Fao technical bods (electrics)

Had a look at a mates sv yesterday. He's had the battery go flat a couple of times when the bikes been left standing a while.
Needless to say I suspect a reg/rec problem. I've checked the alternator off load and its output is fine. but I'm seeing 17v at the battery terminals at 5000rpm, I belive it should be more like 15v?
So my question is, if the output is too high, can this cause the battery to go flat?
I allso checked for earth leakage (no problem there) and the battery was fully charged on an optimate which reported no problems either.
The bike doesn't have an alarm fitted by the way.
I think if it was my bike I'd change the rectifier and take it from there, but I don't want to waste someone elses money if it could be something else?
Second opinions would be much apreciated.

[tigersaw]

If the batteries internal resistance has risen because it is fubar, then it will not present a sufficient load allowing the terminal voltage to rise on charge.
Optimate is a clever gadget, but has no knowledge of a batteries capacity, and will report a sad battery as ok so long as it sees a charge current and monitors no considerable drop over its test period.
Test the battery first - leave the lights on for one minute, should not dim at all. Leave another 5, bike should still start.
Possibly swap battery with a known good one, and repeat terminal voltage check on charge? If still high, then suspect rectifier as you suggest, charging at too high a voltage will cook a battery as you point out.

Interesting! but when his baterry went down I lent him one off of my spare bike which has allways been fine, and that went down. I charged his battery on the optimate and then put that back on to test the rectifier.
I did see something in the manual saying that if the chargin voltage is too high it sends it to earth? Does this mean it stops the battery from charging to protect it?

[tigersaw]

Not sure how the rectifier / regulator works on a modern motorbike, crude ones used to dump the excess as heat, hence the large fins. Didn't realise you had already tried a replacement battery.
When the bike runs, and you measure 17v across the battery terminals, do the lights get excessivly bright as well? I'd expect a terminal voltage that high to start blowing bulbs. Are you sure of your meter?

lights all seem fine, and as far as I know it hasn't popped any bulbs. I suppose a hand on the reg/rec to see if it's getting hot would be good. I was working it the dark and with limited tools though so just riggled out the multi plugs without taking off the tail unit.
I'm thinking now that it is as I said, dropping the charge to earth because it's too high.

check output on alternator between 3 cables it should be same between each (1-2,1-3 and 2-3) about 70v iirc. Probably reg/rec by the sounds of 17v across battery terminals

Checked that, the altinator is at around 70v on all three phases

Originally Posted by shifter

Checked that, the altinator is at around 70v on all three phases

also people post on here that the reg/rec takes the battery too sometimes. Got anyone who can borrow bits off? Ohterwise you'll have to just get a reg/rec and test + try.

Think as you say, it's time for a new reg/rec and take it from there. Thanks for that guys, just wanted to make sure I hadn't missed anything

[yorkie_chris]

Sounds like your regs failed closed. 17v way too high.