Originally Posted by truman
Can I assemble a new battery array of 8 rechargeable C-cells and hope to charge it with the original wall wart recharger, or do I need a different charger?
You have to match the type of cells to the charger. Also, for NiMH it's really worth it to have a "smart" charger that knows when to shut off -- the cells last longer.
Originally Posted by truman
Do I need more than 8 cells? I generally see 12 volt systems rated around 13.7v, or so, it seems. Or is it because rechargeables are rated lower?
The voltage produced by a battery drops as it discharges. For instance, a NiMH cell will produce about 1.4V fully charged and about 1.0V when it is nearly discharged. It will be rated at a nominal 1.2V. So a ten cell pack will be rated at 12V, and will produce somewhat between 10 and 14V in actual usage. In a lighting application it's generally a good idea to "overvolt" and size the pack so that its rated voltage is above the rated voltage of the bulb, so that it provides the full bulb voltage for most of the battery life. So for a 12V light you might use a 11-cell pack (13.2V nominal) or 12-cells (14.4V nominal).
Alkaline cells are rated at a nominal 1.5V so eight of them makes 12V.