This article is from the Misc Bicycles FAQs, by various authors.
From: josh@Happy-Man.com (Joshua_Putnam)
I'd add one more component: a fuse.
If you accidentally short a gel cell battery, you'll
discover it has enough rapid-discharge current to burn up
your wiring, scorch the paint off your frame under the
wiring, etc., and do small spot welding.
My 20-watt headlight's wiring harness has a slow-blow fuse
right next to the battery's positive terminal so that it
stays in line when the battery is draining or charging.
Although the light should only draw 1.7 A, I use a large
enough fuse to allow several times that flow, yet still keep
the current within the rating of the smallest wires in the
harness. (Also, I happened to have two full boxes of
slow-blow 5 A fuses sitting around, so they were free :-)
A short may seem unlikely, but a long winter on the bike
exposes the wiring harness to plenty of inclement weather
and wire flex, so it's not as unlikely as it may seem,
especially if you use the frame as a conductor in a one-wire
lighting setup. (I use two-wire for simplicity and
Otherwise, an excellent article. Thanks for posting it!