Topic: Electrical Disaster - 1975 CB750F

[CHM]

I have a 1975 CB750F and left a battery charger connected for too long.  When I tried to turn the ignition on, I heard a POP and saw a spark from somewhere near the fuse box.  The bike is dead.  When I checked the battery, it was putting our something like 60 volts instead of 12 volts.  The fuses did not blow, unfortunately.  I already checked that with a spare main fuse.  Any ideas which component is blown?  Rectifier versus voltage regulator?  How do I test these components.  Any help is appreciated

Cary Meyers
#176

[bryanj]

Rectifier is more prone to overvoltage but as i remember is not connected by ignition switch but "in line" all the time---regulator has nothing to "pop", youre charger must be bad of the battery made 60 Volt.

For the bike to be totaly dead it has to be the main fuse, ignition switch or one of the main cables (not forgetting the ground one)--- The fuse can look ok but ne disconnected inside, try a New one anyway

[TwoTired]

Can't imagine the battery having 60 volts unless there is a serious lack of electrolyte inside, or an outright open circuit within.

However, if it does, your fuses will probably go poof in a big way.

At 12V the bike normally draws about 10 amps.  This equates to a bike circuit resistance of about 1.2 ohms.  The bike resistance isn't going to change (at least at first application) even when 60V is put on it.  However, the initial current draw will be 50 Amps!  Your main fuse ought to make quite a lights show, being it is supposed to open at 15-16 Amps.  Good thing, too. 60 volts make your 50 W headlight into a 1000 watt device... for a few milliseconds anyway.

The charging system is direct connected to the battery with no fuse.

Check your charging system rectifier and use a known good battery for further checkout.

And get a new main fuse.   

Cheers,

[OldSchool_IsCool]

And a battery tender! Here is a recent thread on the topic.

http://forums.sohc4.net/index.php?topic=44436.0

[CHM]

Thanks for your replies.  I went and checked the battery today and it read 12 volts again.  I checked out all the circuits, continuiy, etc.  Everything checked-out.  I cleaned the cables, reconnected the battery, and everything was back.  I am tossing the old garbage charger.  Incidentally, I have a dry battery.  I wonder if this had anything to do with the overvoltage?  Don't know.  I do know that I do not miss acid dripping down the overflow tube.

[Trevor from Warragul]

Did you have your multimeter set to AC or DC first time round..?

[markjenn]

The 60V was anomalous.  No amount of overcharging or any kind of battery failure will cause a 12V battery to develop 60V.  The basic chemistry of each cell limits the voltage it can develop to about 2.2V per cell (there are six of them in series).

Glad you got it fixed.  If you have a good smart battery charger (I like the Optimate ones rather than the Battery Tenders), you shouldn't have any future problems.

- Mark