Zener diode, new 12V 9aH flooded cell battery fully charged, no lights etc on. Only draw is the electronic ignition. The voltage across the battery terminals reads 15v when it is running at a fast idle (see video in By Gawd thread).
That will surely boil the battery. How to tone it down?? Needs to be 13.8-14.2V ime.
1969 BSA B44VS PC8113 Engine PC8113B44VS Recently acquired (Feb 2011) Aiming to get running and return all parts/configurations to original. First BSA. Have 1970 Honda CB750 too.
The Zener diode controls the voltage. What did the voltage drop to with the lights turned on. How about readings at low rpm and then again with a good rev..
I would be thinking the meter, - is it reading correctly? There will still be a little ac ripple but you are measuring at the correct place, - across the battery itself.
Next would be checking the Zener earth. Are there any floating voltages in the system ? ie some components not earthed correctly or linked to a common ground point. Is the Zener indeed connected to the rectifier output and battery live correctly.
It may be a fauly Zener, you could put the meter over that and see if you get similar results. The voltage should obviously plateau at the Zener operating voltage.
At least you have too much charge. Easier to fix than too little. To prevent boiling the battery you could run with the lights on... or even pulling an alternator ac lead off the input to the rectifier.
It shouldn't be too difficult to work through and fix this one.
Last edited by Kevin (NZ).; 06/05/1212:26 am. Reason: shouldn't
The voltage across the battery terminals reads 15v
Agree with Kevin.
1. Have you tried a known good battery in your multimeter? As I found out just yesterday, even when measuring Volts, my multimeter's reading was high due to a failing internal battery. Or try just another multimeter.
2. Do you always have battery Volts at the Zener's tab terminal?
3. Have you checked for any resistance between the Zener's stud and battery +ve?
However, as long as the Volts don't go higher than 15V, you might be stuck with that limit unless you change the Zener; that's what many new ones have been working to for some years. And, if you change the Zener, there isn't any guarantee that another one won't regulate until a similar Voltage.
Originally Posted By: DaveOfTexas
That will surely boil the battery.
Ime, it won't. But, if he doesn't understand already, you need to impress on the bike's owner that 43-year-old Britbike electrics aren't fit-'n'-forget like his car's or modern Jap bike's; he needs to check the battery water levels regularly, then he'll notice if the levels are dropping.
The Haynes manual says that at 15.5 volts the Zener diode diverts all the current from the alternator, about 5amps, to the heat sink, thus insuring the battery is not overcharged. So apparently you seeing 15v is OK.
15V is at the top end of what a battery will tolerate but it will shorten its life eventually, I have a digital car battery charger and it charges the battery until it sees 16.3V and then stops and use it regularly and my car batteries last fine. For the moment until you csn get it down to 14.5V switch your lights on.