I have a problem with many of the flashlights. When there's a lot of vibration, the springs don't make good contact. Sometimes the light flickers, or dims out. Sometimes I lost a light completely.
A friend of mine had a similar problem on a different application (DIY RC electric cars) and he solve it by using strong rare earth magnets from the surplus store. The technique is to find a magnet of a similar diameter as the battery and place it in between the battery and the spring.
Additionally, he also used sand paper to get the inside of the tube down to the metal. Then used aluminum foil, pinched between the magnets and the battery, hand-shaped to the tube's diameter. This act as a secondary contact just in case.
I'm gonna try this as soon as I get time to stop at the surplus store. Anyway, anybody tried something similar before? Sometimes I just wanna solder the darn batteries...