Technical Information Pages for the Restoration of the heavy military bikes BMW R75 and Zündapp KS 750.


Editor:   ERSATZTEIL- DIENST   Hans-Peter HOMMES       D-41748   VIERSEN   Tiefenstraße 10       Tel. 02162 - 8100933

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Problems with the Generator?


The generator brings power.
When I accelerate, the light becomes brighter.
But when I drive for a certain time, then the battery is empty!
When I ride without light I have a full battery.
Where is the mistake?


When battery, generator and the power plant are in order, then this may be due to a too high power consumption, when the light is switched on.

The generator brings a maximum output of 50/70 watts.


When I assume that the battery is fully charged at 8 amps, a longer trip with light should be possible without problems.

Since the 50 watts are only generated at a certain speed, it may be that the battery is empty after some driving time in spite of a working generator.



The reason for this is mostly the use of bulb with too high a voltage.


Correct items are.


Headlight                          25/25 watt bulb

Parking light                     1,5   watt  (This light was not turned on during normal driving like today) 

Rear light                         3,0   watt

Sidecar light                     1,5   watt

Sidecar rear light             1,5   watt


So electricity consumers with 25+3+1,5+1,5 = 31 watt are connected.


Today consumers are often increased in the wattage:


The head light has a 35/35 watts, the rear light has 5 watts or even 10 and as for parking lights three 2 watts bulbs are used.


This results in at least 46 watts. This means that the generator always has to bring full power, just to operate the lighting.
When the generator brings less at a low engine speed, the battery must supply the missing power.

But the battery gets almost no charge current and is slowly sucked empty.


With the correct light bulbs a normal driving is possible and a 25-watt bulb with a good reflector also brings sufficient good light.


It is also important that the governor is set correctly

Electronic governors without adjustability always bring too little power.
This permanently dim light of the headlights indicate this.


Adjustable governor you can get in our shop                 order no. 3170





The information that I found always says more or less, that this battery is not suitable for our bike because the charging by the generator destroys the battery within a short time.
Unless, the battery is not charged by the generator but externally by a suitable charger.


On this subject I found the following article:

The gel battery is sensitive to excessive charging voltages. Therefore, a battery charger which operates with constant current or constant power charging, can drive the voltage too high to adjust the current. This is not good for the gel battery. Usually it is not clearly visible if the charger drives the voltage too high or not . Therefore you should make sure that the charger contains a reference "also suitable for gel batteries". These chargers will limit the charging voltage or work with a charging curve which does not raise too high a voltage. The gel battery should not produce gas as there is actually no refilling possibility. If, however, the charging voltage is too high the gel reacts and will cause a damage to the battery.
For lead-acid batteries or NC battery a too high charge voltage is also not optimal but can be compensated by refilling water.

A car charger is just like charging by generator usually unsuitable for the gel battery because the currents of these devices are too high.
Too high currents heat the battery. This changes the end-of-charge voltage.
It makes sense to use gel batteries as stationary batteries which are provided with a suitable charger via the power grid.



We offer:

Spare parts + accessories + repair for engine, transmission, etc. of the
Zündapp KS 750 and 600

We offer:

spare parts + accessories + repair for the

BMW R75 and Zündapp


H-P Hommes Ersatzteil-Dienst
Ersatzteile + Instandsetzungen
repairing + restoring

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