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Old 08-28-2022, 06:42 PM   #1
Denny B
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Default Strange Alternator Problem

I posted this on another thread and received little response (I figured it got lost in the thread), so I thought I would post it under a new title...
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Okay…this has got me stymied. I installed a “Mini” alternator in my buggy (35 Amp). When I installed it I really don’t recall my sequence of things but I must have started the engine and then connected a jumper cable (small one) between the alternator output terminal and the positive side of the battery and was reading 14.7 volts at the battery. If you recall, I asked about that in an earlier post.

Fast forward today: I have not run the engine since I originally installed the alternator until today. Today I hard wired the alternator to the battery (alternator output terminal to battery positive terminal) and a really strange thing happened.

When I hit the starter I could tell it was cranking a little slow like there was a drag on the engine (new battery, fully charged) and it took longer than usual to start. When it started it was idling much lower than normal. When I opened the throttle the engine stalled and quit. I tried this a few more times with identical results. I then disconnected the alternator wire from the battery and turned the key and the engine fired right up, ran normal and accelerated just fine when I opened the throttle.

Okay…here comes the crazy part…after starting the engine with the wire disconnected I could re-attach the alternator wire to the battery and everything worked normal. Engine runs and sounds normal and the battery voltage reads 14.7 volts. I turned off the engine, tried to start it again and it cranked slow, idled slow and quit when I opened the throttle. I disconnected the wire from the battery, started the engine, it started right up and ran normal. I went back and forth and few times with this (wire connected & disconnected) with the same results. I could not start it with normal results with the wire connected, but, with the wire disconnected it would start right up, run normal and when I re-attached the wire to the battery I would get charging voltage of 14.7v and totally normal sound and power from the engine.

I have a Predator 212 engine with a V-Power starter kit wired to an ignition switch. I have the alternator grounded to the buggy frame and a #10 wire running from the alternator output terminal to the battery positive terminal.

Can any of you guys solve this puzzle for me?

Maybe a relay that cuts out the alternator while starting the engine?

I could always put a switch on the alternator wire and turn it off upon starting the engine and turn it on after start-up….but I’m sure there must be a better way.
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Old 09-06-2022, 01:09 PM   #2
bronsonj
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Default Re: Strange Alternator Problem

My guess is the engine is trying to start which lowers the battery voltage. The alternator is trying to charge the battery but it is spinning slowly so it's having difficulty and putting substantial drag on the whole system. Then when it does finally start the alternator has some work to do to get the battery back to charged so it labors a bit. If you tickle the throttle cable with the engine under that load it stalls.

The way to fix that would be to have the alternator not charging while the starter is spinning. Then once it's started give it a few moments to charge the battery before hopping on the throttle.

It's a 6hp engine, with one hp being 745 watts the engine output is around 4400 watts at peak and at idle it will be much, much less. 35 amps at 12v is 420 watts, about 1/10th of what the engine can produce at peak power... I think 35 amps of charge may be all it's able to do, especially at idle.

Maybe there's an easy way to wire it so either the starter is engaged or the alternator is charging. Maybe a dual throw relay so when you turn the key to power the starter it disconnects the alternator? Then when the relay is de-energized the starter is disengaged and the alternator is engaged.
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Old 09-06-2022, 01:37 PM   #3
Denny B
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Default Re: Strange Alternator Problem

Quote:
Originally Posted by bronsonj View Post
My guess is the engine is trying to start which lowers the battery voltage. The alternator is trying to charge the battery but it is spinning slowly so it's having difficulty and putting substantial drag on the whole system. Then when it does finally start the alternator has some work to do to get the battery back to charged so it labors a bit. If you tickle the throttle cable with the engine under that load it stalls.

The way to fix that would be to have the alternator not charging while the starter is spinning. Then once it's started give it a few moments to charge the battery before hopping on the throttle.

It's a 6hp engine, with one hp being 745 watts the engine output is around 4400 watts at peak and at idle it will be much, much less. 35 amps at 12v is 420 watts, about 1/10th of what the engine can produce at peak power... I think 35 amps of charge may be all it's able to do, especially at idle.

Maybe there's an easy way to wire it so either the starter is engaged or the alternator is charging. Maybe a dual throw relay so when you turn the key to power the starter it disconnects the alternator? Then when the relay is de-energized the starter is disengaged and the alternator is engaged.
Excellent reply!

I'm working on a solution like what you suggest right now. I will post my final results when I get this sorted out.

Thanks for your input.
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Old 09-12-2022, 08:26 AM   #4
bronsonj
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Default Re: Strange Alternator Problem

Another way might be to change the pulley that spins the alternator. If you change the pulley so the alt spins slower (either a larger pulley on the alt or a smaller pulley on the crank) then it will be less load when trying to start. It will be less load across the entire RPM range. The tradeoff is the alternator will be less able to produce peak charge current.

Normally on a car the crank pulley is about 7 or 8 inches in diameter with the alt pulley about 2.5 inches. That's good for charging with a substantial engine but for more power people use an 'underdrive' pulley kit to reduce the load on the engine and create more rear-wheel power.

Plus, using different pulleys would be a very simple solution.
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Old 09-12-2022, 08:38 PM   #5
Denny B
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Default Re: Strange Alternator Problem

Quote:
Originally Posted by bronsonj View Post
Another way might be to change the pulley that spins the alternator. If you change the pulley so the alt spins slower (either a larger pulley on the alt or a smaller pulley on the crank) then it will be less load when trying to start. It will be less load across the entire RPM range. The tradeoff is the alternator will be less able to produce peak charge current.

Normally on a car the crank pulley is about 7 or 8 inches in diameter with the alt pulley about 2.5 inches. That's good for charging with a substantial engine but for more power people use an 'underdrive' pulley kit to reduce the load on the engine and create more rear-wheel power.

Plus, using different pulleys would be a very simple solution.
Good suggestion.

I've already looked into that and will try that in the future. I've already determined that the alternator turns 2 1/2 revs to every 1 rev of the engine. At 1000 engine RPM's the alternator is turning 2500 RPMs.

I did change the regulator and now the output voltage is 14.2v (instead of 14.7v) and there is noticeably less load on the engine...but, it still lugs down too much.

The drive pulley cannot be changed because it's part of the CVT clutch unit. I am in the process of searching for a larger alternator pulley. The drive pulley is approximately 7" and the alternator pulley is 3". I'll try something like a 4 1/2" to 5" pulley on the alternator and post the results.

Thanks.
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