Motoscrubs.com

I am on the final stretch of a rebuild - Mototrans '62 Elite. I had an Elite in '68 which only went as far as a battery charge would last - I did a lot of pushing! With this bike I am trying to stay as close to as-built as logic and budget allow. But my pushing days are over.

Reading a ton of stuff on here I believe there is direct replacement for the old plate rectifier - a KBPC5006 50A 600V. It would mount well on the bracket for the plate rectifier. So far, so good.



Modern bikes have reg/recs and batteries prefer stable charge voltages ~ 6.8V for AGM. I assume DC voltage from the rectifier varies with engine revs and if so, is there a regulator that would sit between the rectifier +ve output and the (AGM) battery?

Hans is your man - he often gives advice on here are advocates using a cheap Chinese reg/rectifier unit; hopefully he will chime in with advice.

Well, here is the expected advice ;o)

The old rectifier bridge is made from Selenium, an ancient semiconducter material not in use anymore. This rectifier had a low efficiency and high leakage current, and is not durable today due to corrosion and other degrading effects. It shouldn't be used anymore.
Your choice of a modern Silicon bridge is basically right, but overpowered. The lower grade KBPC2502 will do the job perfectly, it is also cheaper.
A regulating device is imho necessary, especially if your Elite has a lead-acid battery, which should not be overcharged at all. A LUCAS 6Volt zener diode from an old brit bike could do this regulating job, but I'm unsure these really exist. But if so and you can find one, i.e. on ebay, beware: you have to convert your electrical system to positive ground !, which is not critical, if you don't run LED main and/or pilot lamps; and the diode needs a heat sink.
If your are good in hobby electronics, semiconductors and soldering, I can provide instructions how to design and build a power zener diode, using a silicon power transistor and a couple of small and cheap electronic components; this will emulate a Zener diode for negative ground.
AGM batteries: afaik they need 7,3 Volt for being fully charged, not 6,8; for standard batteries 7-7,2 Volt is OK.

Hans

ducwiz wrote: A LUCAS 6Volt zener diode from an old brit bike could do this regulating job, but I'm unsure these really exist.

I also doubt Lucas 6V zener diodes were fitted to British bikes as part of the battery charging system, but 12V types were.

Hans, very good of you to take the time for such a comprehensive reply and for your offer.

I came across the silicon bridge rectifier on a thread here reported as working successfully. I chose it on that recommendation. The 25A version you mention is cheaper but I had ordered the 50A for €5, inc delivered by post. If the 24A is more efficient, I could switch? I like the idea of a silicon bridge rectifier as a direct plug-in replacement for the plate (selenium) original. With a battery fitted, that would allow me to start the bike for the first time and test both engine and electrics. My quest for a 6V -ve regulator comes from the experience I had living on a sailing yacht for 8 years: battery management becomes a critical operation when off-grid with a fine line between fully charging and overcharging. For a bike with no starter motor, a float charge of 6.8V would seem adequate avoiding the possibility of gassing and overcharging at 7.3V on a long (ha ha) run. As you suggest, ~ 7 to 7.2 is a good compromise.

But how to get that steady voltage? I don't want to change the wiring, can't find a -ve 6V zener and would prefer an off-the-shelf, repeatable solution.
I can find only this sold for a big range of small scooters and sometimes described as 'mofa':
https://www.ebay.co.uk/str/m2sparts?_bk ... +regulator
but reported by one user as 6.6V output.

If buying a solution is not a possibility, then I would consider building one as you suggest Hans. But I am no electronics hobbyist although electronic component soldering is within my abilities.