a device that regulates current and voltage to give power or in this case, charge the batteries. If you have a background in electronics, and if you are good at soldering and have access to all the components/wires/power supply box, then you can try to make your own. All details about the batteries themselves such as voltage, maximum current, charging voltage¤t can all be found at this ebay link, where you can buy them for about 8-9 dollars each http://www.ebay.com/itm/Tenergy-Tenergy-LiFePO4-18650-3-2V-1100mAh-Power-Cell-Max-25A-w-Tabs-SALE-/390575207953?pt=US_Rechargeable_Batteries&hash=item5af018b611