The guy in the store is wrong.
First of all, the mAh of the battery determines whether or not the battery can stand the drain of a particular lamp without dropping its' voltage (and therefore dimming the lamp).
In your 3 cell Mag Light, for example, if you use 3 hi cap C cells, each about 1800 mAh, the batteries will not support a 1.5 A or 1.7 A KSR (Krypton Star Rechargeable) lamp (i.e. a KSR 1.5A or KSR 1.7A lamp). The lamp will be dim, as the small batteries cannot support such a large load. If you use 3 hi cap D cells, each 4500 mAh, then the KSR 1.5 or KSR 1.7 lamp will be bright; these lamps will last about 90 min with freshly recharged D batteries.
The reason he told you to use a 2 cell lamp in a 3 NiCd cell lamp is because the NiCd lamps run at a nominal 1.2 V. Thus, alkaline 3 cell lamps might run at 4.5 V, but NiCd 3 cell lamps might run at only 3.6 V. This is close to the 3.0 V of a 2 cell alkaline flashlight. However, the discharge characteristics of the batteries are different; alkaline cells discharge in a linear fashion, while NiCds tend to hold their working voltage near 1.2 V (after an initial small drop from about 1.4 V which is where they are immediately after recharging).
What this means is that the NiCds will actually hold a lamp at a HIGHER average voltage. This will shorten lamp life. This is why Carley makes KSA (Krypton Star Alkaline) as well as KSR lamps.
Here is the deal. Unless you have a lot of money tied up in high cap rechargable batteries, forget NiCds. Just use a XS (Xenon Star) lamp. With alkaline cells. The XS lamp only draws about 10% more than a KSA lamp, but is a lot brighter. Plus, the XS lamp will burn 10-12 hours with a fresh set of alkalines.
Despite the higher candlepower rating of the KSR lamps, and their much higher amperage then KSA and XS lamps, I have found that in practice, the XS lamps are as bright as the KSR lamps. They run at a higher color temperature.
Forget NiCds.
Walt Welch
