Bought a motorcycle specific capacitor to replace battery. With battery in system I get 14-15 DCV at the battery. With the capacitor in place of the battery I get 30+ DCV at the capacitor. 12 V light bulbs don't like this very much. Instructions for capacitor were to simply use the same positive and negative connections that went to the battery. Seems like I'm not getting any capacita-ing here. What could be wrong?

 

The dynamo on the motocycle is meant for charging the bike's battery.
If the voltage is not controlled the capacitor will charge to the peak output voltage from the dynamo.
There is nothing useful achieved in trying to replace the battery with a capacitor. The battery can store a lot more electrical energy than a capacitor.

 

I don't believe you are suppose to remove the battery completely !

I guess you have something like this

Designed to put across the battery terminals to take the strain out of the battery when cranking the engine , so extending battery life ... they are 16V max. , so if you really have charged it up to 30V it maybe destroyed ....
You can use a very small battery in conjunction with one of these caps , and so reduce weigh ... the cap should last forever.

12V lights subject to 30V would blow in less then 1 second.

 

What is the motorcycle, and why do you want to replace the battery?

 

What is the motorcycle, and why do you want to replace the battery?

The bike is a 1974 Honda SL125. I'm trying to make a cafe racer out of it. This means making it look as spare as possible. Minimal fenders and seat. No side panels or large battery, etc.

 

I don't believe you are suppose to remove the battery completely !

I guess you have something like this

Designed to put across the battery terminals to take the strain out of the battery when cranking the engine , so extending battery life ... they are 16V max. , so if you really have charged it up to 30V it maybe destroyed ....
You can use a very small battery in conjunction with one of these caps , and so reduce weigh ... the cap should last forever.

12V lights subject to 30V would blow in less then 1 second.

Hi,
The item is an
ACCEL Battery Eliminator Capacitor 151308

Designed to replace battery.

 

Hi,
The item is an
ACCEL Battery Eliminator Capacitor 151308
View attachment 177418
Designed to replace battery.

"... For use with Magneto and Generator systems only ..."
Does your motorcycle you have a Magneto or Generator system?

"... Use ACCEL solid state regulators 201103, 201104, or 211104. "
Did you use one of the above regulators?

 

It looks like that unit is designed for Harley's that have a voltage regulator, which your bike apparently does not have.
If you want to use it on your bike with the capacitor but without a battery, then you will need to limit the voltage with a high power Zener or a Zener/transistor combination.
Do you have any info on the charging current of the generator at high speed?

 

Below is the LTspice simulation of an example circuit to regulate the generator voltage by shunting the excess voltage/current to ground.
It uses a TL431 programmable Zener, and a P-MOSFET (which must be on a heat-sink since it dissipates power equal to 13V times the generator current, yellow trace).
As shown, it regulates the voltage to 13V maximum as the simulated generator current goes from 0 to 5A (X-axis).

 

It looks like that unit is designed for Harley's

It was used for the old Triumph motorcycles. Don't know of anyone using them in a Harley.

 

It was used for the old Triumph motorcycles. Don't know of anyone using them in a Harley.

I was going by this site (click on fitment).

Old Triumph motorcycles don't have a voltage regulator (newer ones added a large shunt Zener diode) so I think there would be a problem with high voltage with those also.

 

Bought a motorcycle specific capacitor to replace battery. With battery in system I get 14-15 DCV at the battery. With the capacitor in place of the battery I get 30+ DCV at the capacitor. 12 V light bulbs don't like this very much. Instructions for capacitor were to simply use the same positive and negative connections that went to the battery. Seems like I'm not getting any capacita-ing here. What could be wrong?

Some British motorcycles were marketed as scramblers and had a capacitor instead of a battery as original equipment - but they were regulated by a dirty great Zener; 15V 100W AFAICR.

A lot of Japanese motorcycles use half wave shunt regulators with 1 or more thyristors killing an AC arm of the bridge rectifier. The other half cycle is free to blow bulbs. The single phase alternators usually have 3 windings, 2 of which are in parallel and only switched in when you use main beam. smaller bikes just switch in a dummy load resistor when not using main beam.

The zeners are commercially available in both polarities, but from motorcycle suppliers will be positive earth.

 

The zeners are commercially available in both polarities, but from motorcycle suppliers will be positive earth.

I didn't find any in a quick search.
Do you have a source?

 

https://www.ntepartsdirect.com/PRODUCTLIST/SEMICONDUCTORS_ZENER_DIODES_50_WATT

 

I didn't find any in a quick search.
Do you have a source?

I'd been looking in the Philips catalogue - it was NXP last time I looked.

 

https://www.ntepartsdirect.com/PRODUCTLIST/SEMICONDUCTORS_ZENER_DIODES_50_WATT

Okay, thanks.
But at 30 buck$ they are a little pricey.
The circuit in post #9, that does the same job (with higher accuracy), should be a lot cheaper.

 

Okay, thanks.
But at 30 buck$ they are a little pricey.
The circuit in post #9, that does the same job (with higher accuracy), should be a lot cheaper.

The original Lucas Zeners are probably much worse. Ant they're polarity oriented for positive earth - so you might need an insulating kit.

A shunt regulator is probably as good as it gets with a PM alternator - the alternative is constructing an; "amplified Zener" with a dirty great power transistor.