Powerdynamo brings new ignition & light
to your vintage motorcycle



Technical Help



Battery is boiling ...

First the battery was always empty with the old and aged generator, and thus led to the installation of a PowerDynamo system. Now some see the opposite thing happen. The battery is boiling. Unsightly acid discharge, possibly even over the fresh paint or chrome are unwelcome side effects.

What can I do?  What happens? Is the new regulator faulty?

There can be several causes for malfunctions. One of the options surely is that our regulator could be defective. But in our experience that is really extremely rarely the case. Usually there are other reasons. This page is intended to give you guidance.

Many clients, having installed the new systems are keen to check voltage (always good) of the PowerDynamo's regulator (without a battery connected). Some of them than take to the phone immediately to tell us that they get less than 10 volts on the 12 volts system. And even worse, the more they rev the engine, the lower the voltage gets, in cases right down to 6 volts.

What happens? Lights shine brightly, but the meter shows low voltage. The meter is fooled! Fooled by the frequency of the generator (which changes with engine speed). Many - notably fast digital - meters have a problem with this. (More information on this see our page fooled meter.)

Others measure more than 14V and get misgivings, because of the 12V system. Well, at 12 volts the battery would never get charged, 14.4 volts is the correct voltage for a 12 volts battery. A load of up to 15.5 volts is not abnormal and is tolerated by the battery. If you switch off the light (which should be on for measuring please) than voltage can even rise to over 16 volts. A regulator needs some load to work correctly. Only when the voltage is even greater, a regulator failure is present.

The battery is getting sucked empty ...

He who had charging problems with his stock system is still suspicious and likes to check and surely see a fully charged battery now.

More often than not he will discover however that the battery does not charge well or not at all. Why? The poor thing is simply dead, killed by the stock systems negligence and/or old age.

Still the question:

What can I do?  What happens? Is the new regulator faulty?

Of course, any battery has a limited life and about after 2-6 years it passes the moon. It cannot take and store the charging current and starts to boil, converting the current into heat by electric-chemical reaction. The acid will be vapourized and lead off over the vent openings. This especially occurs with a battery whose life was previously characterized by energy shortages as a result of defective dynamos. So a late consequence of the old evil. Even a long winter without care is doing its destructive work. In that case you should check the battery (or even better to get it checked at a more specialized garage).

Our regulator/rectifier is checked repeatedly before dispatch and rather robust. However, it is a technical product and thus can also fail. Above all it will burn (and instantly so) if connected with wrong polarity or subjected to a short circuit (e.g. by a wire accidentially connecting to ground or a defective consumer such as a horn). In those cases you can normally smell what is wrong. The regulator has a destinctive smell of burnt plastics, notably at the cable outlet. If it smells there - it was certainly dead as a result of one of the above situations!  For more info on checking DC regulator output see here

But there are things like "semi" short circuits, those do not blow the fuse or char the cable's insulation, but they consume a very high current. Culprits are usually the main switch, the battery, the horn or on an (bad) earth connection. In any case power is lost. If too much power is lost, at some point the voltage drops because the generator cannot meet the demand anymore.

How to find such a "semi" short circuit?

First disconnect the (red) "+" and (brown) "-" cable from the Powerdynamo regulator/rectifier. Connect a headlight bulb to those regulator cables (using some "helping" wires). Now: Start the engine. Does the bulb shine brightly or does it not? If it shines, the regulator and the generator are well. If not, there is either a defect in the regulator or the generator or at worst both. 
If the light shines brightly, stop the engine, re-connect the temporarilly disconnected wires, start the engine again and switch on the light. Does the motorcycle's headlight shine as brightly as your testing bulb before?

If now the light is "darker", than power gets lost somewhere in the circuitry. In our experiences mostly in the main switch, on a "bad" earth connection or inside the battery (which is been only heated).

How to check my battery?

The first thing you should check is whether and how much voltage the battery gives. For this the battery should be fully charged. Start the engine and run at about 3,000 rpm with the headlights on. Review with the help of a multimeter, how much voltage is present between the battery terminals. With increasing speed voltage should increase and reach a maximum value of 14-15.5 V DC.

The following table gives an orientation on the relationship voltage - charge state.

charge state

6 Volts

12 Volts

voltage per cell









































0 %




Voltage test under load
The test requires a battery hydrometer (see below) showing an acid density of 1.20 to 1.24 (i.e. the battery is charged to 50% to 60%) in all cells results.

  • Set the multimeter on voltage display. The red sensor cable comes to the positive (+), the black cable to negative (-) battery.

  • Now switch on as many consumers (lights, indicators etc.) as possible. But the engine remains off!

  • During the process, you observe the meter and you get results that provide a starting point:
    battery okay: 9-12 volts (4,5-6 volts  for a 6V system)       
    weak battery
    : < 9 volts (< 4,5 volts)

For further informationen consult the website of a battery producer e.g. at the Varta webpage, from which we took the above shown information.