Voltage plumet when turned on

[ToddS]

Hey GS Resources,

My 1980 GS450 reads about 13.3 volts when turned off. As soon as I turn the key, it goes down to 10.3v. Unplugging the headlight only makes it go up a volt. The bike runs great if I jumpstart it with my charger.

Any ideas? Is this normal?

[Boondocks]

Make sure that the bike's charging system is working properly and buy a new battery.

[ToddS]

The charging system is working. Would that make a difference even though I try to start the bike right after a full charge?

The battery I'm using holds a charge for weeks or longer. What's the best way to make sure it's good?

Thanks

[brveagle]

Sounds like you either have a short (unlikely) or your battery is toast. It may just be low on water. Either way, it points to the battery

[duaneage]

The battery MIGHT have a weak cell. Test the specific gravity with a tester from a auto parts store (they are cheap) and see. The clymers has a specific gravity table for batteries, that should tell you.

With out the engine running the battery will go down a lot with the key on. You are powering lights and coils, that draws about 5-6 amps. Starting takes 10 amps. The charging system puts out around 250 watts, around 15- 20 amps depending on engine speed.

[ToddS]

I picked up a new battery and now it starts up and runs better than ever. I read in another thread that a dead battery can read 13v just when there's no load on it. Funky.

Thanks for your help GSR!

[nabrams]

I wouldn't declare victory just yet if I were you.
I'd check what my charging voltage is when the bike is at 5000 RPM.
If it's less than 13 volts then your battery will slowly discharge over several rides.
If it's more than 15 volts then you'll boil your battery dry over several long rides.
These bikes will allow up to 17.x before blowing the fuse.

[ToddS]

Good point. I'll test it tonight and see if it's in specs.

[ToddS]

Just checked it and it's 13.7v idle and 14v at 5000. Halejaluyah or however you spell it.

[jm_foote]

Just checked it and it's 13.7v idle and 14v at 5000. Halejaluyah or however you spell it.

Ohhh, 10.3 volts NOT RUNNING. Yeah, it sounds like a defunk battery.