In A Nutshell
Currently, the best rechargeable C-size batteries are made by SunLabz. These batteries are low self-discharge NiMH (Nickel Metal-Hydride) cells, and they have a high capacity of 5000 mAhs (milliamp hours).
Unfortunately, Sunlabz stopped making C-sized batteries for some reason. However, these cells from Tenergy are a good choice too. They are low self-discharging cells.
Rechargeable batteries have made some real improvements in the last five years. Most notably, you can now get rechargeables that don’t go flat after a couple of months! The technology is called “low self-discharge” and means rechargeable batteries can hold 80% of their charge for a year. Low self-discharge batteries (LSD) NiMH cells were first introduced by Sanyo in their Eneloop AA cells in November 2005. Since then, other battery manufacturers have adopted this technology. Then in 2010, companies like Maha started releasing LSD NiMH cells in C and D sizes.
An alkaline C cell is typically rated for about 8000 mAh, but only at a very low current (around 30 mA). When used at a load of 500 mA (such as flashlight), the capacity of an alkaline drops to about half. Basically, the advantage that alkaline cells have over NiHM cells, is that they have a shelf life of about 10 years, with minimal self-discharge.