How do I check my alternator in situ?

[Mudhook]

I have a suspect alternator, I think about 55A rated output, and I don't get a good charge on my main 'leisure' battery. Starter battery seems OK but doesn't get much stick. How can I check my generator output without taking it off and sending it ashore? I have a multimeter but nothing more sophisticated. Engine is Volvo MD7 in 30-year old Sabre boat. Generator type unknown (it's green!). Yes, the Alt light does go out.

My local garage bloke (usually very good) told me to monitor volts across the battery, start up and watch the volts rise to 14 or so (or not). I fear this only checks that some power is going in. How can I confirm how much?

Regards, Mudhook.

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[tcm]

erm, well, it's the voltage that matters innit. A nice lot of volts less than 15 volts and more than probly 13.5 and that's all you need really. You need an inline ammeter to check the amps, but the volts will ensure there's always a trcikle charge with engine on. If there's a decent voltage going in but still flaky lectrics - then the battery is not holding the charge and you need new ones, i bet . But gettem checked before dumping them/it.

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[jfkal]

Two ways to do it. Borrow a AC/DC "Clamp on Ammeter" from your friendly neighborhood electrician (Look s like one of those clips of a jumper cable with an integrated instrument). You just clip it over the + lead coming from the alternator and you will see the actual charging current. For maybe 20 USD you can buy a small ammeter with a 100 A shunt which you can permanently install between the battery and the alternator (shunt in series between alt and batt, ammeter in parallel to Shunt).
Get a smart alternator reg. to make best use of your alt. The charging can be quite frustrating even with new batteries and everything in order you will get no more than 80 % with a standard reg. and it takes ages.

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[MainlySteam]

Go along completely with TCM and add if you do not have an external battery sensed multistage regulator, and it sounds as if that is so, it is unlikely you will get a good charge. If after multiple starts in normal use (ie start, run for a while stop, etc) your engine start battery is not losing output, then the alternator is probably, but not necessarily, ok.

First check is always the battery's ability to accept a charge. The charge current, depending on state of discharge and the battery type, will likely be more than your multimeter will be happy with. In which case I would suggest taking the services supply battery out and getting it checked by a reliable auto electrician (or if multiple batteries, and it is too difficult to cart them all around, take one, if good, take the next, etc and if bad one found replace the lot).

John

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[oldharry]

The standard automotive 'in situ' test is, after ensuring the battery is reasonably well charged (12.7 - 13 volts), to switch the headlights on for 5 minutes (around 140watts = roughly 12 amps). Switch off, connect a multimeter across the battery. It should return to around 12.5 volts within a couple of minutes, much less indicates a suspect battery. Start the engine and monitor the voltage rise, which should come up to around 14 volts within a couple of minutes. Anything above around 14.7, or below around 13.8 suggests the alternator is suspect. If the charge warning light glows very faintly at running revs (too dim to see in daylight, but just glowing) it is likely that one of the diodes has blown and output will be down.

The problem on the boat is it does not have headlamps, so that loading the battery correctly may not be so easy.

Otherwise take the alternator off and take it to an automotive electrical specialist to have its output checked properly under bench condtions.

A green generator is either its been painted at some point, or a Dynastart. If its a Dynastart and the starter part works OK, then the problem is almost certainly the control box, not the Dynamo. If you are not sure, does it look similar to the alternator in your car? i.e. does the casing diameter measure roughly the same as the front to back depth, or does it look like a metal tube perhaps twice as long as it is wide? If so its a generator or a dynastart, and you may well have problems getting spares.

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[Mudhook]

Thanks to all for the useful info and insights.

Definitely not a Dynastart; it's a car-looking alternator but oversprayed by a previous owner in their wisdom.

Volts on the ameter next to my switch board does go to 12 or 13.5, but the battery doesn't seem to 'take' much even after an hour.

Blown diode theory is interesting; Alt light doesn't 'glow' but sometimes you have to gun the motor to get it to go out.

No, I don't have any sophisticated charge-control system.

I wil perform further tests this weekend. Meanwhile, no night sailing till I've resolved it. And a big electrical upgrade this winter I think!

Thanks to all.

Regards, Mudhook.

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[charles_reed]

I wouldn't waste any more time checking - take it out and to an alternator specialist. It will probably need a new diode and bridge - cost between £25 - 40.

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[oldharry]

If the alternator is only pushing a max of 13.5 volts the battery is getting very little charge, and the alternator is the prime suspect. However, before you rip it out just check that not only the positive cables, but the earth return (usually through the casing) is sound. It sometimes happens that corrosion between the alt casing and the engine makes a bad contact, and reduces the alt output. 1 boat I dealt with recently we had to install a wire from the alternator chassis back to the battery negative before we could get full output. Its just worth checking before you get your wallet out....!

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[philmarks]

Get hold of Nigel Calder's book - Elevtrical and Mechanical doh I can't remember the rest. Trouble shooting checklist for alternators. Very good. Worked well for me, but you may need to dismantle, in which case Charles's suggestion may be the best.



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[Alex_Blackwood]

Have you checked that you don't have a slack or slipping drive belt? You should be getting a voltage of around 14.2 at the alternator terminals at normal engine revs. with every thing connected.

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[johnsomerhausen]

e you said ythe engine is an MD7, it must be either an A or a B, both of which were originally fitted with alternators (and starter motors). If your ammeter (I hope it's a digital one) indicates only 13.5 V wjem the alternator is producing juice, that would iondicate that the regulator is either defective or an automobile unit hich cuts off at 13.8 V instead of 14 V for a marine regulator.
john

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[oldharry]

If the alternator can not raise 14 volts on normal charging (i.e.with no ancillary electrics on) then there is something wrong, and it needs repair or replacement. The battery will not be receiving enough current to charge it properly if the voltage is any lower.

Either the alternator is duff or the voltmeter. And s..s law says it s the more expensive bit every time!

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by oldharry on 14/08/2003 08:38 (server time).</FONT></P>

 

[Guest]

Tenner at Halfords ....... Battery tester with LED's

I thought it was a joke and thought what the hell ... buy it and try it. Well it worked and showed that my system was only charging low. Cleaned up terminals, replaced a few cables and then got all led's on ..... showing that charging is good.

Instrument is about 6 inch long, inch wide, 1/2 inch thick with two sets of led's ...... top set are for battery stae only with engine off. Bottom set for alternator charging.

Never leaves the boat now.......


<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !

 

[oldharry]

Battery checker...

"Tenner at Halfords" - typical of that outlet! .... £4.99 from Maplins (and elsewhere), and with 6 LEDs for more accurate readings. But agreed, its a useful piece of kit for keeping an eye on things.

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[anthonyyearsley]

At the start of the season I was suffering low voltage on charge, about 13.6 volts which is not enough to pull in the relay to charge the auxiliary battery. The regulator on the alternator, some 13 years old, was faulty. This was changed by local car electrics shop to restore 14.5 volts. After a 4 week cruise to Brittany, I found the problem of low voltage had recurred. After much checking with a digital voltmeter I found a voltage drop of just over 1.0 volts on the negative lead to the batteries due to the holding screws in the battery connectors not being tightened hard. There were no obvious problems with engine starting or supply voltage to the boat. The problem did not show up with resistance testing. Regular tightening of the screws will be actioned!

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[Guest]

Re: Battery checker...

I am a regfular of Maplins, Halfords, Caravan Shops etc. etc. looking for cheap alternatives....... AT the time of buying my tester, made by Draper ....... it was not available as I found those years ago !!!!! I'm sure that its available all over now !!!!

But its useful tool ...... gives battery stae and alternator charge indication.



<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !

 

[Mudhook]

Thanks for all the input. Lots of useful stuff here. In the end I found low acid level in the service battery and a very poor (loosely bolted) connection in the common negative line. All rectified and now working fine.

I think the overall lesson is that although alternators can and do fail, the first place to look is not the alternator but the batteries and wiring to 'em.

Regards, Mudhook.

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