Topic: overcharged battery 15.8 volts

[neil10]

Hi charged brand new battery on k4 cb 750
ran fine first up but now blows main fuse
multimeterd battery pulling 15.8 volts
open to suggestions

[Frostyboy]

It would have to be related to a regulator problem surely? I don't know if 750 has similar regulator to 550, but there's an adjustment procedure in the 550 manual I downloaded from the top of this forum.
I'd assume there's one for your bike there too. 

[neil10]

Sounds fair  how about if I charged battery for to long

[Frostyboy]

Sounds fair  how about if I charged battery for to long

I don't believe so. Do you have 15.8v with bike off? If so, hook up a load to discharge it for a while to pull it back down to 12.4v. Then start the bike & see what voltage is with engine running. If it goes back up to >15v start looking at the reg I'd say.

[neil10]

Thks what is normal

[Motoguy23]

I must also chime in that on some of the older bikes they don't start charging until 1500 RPM so you might have to set your idle higher say 2000 RPM and check it.

[Frostyboy]

I'd expect 12.4-12.6 fully charged with everything off.
Below is a copy from the 550 manual for reference.
Cheers.

[neil10]

Good do you think this would cause blown fuses

[Frostyboy]

Quite likely, however we are now entering into more technical questions than I'm qualified to answer. Hopefully TwoTired will look in on this thread and offer his thoughts.  

[falconhockey]

my idle is always set at about 1800 or so... is that normal, it idles really well there so  ihae just left it. i supposed i could turn the screw a little to lower it. whats normal

[Motoguy23]

Well the manual says for cb500 that it should be floating around 100-1200 I believe

[TwoTired]

Sanity check...
Stock bike electrical's draw about 10 Amps through the 15 A fuse.  Let's assume the voltage average is about 13.5 V.
Using ohm's law R = E/I  We get 13.5/10 or a bike resistance of 1.35 ohms.

Now assume the same resistance but higher voltage of 15.8V  E/R = I,  or 15.8/1.35 ohms gets 11.7 Amps through the main fuse.

If the headlight hasn't been upgraded to higher watts, and the coils are still 5 ohm, and the fuse terminals are clean and non-resistive, the fuse will run a bit hotter, but should not blow with the higher voltage in this case.

The higher voltage will eventually damage the battery, though.

Cheers,

[Frostyboy]

Stock bike electrical's draw about 10 Amps through the 15 A fuse. 

Cheers TT, that's the bit I wasn't certain about.
Neil, it looks like you're going to have to track down what's blowing that fuse.
Does the fuse blow with the key on without the engine running? Or does the engine have to be running?
Will the fuse stay intact long enough to measure battery voltage with the engine running?

[neil10]

fuse blows immediately or within 5 secs
but doesnt blow if i put kill switch on

[Frostyboy]

fuse blows immediately or within 5 secs
but doesnt blow if i put kill switch on

Okies, now remember I'm not a sparky, but what I'd do now is remove the 15a fuse & connect a reasonably high wattage bulb across the fuse contacts. Turn the ign on. The light should glow brightly when the short circuit occurs. Start wriggling the loom or disconnecting wires until the light goes out. That hopefully will narrow the field in finding out where the short is. Good luck.

[Bodi]

The only thing connected to the kill switch is the ignition coil power. Possibly a coil 12V wire is pinched somewhere or some other wiring problem. Disconnect the coil power wires (the small wires that do not go to the points) and see if the fuse blows, if it does then you have a wiring problem. Not, probably a coil gone bad. Plug them in one at a time to see which.

[neil10]

thks