07-02-2025, 09:17 AM
Hi Jamie
The resistance figures mentioned above are still the best information I have for the "Half/Summer Charge" resistors. The 6 volt third brush system relies on a combination of the lighting switch position and driver choice to set either full or half charging current, roughly 8 or 4 amps. The third brush setup stabilises (to some extent) the charging current with changing engine/dynamo RPM. The main aim of Half/Summer charge is to avoid overcharging the battery during long runs with no lighting load. The battery acts as the system voltage stabiliser, as its terminal voltage does not vary a huge amount with different charge currents.
I assume you are going for a 12 volt conversion. To allow for the higher system voltage the ideal would be to re-wind the field coils to double their resistance. Historically this was considered too expensive, so folks just put a resistor in series with the standard coils. A bit wasteful of power, but it did the job. If you want to retain the ability to rein back the charge rate by reducing the field current you will need two selectable values of resistor. This is the best way to avoid overcharging.
I have seen high power shunt zener diodes used in motorcycles to limit the system voltage, these being bolted to the frame as a heatsink. All that happens of course is that the zener diverts some of the current that would otherwise flow into the battery. The dynamo will still be producing full whack, which it may not enjoy especially on a long run in hot weather. I have not seen a zener proposed or used in a 12 volt Seven.
Although 12 volt conversions were all the rage at one time, the advent of LEDs for lighting has removed one of the reasons for their adoption.
The resistance figures mentioned above are still the best information I have for the "Half/Summer Charge" resistors. The 6 volt third brush system relies on a combination of the lighting switch position and driver choice to set either full or half charging current, roughly 8 or 4 amps. The third brush setup stabilises (to some extent) the charging current with changing engine/dynamo RPM. The main aim of Half/Summer charge is to avoid overcharging the battery during long runs with no lighting load. The battery acts as the system voltage stabiliser, as its terminal voltage does not vary a huge amount with different charge currents.
I assume you are going for a 12 volt conversion. To allow for the higher system voltage the ideal would be to re-wind the field coils to double their resistance. Historically this was considered too expensive, so folks just put a resistor in series with the standard coils. A bit wasteful of power, but it did the job. If you want to retain the ability to rein back the charge rate by reducing the field current you will need two selectable values of resistor. This is the best way to avoid overcharging.
I have seen high power shunt zener diodes used in motorcycles to limit the system voltage, these being bolted to the frame as a heatsink. All that happens of course is that the zener diverts some of the current that would otherwise flow into the battery. The dynamo will still be producing full whack, which it may not enjoy especially on a long run in hot weather. I have not seen a zener proposed or used in a 12 volt Seven.
Although 12 volt conversions were all the rage at one time, the advent of LEDs for lighting has removed one of the reasons for their adoption.