Capacitors In Your Charging System - They Really Work!"

- by Pete Snidal (C)2001
Over the years, I've run into a number of British bikes with the big spring-mounted Lucas Capacitor under the seat somewhere. The story goes that they "replace" your battery, allowing you to run without one. Not being one to use my old Brits in competition (any more), and hence not having to worry about overturning the battery, and not riding a chopper, for which I want to achieve the "clean look" of no battery, I never paid them much attention. Besides, they seemed a little expensive to me anyway.

But a few years ago, I decided it would be nice to steady down that oscillating ammeter, and to this end, I tried putting a likely-looking capacitor from my radio junk box into my charging circuit. It didn't make any difference to the wildly-waving ammeter needle, but it did have one effect, which I discovered a while later.

My bike starts with a dead battery! Since then, my battery has gotten old, and it doesn't hold much of a charge, or for very long. Last fall, I noticed once that the charge was so low it barely lit the running light; when I turned on the headlight, they both went out. So I was a little apprehensive when I turned them both off and kicked it over (key on, of course) and it started first kick! I rode it home, and by the time I got there, the battery was up enough to run the lights just fine.

When I rolled the bike out of its spot in the shop this spring, the battery was stone cold dead. I'd trickle charged it a couple of times over the winter, but no matter; this time it would take no charge at all. Living as I do mail-order distance from the nearest motorcycle battery, I attempted revivification. I drained out the acid, and replaced it with fresh, and this got things so that the battery would take a 4 amp trickle charge for an hour or two, and light the headlight after a 24 hour wait. But the voltage was down after 24 hours of standing to less than 11 V. And after 48, it was down to under 7 - barely enough to light the light. The next day, it was down so low it wouldn't even do that, so I thought this would be a good time to see what effect the capacitor was having on the system.

I pulled a plug, leaving it connected sitting on the cylinder head. Turned on the ignition and kicked it over. The plug gave a series of lovely bright blue sparks! This with a dead battery still connected! Am I now a believer in capacitors? You Bet!

What Kind of Capacitor?

The one I used was just an electronics part, a Mallory 40000 ufd electrolytic with a 40VDC rating (peak 50V). Size is the determining factor in capacity with these things, apparently, and it's physical size is about 4" X 1 1/2". (10 cm X 35) You can use one like this, or you can buy the Lucas one, if they're still available. Either way, I won't be going anywhere without one anymore!

If you decide to use one, be sure to observe polarity when hooking it up - it's marked, and if you use an electronics one, you'll have to solder the wires to the terminals - the Lucas ones come with spade connectors. Other than that, there's nothing to it. I strap-tied mine to my wiring loom in a fairly vibration-free spot, do that and Roberta's your Auntie! ---