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Bikes, Pictures, Stories & more => Other BSAs, Other Bikes, Machinery & Tools => Topic started by: BSARGS650 on 20.02. 2021 13:33

Title: BSA WDB40 Charging Battery Choice Please Help
Post by: BSARGS650 on 20.02. 2021 13:33
Please, may I consult the wise and enlightened on this wonderful forum in the cosmos? I appreciate that my question is not strictly A7, or A10 related, but, there is so much wealth of trusted knowledge and wisdom within, this is the best of places to raise my question.

I am starting to restore a BSA WD B40, in future; I intend to replace the Lucas PZU5A 12 volt battery with a maintenance free AGM unit.  The current charging system is alternator, bridge rectifier and a reverse biased zener diode in parallel with the battery.  I presume the avalanche effect of shunting excess current with low loading at around + 2000 RPM would be around 13.0 to 14.5 volts – not a sophisticated system.

My Harley-Davidson Sportster has an AGM battery, but, I suspect the regulator, which incorporates the rectifying and shunting has more electronic sophistication for monitoring and controlling the charging circuit where AGM batteries are concerned.  For instance, during long periods when the machine is not ridden, one cannot use a typical battery charger; it has to be of the intelligent type.  My Sportster was bought new in January 2006, its original battery being replaced May 2010 and is still performing well.  It is connected to a battery tender – intelligent charger for periods of no machine use.  The AGM type, in my experience, gives long reliable service.

The simple charging circuit on the BSA WDB40 may not be compatible for the AGM type, as the charging would not be too different compared to that of a typical battery charger which cannot be used for AGM type?  Am I worried about nothing and all will be OK?  Is it not possible unless a more sophisticated regulator incorporating bridge rectifier, zener diode and shunting circuitry is used?  Is a suitable regulator available? 

Title: Re: BSA WDB40 Charging Battery Choice Please Help
Post by: Black Sheep on 20.02. 2021 15:43
Anyway, on my currently dismantled WDB40, I long ago ditched the rectifier/zener diode setup for a solid state 12v voltage regulator. My one came from AO Services but of course others are available.
P.S. Beware of letting your son do some fettling on your bike. Mine was delivered back in several boxes. *cry*
Title: Re: BSA WDB40 Charging Battery Choice Please Help
Post by: Triton Thrasher on 20.02. 2021 18:01
If the rectifier and Zener are working, they will charge an AGM battery.

Presuming the charging voltage doesn’t work as well as measuring it.

If you want a rectifier/regulator black box, these work well. (
Title: Re: BSA WDB40 Charging Battery Choice Please Help
Post by: BSARGS650 on 21.02. 2021 10:59
Many thanks for taking time to reply and advice given......