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Old 07-06-2010, 11:45   #1
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RPM for Effective Charging

Hi:

I have a Volvo D1-30, and with my generator out of commission, use it to charge my batteries. I note that it sends more amps to the batteries as I increase the RPMs in idle.

Is there an optimal RPM? From an engine point of view, am I better off at 1500 RPM or 750-800 idling RPM?

Any advice on the tradeoffs?

Thanks!

/jon
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Old 07-06-2010, 15:07   #2
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It varies depending on the current charge level in the batteries. if the batteries can accept it and the regulator allows it then faster engine rpms equlas more amps. But if the batteries are fairly charged then running the engine faster isn't getting more amps to them. When I do it I adjust the enginespeed and monitor the amps going into the batteries and speed up/slow down so the amps stay the same. Running the engine at 1500 rpm to get the same amps as 1000 amps is just a waste of fuel.
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Old 07-06-2010, 20:26   #3
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When a magnetic field passes a wire a voltage is induced. To get a higher voltage you can pass a higher magnetic field or pass the same magnetic field past the wire faster.

In an alternator you are limited in the voltage you can handle by the physical characteristics of the wire in the alternator. With a low battery charge state, at idle you have maximum magnetic field to demand but are not passing the filed fast enough hence the need for more RPM.

The alternator is designed/sized for a particular average RPM. That RPM is above idle in most applications. Once your reach 1200 or so RPM the alternator should be regulating at max capacity so any further speed is not necessary.

Short answer - Yes - in most cases idle RPM will not give you optimum charging rates. 1200-1400 will likely be the right zone.
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Old 07-06-2010, 20:46   #4
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On our Privilege 39 catamaran, we had a three stage smart regulator and a proper sized pulley for charging the high output alternators. We did not see any benefit from running higher RPMs during battery charging. The Balmar 3 stage regulator only had maximum power output for about fifteen minutes, and running the engine faster wouldn't have made any difference. It was the acceptance rate of our batteries that limited how much bulk charging could be done.

One of the reasons we had such a short period of bulk charging was because we had wind generators running in the trade winds, so we rarely ran our batteries down to even the fifty percent level. If we had done deeper discharges, then probably the bulk charging would have lasted longer.

We set our charging RPM by looking at the amp meter. We pushed up the throttle until our amperage output no longer increased. Usually, we charged at 1200 RPM or less.
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Old 08-06-2010, 00:47   #5
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Quote:
Originally Posted by maxingout View Post
On our Privilege 39 catamaran, we had a three stage smart regulator and a proper sized pulley for charging the high output alternators. We did not see any benefit from running higher RPMs during battery charging. The Balmar 3 stage regulator only had maximum power output for about fifteen minutes, and running the engine faster wouldn't have made any difference. It was the acceptance rate of our batteries that limited how much bulk charging could be done.

One of the reasons we had such a short period of bulk charging was because we had wind generators running in the trade winds, so we rarely ran our batteries down to even the fifty percent level. If we had done deeper discharges, then probably the bulk charging would have lasted longer.

We set our charging RPM by looking at the amp meter. We pushed up the throttle until our amperage output no longer increased. Usually, we charged at 1200 RPM or less.
I have an Ample SAR running our 110 amp alternator, and I use the volt meter in the cockpit to give me an indicator of when I'm charging...seems like about 1000 rpms is when it starts kicking in for us.
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Old 08-06-2010, 02:31   #6
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you should be able to get output figures for your alternator at various revs(alternator) and may find a smaller pulley on Alternator will give a boost in amps at low engine revs
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Old 09-06-2010, 18:08   #7
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Jon-
Some manufacturers (like the Delcos) publish alternator specs on the web or will give them to you if you ask. Others (like Denso) won't tell you the time of day.
But every alternator has output curves, will produce xx amps at idle rpm and xx more at higher rpm. Below 1000-2000 shaft rpm (on the alternator shaft) most produce almost nothing, and full output may take 4-5000 shaft rpm. Somewhere between 6-18,000 shaft rpm and the alternator will literally burn out, so "more" is not "better".

If you can get the numbers for your alternator, an ideally sized pulley for the alternator would make it turn quickly at low engine speeds (idle to cruising) in order to get optimum charging, but the pulley must be sized so that at "battle speed" you still won't turn the alternator fast enough to burn it out. If you can't obtain a stock pully (from an alternator shop) that is the right size, expect a machine shop to charge you $100-200 to make one up, depending on whether you want lightening holes, anodizing, etc. in it. If you don't anodize it, or pick an alloy the machine shop usually won't have on hand, salt air can leave lots of white powder on aluminum, real fast.

Done this, and can tell you that simply changing a pulley CAN cut your charging time in half.
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Old 09-06-2010, 19:30   #8
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Most sailboats have a 2.5:1 ratio between the engine rpm and the alternator. If the engine is running at 1,000 rpm then the alternator will be running at 2,500.

If you take a look at the Balmar alternator curves which may not be applicable to your Volvo, you will see that at 2,500 alternator rpm you are producing about 50 amps.

Other alternators produce less. I know that a stock Hitachi 80 amp alternator on a Yanmar will produce maybe 25 amps at 1000 rpm.

All of the above assumes that you have depleted batteries and a regulator that will put out maximum amps into the load. In a stock alternator that is never the case.

David
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Old 09-06-2010, 19:53   #9
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Jon...

Good advice above.

The D1-30 comes standard with a 115A alternator and a built-in "charging sensor". Read "internal regulator". I assume that's what you have.

From a practical perspective, you wanna charge the batteries as best you can during the engine run-time. Hopefully, your solar panel system is working; sorry to hear about the genset.

One thing you can do when charging at anchor is to run the RPM up while you watch the AMMETER, not the voltmeter. When you get maximum amps that's the RPM for you.....at least for awhile. Watch the amperage. After awhile -- because the batteries get charged up and will accept less amperage and because the alternator heats up and won't produce as much amperage -- you'll see the amperage drop, even though you have the same RPM. Just adjust the RPM downwards until you begin to see the amps drop a bit, then leave the throttle there for awhile.

So, you'll be cutting the RPM back in steps, maintaining just enough RPM to give you the most amperage output to your batteries for the conditions encountered.

Hope you guys are having a heck of a time. Wishing you full sails and dry bilges,

Bill
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Old 10-06-2010, 14:07   #10
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Quote:
Originally Posted by Ex-Calif View Post
In an alternator you are limited in the voltage you can handle by the physical characteristics of the wire in the alternator. With a low battery charge state, at idle you have maximum magnetic field to demand but are not passing the filed fast enough hence the need for more RPM.
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The current (Amps) that the alternator can supply is limited by the carrying current capacity of the main wiring of the alternator. The voltage produced by the alternator is related to the excitation of its field by the DC supply of a voltage regulator.
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