Battery Not Charging

[RazeOrion]

So, I recently got my 1980 XS400SG running after replacing my spark plug cables and all my fuses. My new problem is that she isn't charging. Battery drained from 12v down to 7v after idling for a little under a minute.

Battery is new-ish and holds charge. I need some advice on how to test my stator if the bike won't run longer than 30 seconds.

 

[tstidham]

I'd say the battery has a bad cell if it's dropping to 7 volts in 30 seconds. When I first got my 78, I didn't realize the voltage regulator was bad. I could ride a mile or two before it started sputtering. And with the headlight turned off (the older bikes had switches), I could go several miles. I would probably start with a new battery then check to see what voltage you see at the battery when running.

 

[RazeOrion]

I'd say the battery has a bad cell if it's dropping to 7 volts in 30 seconds. When I first got my 78, I didn't realize the voltage regulator was bad. I could ride a mile or two before it started sputtering. And with the headlight turned off (the older bikes had switches), I could go several miles. I would probably start with a new battery then check to see what voltage you see at the battery when running.

Well the battery holds a charge fine.

Wouldn't the battery at least maintain a charge if the charging system was working, even if the battery had a bad cell?

 

[tstidham]

Not necessarily, I have seen batteries that failed in such a way as to pull so much charge voltage that it overwhelms what the alternator or regulation can provide. Our alternator is only like 230 watts or something so it doesn't really charge at idle anyway. Plus you usually need to blip the throttle to get the field voltage up for initial "excitement" to Start charging

 

[RazeOrion]

Not necessarily, I have seen batteries that failed in such a way as to pull so much charge voltage that it overwhelms what the alternator or regulation can provide. Our alternator is only like 230 watts or something so it doesn't really charge at idle anyway. Plus you usually need to blip the throttle to get the field voltage up for initial "excitement" to Start charging

**UPDATE**

I can start the bike with the battery hooked up, and she stays running with the battery disconnected. Tested the battery itself, and it does seem to be bad.

Rated for 85 CCA, only getting 48
Battery health on tester say 20%, so the battery is definitely having issues. Thankfully, my local CycleGear has my battery in stock for $49.99

**My new problem is she now dies if I give her any amount of throttle**