KZ750CSR Twin...Will it run without a battery?

electrical circuits are not my strong point on these old bikes. We tried to start this bike up without a battery (kick start) and she wouldn't fire. So we put a jump box to it and she started on the first kick. We took the jump box off and she got really rough and all the lights dimmed way out.

I didn't get a chance to really look into stator output, but I know some bikes just won't work without a battery....is this one of them?

Don't run it without a battery. The battery acts as a giant filter capacitor to keep the system voltage steady DC. Running without it will hit your system with voltage spikes that can really kill things quickly.

Guess how I found that out.

Can a properly sized capacitor be used in lieu of a battery? I'm not sure if there are any advantages to that, other than packaging purposes for custom bikes, but I just wanted to know for information purposes. I'm assuming that the capacitor would need to be fairly large.

There are two forum members to ask that question to:
"Loudhvx"
"Otakar"

Both have knowledge about replacing the battery...

Theoretically, a very large cap could work (and some are available for that purpose) but understand: in normal operation (lights on, ignition running) the bike's alternator does not put out enough juice to supply the bike at idle or low RPM. Using a cap instead of a battery, it could die at idle from starving the ignition out. I think most bikes that run without battery have a bare bones system without lights and accessories.

So I guess something like a chopper with only a headlight and taillight, and points ignition only. Possibly a drag bike with only the ignition to run and very minimal time spent idling would work with a cap.

Drag bike probably since they never let the revs drop and idle. I don't think a street bike with any kind of lights could run off a capacitor but I never tried it.

I don't run a battery on one of my bikes, and I use two 55W halogen headlights. It idles just fine and has plenty of voltage to idle. Even if the voltage is a little low at idle, it doens't matter since there is no battery to drain, and the capacitor doesn't care if the votlage is less than 13v.

The only thing is, the lights flicker a little at idle due to ignition pulsing, but remember, that's with two 55 watt halogen headlights. I don't run any gauges, but they are nothing compared to a 55W headlight. (The bike originally came with a single 55W headlight.)

It only works on bikes with a permanent-magnet alternator. The single-phase units may have little less power to wirk with, so you may have to stick with a single headlight. My ignition modules use a little less power at idle, so that helps, but it's a large amount. I've tried it on my other 550's as well, and one of them had two 55W halogens and a stock igniter.

The only issue is that it will only start if you have the lights turned off to start. That's because lights act like a short until they heat up and start glowing. Once the bike is running, you can turn the lights on and the momentum of the engine will prevent it from dying during that brief moment when the lights act like a short. So you obviously need a way to turn the lights off with igition on. I made it automatic with a relay. Now the start bnutton is used to turn the lights on.

Also, if there is a brown wire on the reg/rec, it needs to be tied directly to the output white wire on the reg/rec for safety.

Details and diagram here:
kzrider.com/forum/11-projects/221140-ano...at?limit=20&start=60

Okay, we got about 30~40vac at idle off the stator. Climbs to nearly 100vac when we hit the throttle. Does this sound good? I'm not sure what I should read on these single phase stators.

It's more complicated. The no load stator voltage doesn't mean much, it's the voltage under whatever load it's trying to drive (ignition at minimum, headlight/tail light if present) that will tell the story. The AC voltage peaks from the stator are rectified by the diodes and then "filtered" by the capacitor, but the DC system voltage is dependent on the RMS current the stator is being asked to deliver and what it's effective internal impedance is under load. Basically, with the capacitor in place and the bike running (lights on) read the system voltage at idle. If it's 12V, the stator has enough to handle the load. Less, it probably does not.

We're not going to run a cap. We're running a battery. DC voltage was tested with all lights and a battery...under full load. We only had about 9vdc at the battery...but even with a power supply hooked up (putting out 12.5vdc) we only saw about 9v at the battery...we think that issue is the battery itself, though.

Why not take the battery to your local parts store and have it tested? They usually don't charge anything to do that, and it would eliminate the battery as a possibility.


Thanks for the pointers BH and Lou. I would want a super stripped down electrical system if I ever do put together that chopper frame that I got. I'm thinking something like your ignition system, single headlight, tail light, horn, and maybe a tach at most. Kick start only, super bare bones bike.