Rotax killing batteries in half a session

I have an older Rotax 125 (~2010) which has killed 3 batteries in 10 ~30 mins each. The batteries were new, the kart started just fine. Then at some point during the session the kart starts missing and 30 seconds later it’s dead, won’t crank, putting a voltmeter on the battery, it’s at ~10V and drops to zero as soon as the starter button is pushed. Can’t be recharged, won’t go past 10V.
The batteries were cold, didn’t seem like they delivered a bunch of amps.
Out of the 3 batteries, 2 were of different brands.

Any idea what could cause this? I thought the first one could be a defect and internal shorting of cells due to vibrations maybe.
Recently when I had trouble with the engine I noticed a spark between the engine and chassis. Could a high tension issue kill a battery? My ground wire checked OK.

Is there a way to test the ignition system on a Rotax?

Start with a new battery from a large kart parts distributer that’s rated at 9 Ah and charge it as recommended. Also be sure you have the correct charger for the battery. The 7.2 Ah batteries are good for emergency exit lights.
Where do you see the arcing because that’s not normal? Guessing that you have a leak at the spark plug boot or along the high-tension lead from the coil. You could try lightly spraying water on the engine or wrapping the boot and coil wire with tape as a trouble shooting technique.
Where do you have the ground wire attached to the engine?

What’s the voltage at the terminals when the engine is running? I believe the Rotax should be recharging battery as it runs, so I would expect to see a voltage north of 12v.

Tony,I saw arcing one evening, and it was in the starter area, between the engine and the frame. Only one arc on the last stroke when the engine died.

The ground wire is attached to the engine on the coil mounts, so on the end of the gear cover.
So, from the back of the gear cover, to the coil. There are 2 other grounds from the harness connected to the other coil bolt, also on the backside of the gear cover.

Both batteries I tried were 9Ah.

James, I haven’t checked the voltage when the engine is running. I’ll have to buy a new battery to try.
My understanding was that Rotax engines run completely on the battery, they don’t recharge It nor even provide their own electricity for the ignition.

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The Rotax 125 i had did not charging capabilities with engine running. You will want to confirm your charger is working well and has plenty of time plugged in to charge. Maybe your positive cable is grouding on the frame? A shirt to ground will disipate the battery for sure.

For sure there’s something amiss, last post is correct in that Rotax doesn’t charge the battery whilst running.

Something is not only dissipating the battery but frying it also if it then can’t be charged after, shot in the dark but I’d try a different coil.

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Yes, that’s my plan, once I get my hands on another coil. I don’t understand how the battery would lose all capacity in a few minutes without dissipating any heat. I think even these small batteries would melt the harness if there was a short, and the battery would probably get hot. Maybe high tension can fry it like that.
I have a clamp-style DC ampmeter so I’ll try to measure how much current is flowing out of the battery while running.
Another thing I could do maybe is bench-test the ignition system, if I can find out what the crank signal looks like.

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Ah, I forgot the Rotax is total loss. I’ve been playing with pit bike motors too much lately I think.

I do wonder if you’ve got a stator issue, perhaps shorted internally.

Curious to see the Amp draw you find.

I just did a test on the stand, with the kart jumpstarted from a car battery.
Amp draw while cranking was ~30A.
Then idling at ~2K rpm, it was about 0.25A. Revving it to 6~7K was ~0.78A.
So it seems to draw roughly a quarter amp per 2000 rpm, and at 12.000 rpm it should be 1.5A, at least with no load.
A brand new fully charged 9Ah battery should be able to deliver that for a long time.

Everything seemed normal.
Maybe something’s happening at high RPM.

I’d check the wires and starter switch. Wiggle and vibrate them all to check for intermittent contact. Most likely the starter is getting spun up while running, draining the battery.


I believe the batteries have been killed from vibrations. First, from the 3 batteries I used, 2 of them were probably not meant for karting and more for UPS or small electric cars for kids.
I realized my engine had more vibrations than normal (after driving a friend’s Rotax). The 3rd battery died during a small off-track excursion (probably vibrations too).

I’ve been running a Nermak LiPo battery now for some time now, and no issues.

I don’t know how the cheap batteries are made but I guess there’s something connecting all the internal elements together, and that whatever it is, it broke due to vibrations.

I’m going to switch my bottom end as I have a spare and will rebuild the other one and hope to end up with a smoother-running engine.
I’m not sure what caused the vibrations, the balance shaft looks to be timed properly.


Why not using the original rotax one. I am driving this one for years without any issues