Alternator robbing HP, what's the right idle? - SailNet Community
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post #1 of 44 Old 2 Weeks Ago Thread Starter
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Alternator robbing HP, what's the right idle?

Hi all,
I Have a 120 amp alternator and balmar ARS-5 regulator on an old volvo 36hp engine.

When the alt starts (and my batts are low), my engine is just barely able to keep up. It will rock around on the mounts. I end up taking it out of gear and throttling up when trying to maneuver around waiting for bridges, or raising the anchor, etc.

If I know I'll be motoring far, I'll end up just turning the key [switch] (depowering regulator) and the alt spins free and no issues maneuvering.

I've adjusted the regulator's belt load to 20% less.. so I only see 100amps.
I've also increased my idle speed a bit on the diesel.
I'm considering raising the idle speed higher, but concerned that if I do that I'll be idling too high when my batts are full. Therefore flying through a marina unable to slow without taking it out of gear periodically.

Lmk your thoughts.
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Re: Alternator robbing HP, what's the right idle?

You may need to add a cut out switch for your alternator and use it at low RPMS need for maneuvering into a slip. When you are motoring or say anchored and want to pump amps into the batts have the switch close and the amp working.
150 amps may be too much load for your engine.

I have the same engine and a 120 amp alt but it rarely puts out anything over 40 amps... basically because the batt capacity is quite large and they are usually quite charged. I don't have a 12v refer and when the engine is cooling the electric draw is quite low.. only enough to engage the compressor's clutch. The heavy compressor work is done by the fan belt and THAT can be difficult for the engine at idle speed.

You really are not using the engine to push at very slow speeds... just for docking, mooring etc. You can do without the alternator running. My electric windlass CAN draw a lot of amps. But again if you use it to get the boat making way, then the windlass is not heavily loaded and will not drawer as many amps. Letting chain run is little strain or draw on the batts. But you have a manual windlass if I recall correctly.

You really don't want to let your batteries go down and want a big mouthful of amps. Your alt sources should keep the batts close to full and you should have as large capacity bank as possible... so ANY draw is a small percentage of the capacity.

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Re: Alternator robbing HP, what's the right idle?

We had a 120A alternator on a Volvo 29hp, derated to 100A with a Balmar regulator. There was no noticeable loss of power when it was charging - certainly not to the degree you have described. I don't know the answer to your issue, but don't think it is solely the 100A alternator itself if it is working correctly. Have you taken the belt off and made sure the alternator bearings are free? Maybe shoot it with an IR gun and see if there is a temperature anomaly?

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Re: Alternator robbing HP, what's the right idle?

I also think something else may be up.
Recheck belt tension...unstated type..
What mark posted
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Re: Alternator robbing HP, what's the right idle?

My engine will struggle at idle when I turn the refer "on" and the clutch is pulled in and the compressor becomes a sizable load. Therefore I don't use the refer when the engine is at idle speeds and low RPMs. An alternator could do the same thing... it should do the same thing if it is putting out high amps.

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Re: Alternator robbing HP, what's the right idle?

If I recall correctly, SO has an engine driven cold compressor. A refer compressor is a ton of drag on an engine. I think there is something else wrong here. Either the alternator is bound up or the engine is starving for fuel or air, when under load.

With the Balmar regulator, the bank is not going to cook at high idle. Depending on how big the bank is and whether it's an AGM, I'm surprised it's ever trying to make the full 120amps, let alone 100amps.

Still, I think there is something more fundamental going on here.

Just curious, does this setup have a dual v-belt or a wide single flat serpentine?
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Re: Alternator robbing HP, what's the right idle?

Quote:
Originally Posted by Minnewaska View Post
If I recall correctly, SO has an engine driven cold compressor. A refer compressor is a ton of drag on an engine. I think there is something else wrong here. Either the alternator is bound up or the engine is starving for fuel or air, when under load.

With the Balmar regulator, the bank is not going to cook at high idle. Depending on how big the bank is and whether it's an AGM, I'm surprised it's ever trying to make the full 120amps, let alone 100amps.

Still, I think there is something more fundamental going on here.

Just curious, does this setup have a dual v-belt or a wide single flat serpentine?
You are correct. I have the same engine and a 2 - v belt driven compressor on a separate "power take off" dual pulley. I believe Scott has a similar arrangement less the PTO , and compressor.

The engine as a single V belt which drives the cooling circ pump (upper left) and the alternator (upper right), I have a beefed up adjusting bracket for the alternator. My alt is 120 amp w/ a Balmar 614 regulator. I also have a house bank of 500 ah (AGM) and a separate start batt, Alt charges house bank, House bank charges the start bank w/echo charge. 110 watts of solar keep house bank topped up.
I never ever see more than 40amps from the alternator and it ramps down pretty quickly to float.. and is little load on the engine.

No load idle should be 800 rpms
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Re: Alternator robbing HP, what's the right idle?

Man, that is a massive flywheel! I'm surprised anything bogs that down.

Mark

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Well Jeffrey's certainly looks prettier!

Dont think there is an issue TBH. The belt tension is good, and the alt spins freely with the belt off. I would expect 100 amps to put a significant load on the motor. I also adjusted the regulator to have a minute delay before applying the load. Once that minute is up however, it's a matter of seconds before it's doing 100amps, that's when the motor struggles. After the initial bulk charging, it moves to absorption and it idles smoother doing aprox 60amps, and so on for float charge doing minimal amps.

Like I mentioned earlier, I just turn the key off and the alt stops the load. I guess I'll just keep doing that when I need to instead of adjusting idle.

We have 450 amp hour bank. If were staying put for a couple days, we get down to around 65-70% battery level pretty easily, especially with little sun or wind. We have 12v refer, and use the inverter a good bit for cooking, ice making etc.

My initial thought was to raise the idle speed to accommodate the load, but worried that once the batts are topped off, I'd idle around 1200rpms, and idle forward would be faster than desired.
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