I'm not crazy about NiCd or NiMh batteries. So I decided to mod my radio box to use the wasted room and install 2 - 2600 mah (total 5200 mah) 7.4 Lithium Ion batteries and a duralite switched regulator. The new switch was also mounted on the top of the box like it should have been in the first place.
I used a dremel cutoff wheel to cut the box on the edges and bottom.
A piece of thin aircraft plywood for the bottom, some CA glue and 20 minute epoxy putty. This also saved almost 5 oz of weight. The Li-on batteries with the regulator weigh 8 oz the 4200 NiMh battery pack is over 13 oz.
I also modded my Nomadio Sensor radio for a 2 cell Li-on pack. I still use the stock Baja charging plug as I made an adapter for it to use my Li-on charger
The advantage is higher capacity, less weight, no cycling or memory issues and the batteries don't self discharge. Anyway, I always like to have the electronics my way because that's the only connection between me and my car. Here's a link to the picture page.
http://www.bashbuster.com/radioboxmod.htm