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Old 26-04-2011, 11:37   #1
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Best Diesel Charging RPM

What is the recomended charging RPM for a Yanmar 4JH3E under no load other than that of the alternator. I have heard numerous conflicting opinions. TIA
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Old 26-04-2011, 12:28   #2
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Re: Best Diesel Charging RPM

The best rpm for charging out of gear is zero.

Seriously, your post begs a lot of questions: type of alternator, regulator, size of battery bank, how long you plan to do this.

Lightly loading a diesel isn't particularly good for it. But I've done it as I am sure thousands of cruisers have done it. Done moderately, say 10-20% of total hours done charging, with lots of motoring between charging is probably ok. But the opposite, lots of charging with little loaded motoring is not.

Also look at your alternators output curve. Balmars typically plateau at about 3,000 alternator rpm. Most Yanmars have a 2 or 2.5 : 1 ratio, so that means 1,200 to 1,500 engine rpm. Maybe that is the answer you really wanted.

David
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Old 26-04-2011, 12:52   #3
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Re: Best Diesel Charging RPM

From the "production of amps" point of view... 1,200 RPM, out of gear, would work fine. From the point of view of less carboning up the engine, higher RPM and if at anchor, running in reverse as well, would be best.

We are solar powered 95% of the time, (At anchor.) The other 5% of the time, When doing overnighters, we avoid an energy deficit by motorsailing for 45 minutes or so, at about 6:00 AM, (when batteries are lowest, and solar hasn't yet kicked in).

By motorsailing at 2,600 RPM, we get our amps, a slight speed boost, and the engine is happy too.

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Old 26-04-2011, 13:04   #4
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Re: Best Diesel Charging RPM

We have a 3JH4E and Yanmar recommends (in the owners manual at least) racing the engine every two hours of light loading. Up to max RPM and back down 5 times.
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Old 26-04-2011, 13:08   #5
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Re: Best Diesel Charging RPM

off topic but spoke to a farmer that used small yanmars to run refrigeration compressors under light loads .. got way more than 20000 hours out of them.
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Old 08-06-2011, 08:32   #6
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Re: Best Diesel Charging RPM

Is there anymore that could be said on the topic of charging batteries with a diesel? Im kind of looking for a "Best Practices" of charging batteries on a boat with its diesel. And since like most things the practices vary from person to person and hardware to hardware, is there a place/thread/forum to gather some cruisers procedures or rituals when it comes time to charges the bank(s)?
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Old 08-06-2011, 08:58   #7
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Re: Best Diesel Charging RPM

Your best option to prevent carboning up over time, is to run in gear (reverse) to put a load on the engine, and @ 2,500 RPM or above. (According to Yanmar) Since this puts a HELL of a load on your ground tackle, and would be loud, it lacks practicality.

I would come as close to this as practicality allows, and suits your situation, or charge at just above idle, and deal with a carboned up engine some day...

M.
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Old 08-06-2011, 09:35   #8
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Re: Best Diesel Charging RPM

There is no perfect solution. Diesels that have been run at near full load for most of their lives will last longest. The best thing to do is to minimize how much time you spend recharging your batteries with no load on the engine. Running your engine fast with no load is still running your engine with no load and is not good for your engine.
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Old 08-06-2011, 18:09   #9
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Its all bad...

I put a voltmeter across my batteries when I last did some serious charging with the engine. I was getting a good charge at quite low rpm's with my John Deere 4045D. In the range 1200 to 1400 from memory - notes are on the boat and its cold and windy here.

Problem is - if the batteries need charging then the crew ain't gonna be happy unless they are charged. Count the cost of flat batteries and an unhappy crew against the tiny amount of damage caused by running the engine unloaded. I wouldn't do it for long with a brand new engine though.

Its really glazing that we're worried about, right? So giving the engine a nice long run at operating temperature and solid load every so often should take care of most of the damage?
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Old 09-06-2011, 07:51   #10
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Re: Best Diesel Charging RPM

thanks all!
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