Battery Charging at Idle

[Warren Kaplan]

I went aboard Sine Qua Non for the first time since she was launched, on Sunday. Just the usual putting things in order chores and nothing else.

In the course of all this I fired up the engine to see if all was well. I let her run for about 20 minutes. I noticed that it seemed to charge the batteries quite well when I checked them after I shut the engine off.

The engine was never out of neutral. I believe I read somewhere in the past that charging an engine with the engine in neutral doesn't work very well. I guess all I'm asking is if that is an old wives tale?
I know it may not be good for the engine, but that's not what I'm asking. Any thoughts?

Thanks

[Brian2]

Were your batteries fully charged before launch? If so they should still be well charged if you haven't drained them at all. Also, you have to wait several hours after charging to get an accurate reading as there is "residual" charge left over from the charging source. If your batteries were low, your engine should charge them somewhat at idle. I can hear my engine lug down and then back up if I have just started the engine, and then the alt kicks in, if the batteries were low after a night of using power. I get at least 13.2 volts at idle. I was told by the alt guy that rebuilt my alt that the alt will put out voltage, or amps, but not both at once.

[Gary M]

Warren,
I'd be interested in any other posts, but the way I've always looked at it is the alternator is spinning when your in neutral and that's what counts.

I've never put a meter on it, but its logical that the alternator is putting out a little more at higher RPM's but you do have a voltage regulator that keeps that in check.

I've heard of plenty of cruisers who say they run their engines at idle everyday to charge their batteries, and in a recent overnight race you were allowed to run your engine at idle for one hour to recharge your batteries.

Here's one that I've heard. "The more electricity you are using while charging the batteries, the faster they will charge." In other words, when your running your engine to charge your batteries, turn on all lights, radio, stereo and so forth. No, I don't know if its true, does anyone else?

Gary

[Brian2]

If you turn on a lot of stuff to increase amp output, the alternator will increase the amp output to compenstate. However, the increased amps will go to the increased power needs, not to charging the batteries. When charging the batteries, you are replacing amps that have been consumed. If your batteries are not low, amp output is small. If they are low, amp output is greater, and as they recharge, the amp output decreases.

[Chris Reinke]

Warren - I can not claim to be an expert and would certainly welcome any other opinions.....but I can offer what my Alt guy told me. He advised that some internal voltage regulators have a hard time determining what is needed when the batteries are allowed to run very low. He said that at idle speed the regulator may not realize the charging need and not energize the alternator to provide power. This would be noticed at your amp meter, or by a tachometer that does not respond if your tach sender is alternator based. A slight increase in RPM's should provide sufficient amperage for the regulator to note the difference and determine any charging needs. It only takes a few seconds at the slightly higher RPM's for the regulator to note the requirements. Once the alternator is energized and the regulator if functioning properly you can reduce the RPM's back down to idle without any change.

[Steve D]

From my undestanding, an alternator will not be outputing many amps when the engine is just in idle. So then you must run up the RPM's. Of course that is not good for an engine to run at a higher RPM without a load on her, so many people put the tranny in gear to develop a load and run at higher RPM's

[Bill Cochrane]

I noticed that it seemed to charge the batteries quite well when I checked them after I shut the engine off.

Warren, how did you check the state of charge? If it was voltage, you got a false reading...batteries will read high for several hours after the charging current is removed. (they will also read low if they've recently been loaded.) I believe that specific gravity is not as sensitive, and of course a meter like the Link 10 that measures AH in and out will get you pretty close (it doesn't track self-discharge so if you've let the battery go without a charge for a month or so it'll be down and the Link won't know that).

I think Steve D is on the mark, it's not charging the battery in neutral that's an issue, it's that the diesel much prefers to be run under load and at a good rpm (I've heard 1800-2000 recommended for my 4.108, that's about 60 percent of redline for extended running.)

[Troy Scott]

Folks,

I'm imagining a practical product that would allow sailors to run the alternator at higher RPM with the engine at idle, but return the gearing to normal for running. I guess it would be a two-pulley alternator, or something like bicycle gears. Surely there's a simple, practical way. Has this been done?

[Russell]

At just idle speed altinators do not put out much at all. I run my engine to charge my batteries all the time since I am almost always anchored out. Typically you need to get the engine up above 1000 rpms to get any decent output.

[Boyd]

Hi all:

Alternator ouput is a function of rpm. At idle according to my Link 10 the alternator puts out so little amperage that it shows negitive numbers with just the load of the instuments. If the refrigerator kicks in its -5 amps. I need to be above 1000 rpm before it puts out 10 amps. I get 45 amps output at 2200 rpm with the house batteries at about 75% charge level. This tapers off as the batteries become charged but this takes over an hour to happen even with low batteries. My alternator is rated at 75amps but I have never seen it put out even close to that. The alternator shop told me that under real world conditions its not likely I will ever get full output.

Charging the battery at idle is generally accepted as a bad idea for the engine. It never gets up to full temperature and carbons up. If at anchor I put the engine in reverse and the rpm's to about 1200. But under these conditions it would take forever to put any meaningfull amps into the house bank. Figure 12 amps into a 400 amp-hr bank that is down 100 amp-hours. This is about 12 hours if you account for the tapering output as the batteries charge.

It would under these conditions top off the the group 24 starting battery fairly quickly.

Boyd
s/v Tern
CD30 MkII
Fort Lauderdale, Fla.

[Brian2]

I, too, have the Link 10. I love it. However, don't forget that you have to "excite" the alt before it will put out anything. When I start my engine, if I haven't given it much throttle, the tach doesn't read, nor does the alt put out. I rev slightly to excite the alt, the tach starts registering RPMs, and the alt starts charging, and I throttle back to idle, which is about 1000 rpm for me. Before the alt kicks in, you are still consuming amps from the batteries. After the alt kicks in, the alt provides amps for the load plus recharge. If you are registering -5 with the engine running, I would think that you haven't excited the alt yet.

[M. R. Bober]

Warren,
If you have the 8hp Yanmar, you'll know when the batteries are down. The alternator can use 60% of your 8hp (probably closer to 6hp).

Once the voltage regulator "eased" a bit TIAMARI would--when running under power--start to pick up speed (no change in the rpm).

I would not worry about damaging a small Yanmar by running it under a light load. Those little guys are tough.

As always every best wish...now go sailing!

Mitchel Bober
Sunny Annapolis (where the bilge pumping at my house continues to run under load), MD

[Dan & Pat]

I'm going to have to jump on the Boyd bandwagon (again) with this topic. My area of expertise is computer engineering, but I had to go through a lot of electrical and mechanical theory to get here . And, even though I'm a teacher, above all, I'm a learner too. I won't take anything on say-so, I have to understand it. (OK, so that's the anal part of my makeup). Here's a little ELE 0101

Many folks are of the understanding that the alternator, whether on an automobile or marine application, can be used as a substitute battery charger. That is simply not the case.

When your car/truck/van/boat motor is being normally operated, the alternator produces current at a rate designed to provide the the electrical components with the juice needed to perform at optimum levels. The ignition system uses the most, but there are other components that have to be supplied with electrical power to keep everything going smoothly. More juice when you need it, and less when you don't. The alternator actually produces A/C current which is physically easier to 'generate' than D/C current, but alas, there's no way to store A/C.

All batteries hold D/C voltage so you therefore need to convert the output from the alternator. The device for this is called the rectifier. The rectifier is a series of diodes. Diodes are nothing more than electrical one way valves. By itself, an alternator has no method of controlling its output, so it will over-charge the battery. To prevent this a voltage regulator is introduced. All regulators work in the same way this is by simply regulating the supply based on demand to the rotor or computer controlled ignition system.

In a pinch, jumpstarting a vehicle/vessel with a good battery will get you going, but be aware that you're putting about a 150% strain on your alternator by running the motor with a dead battery. It just isn't designed as a battery charger, and is very inefficient when used as one. If you know why the battery died (Duh, left the lights/bilgepump on for two days..), then CHARGE IT with a BATTERY CHARGER already! Boyd's math "take about 12 hours" tells it all. A $25 K-Mart 2-10 battery charger will charge a battery in about 4 hours, even if it's down to 25%. The difference is that the battery charger isn't trying to keep a motor operating at peak performance like the alternator would be.

If, on the other hand you don't know why the battery died, thats when the forensics begins. Is it just old and won't hold a charge? Is it dying slowly over prolonged periods of non-use? Or, is there an electrical drain somewhere that depleting it? Will a new battery solve the problem?

I'll get off the soap box now I hope this helped, and Good Luck!
Dan

[Bill Goldsmith]

All of the replies have talked about whether or not charging at idle is an effective way to charge the batteries. I think the answer is quite variable, and depends on the level of discharge and the rated output of the alternator. I know that when my batteries are low, charging at idle can tend to lug the engine.

I **think** Warren may be referring to the advice of most diesel manufacturers to avoid long term idling as not great for the engine. I believe there is concern that prolonged idling can result in residue adhering to the cylinder walls, as the engine does not get up to operating temp.

I doubt that occasional topping off at idle would be a problem, especially if the engine is run at cruising speed and temp regularly.

[bottomscraper]

Folks,

I'm imagining a practical product that would allow sailors to run the alternator at higher RPM with the engine at idle, but return the gearing to normal for running. I guess it would be a two-pulley alternator, or something like bicycle gears. Surely there's a simple, practical way. Has this been done?

Sounds like a good idea. I suspect it might suffer from the too many moving parts problem. It would probably need to have some way to keep it in low gear while the engine is starting and warming up so you don't stall the engine with the heavy load. I also wonder what the tradeoff is between running the engine at higher speed with lower load vs. at idle with higher load. Which does more harm to the engine over time? I guess the real answer is a dedicated generator but most of us don't have room for one on a CD. We have a 100amp alternator on Mahalo. Since we also have engine driven refrigeration we usually run the engine at least 1 hr /day anyway. We run it at about 2000 rpm, it gets the engine up to temperature and does a fine job of keeping the batteries charged. If it didn't we would be in trouble since we keep the boat on a mooring and only get to a dock once or twice a season for water and fuel.