Battery charging at high current!


XS400 Member
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Cambridge ON
Hey all, first post here.
Have an 81 xs400 heritage special that I have slowly been getting ready for the road. Over the course of tuning the carbs I burnt out my first battery that I had bought for it. Didn't worry too much as the voltage never quite got to 14.5 V and it was a cheap battery besides. Anyways on after going through all the electrical tests described in the Haynes manual I thought to check the charging current by running a meter in series with the battery. Here were the readings:

Key on not running: -5 amps (do not currently have a headlight or turn signals hooked up).
Idle:+/-2 amps
Above 2000 rpm: 6-7 amps

I uploaded this video to youtube as it is not exactly as described:

Wow! No wonder I fried that first battery. I have actually been jumping the bike with a car battery (bike battery removed) as I have not wanted to hook up my nice new Yuasa battery until I could verify that my charging system was ok. I did however replicate this test with the same results on the smaller battery just to be sure the battery some how wasn't the issue.

Few more details on the results on my electrical tests:

Field winding: 3.4 ohms (4 ohm spec)
Alternator 3 white wires @ first connector: 0.8 ohms each (0.72 ohm spec)
Alternator 3 white wires @ rectifier connection: 1.1 ohms each

I decided against rewiring the connections from the alternator as I thought the 0.8 ohms was close enough to spec, they looked to be in ok shape, and I didn't think new wires would increase the field winding rating to the correct 4 ohms. Obviously this is not ideal though.

Rectifier diodes are all good.

Bench tested the regulator and verified that it was cutting power at 14.5 V. Given that I have yet to see my bike put out more than 14.3V I think it is likely not doing anything.

I see a lot of discussion on the voltage that the bike is charging at, but not much on the current. Does anyone know how the current is supposed to be regulated? 7 amps is way too high for these little batteries, and it didn't take too much run time to get my first battery to the point of not even being able to take a charge on the trickle charger. Any input is appreciated.

While we are discussing electrical, the ignition coils primary resistance comes in at 3.1 and 3.3 ohms. This is out of the 4 ohms spec, but given that I have ok spark should I be concerned?

tl;dr battery charging on bike at too high of current, how can this be regulated?

Thanks everyone,

Thats so obvious I can't believe that didn't occur to me :doh: - especially as I saw the brake light drawing a good 5 amps. Doesn't exactly explain why the first battery died as I had a headlight at that point but oh well.
I'll report back tonight.
Well I got a bit delayed by the development of horrible pops from the exhaust. Turns out my one spark plug died.

With the headlight I am still drawing a steady 5 amps. Is it possible my headlight is too efficient/modern? I am using a 9003 bulb (55W).

Would my alternator put out less power under load (ie actually driving around?). I have just been revving the thing in neutral.
That 9003 should draw something like 4 amps. Make sure all the battery cable connections are clean and tight, especially the ground wire on the engine. I know on mine, I went through every harness plug and did a few disconnect and reconnects to ensure no internal connector corrosion was causing voltage drop. Any that I could wire brush or scotchbrite clean, I did. Including all the headlight bucket connections. Maybe you have some excess voltage drop causing the regulator to think the battery is lower than it really is.
Good idea. Looks like someone has done some rewiring prior to me owning this thing. No physical fuse box (just inline fuses). I'll go through and clean up all the important looking connections, make sure everything is snug.
Also, did they purposely design that battery tray to be hella annoying to take out? Made a few cuts on it and now it comes in and out no problem.
Hey that was good advice about cleaning up some of the electrical connectors, now I am getting too much power! Reading upwards of 15V across the battery. Current now coming in between 3-4 amps on the throttle. I guess my regulator isn't so great after all. Seems to occasionally cut in as I will see it flash to reasonable voltage and current levels for an instant.
Does anyone know off hand if for the 81 model I should be looking at using a R296 or a R292 regulator? Some of the other threads have me mixed up.

Edit: its the 296
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Would anyone be willing to measure the current they get on their own bike? I replaced the regulator which has gotten the voltage to behave, but I am still seeing 4-5 amps go into the battery. This would mean putting your meter in series with the battery. If you have a smaller meter with a 10 amp fuse I would suggest only using the kickstart as the starter may surpass that. Make sure your connections are as secure as you can get them as you are transmitting a good bit of power through tiny probes.

I would like to look into replacing the field coil or stator at this point based on the slightly out of spec values I have, but these things are hard to find, and I don't want to take anything apart until I can get a comparison with a working bike.

I went out and measured mine, keep in mind I have the dodge voltage regulator, a transpo c8313 adjustable one set to 14.2v (it's default setting). I kicked it over, let it warm up some, then took this picture at 2500 rpm, headlight on. Showing 2.3 amps. At idle, it goes into the negative by up to .5 amps


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Hey man thanks, you've been a huge help here. Those numbers look much more reasonable. I called a local motorcycle shop and they thought it sounded like an issue with the stator, so thats what I'll try and replace next.
I would check your ac voltages coming out of the alternator as well (the three white wires to the rectifier). The service manual has the voltage specs, but it seems to me yours is putting out plenty of power and maybe it's a rectifier or regulation issue?