How to test Alternator output?

[Ben998]

Hi All,
Could somebody please enlighten me as to the best way to test the output of the alternator. Ive got my multimeter ready and poised...
cheers,
Ben

 

[michael_w]

Volts or Amps?

Volts-test across the positive output and ground (ie engine block)
Amps-easiest to take to auto electrician's shop if you don't have an ammeter fitted.

Don't try the amps function on your multimeter. It's not man enough.

 

[ShipsWoofy]

Measure the volts across the battery terminals with the engine running, if the alternator is sufficiently charging it should read approx 14.4 volts. You may have to give the throttle a bit as tick over often sits around 12v.

Current, if you had a decent meter it would only be point in time, a 45A alternator is maximum 45A, generally a lot less hence why many yachts fit extra charging circuits to maintain a higher charge rate. I think these can measure amps fairly successfully.

In general, I think you can assume the alternator is working if it is giving full charge measured by voltage.

If you are worried about the battery, measure the volts during engine start, a good battery will drop to a minimum 10v probable less (i.e. between 10v and 12v) (min max gets confusing here!!!!), a dead battery will fall to around 7v during start up.

Hope this helps a bit.

 

[boatmike]

Hope this is not too simplistic but if you have not got a voltmeter fitted you really should fit one as it will tell you the charge state of your battery. Also a properly fitted ammeter will tell you exactly what charge/discharge your battery is experiencing. These two instruments are in my opinion essential to any boat system michael_w is right that most small multi-meters won't give you a high enough reading.

 

[pvb]

Just to clarify...

[ QUOTE ]
Measure the volts across the battery terminals with the engine running, if the alternator is sufficiently charging it should read approx 14.4 volts.

[/ QUOTE ]
Should add that you'll only see 14.4 volts or thereabouts with an ordinary alternator if the battery is pretty well charged. If the battery is quite discharged, it can "pull down" the voltage you measure at the battery terminals.

 

[nedmin]

You cannot test the amps with a multimeter!!! The multimeter, set to amps ,needs to be in series with a load!! otherwise you will blow your meter.Most multimeters will only read about 10amps.so you would need a 10amp load.At 12volts that needs to be 1.2ohms.As michael_w suggests take it to a auto shop.much easier.

 

[Guest]

The purists will laugh at this .... but having had a duff alternator diagnosed using this tool .... taking it to auto-electrician who reapired it ..... I refitted it and tool shows brilliantly the difference .....

9.99 at Halfords ...... a Draper Battery / Alternator LED tester. Clip one lead to battery +eve, other lead to -ve respectively ..... It has two sets of LED's .... lower set give battery condition with engine off etc. Upper set give alternator charge to battery when engine is at fast tickover ...

I even use it to test probe 12v cables etc. instead of fiddling around with larger multi-meter etc.

Yes I agree its not best and its not super accurate .... but when all you want to know is charging bad, moderate or good - then it does the job .....

I even thought of permanent rigging it via a on - off - on switch so that I could check each battery without opening up battery box and clipping on etc.

 

[Phoenix of Hamble]

Nigel,

Sod laughing.... that sounds really usefull......

 

[aztec]

read the other posts and agree with all of them....but another thought...

i have a snap on tool which does just that, it's an ampmeter that works on induction, all you do is clip it to the outside of the wire/cable that you're measuring (no electrical connection), it will tell you the current flow and seems to be quite accurate, reads charge and discharge like all the others.

i also have one for starter motor currents, iv'e used that to successfully diagnose winding problems on "lazy" starters.

can't remember what i paid for it, but if you look around i'm sure you'll find one. i don't use it very often as most faults can be diagnosed using methods already stated, but as a quick check it works fine.

hope this helps.

steve.

 

[pampas]

As various posts have suggested, but if you wish to test its full rated load,no way unless you have access to a bench load tester. Many good auto Electrical repair shops have this equipment and for a small fee will carry out the test for you.

 

[oldharry]

[ QUOTE ]
The purists will laugh at this ....I even thought of permanent rigging it via a on - off - on switch so that I could check each battery without opening up battery box and clipping on etc.

[/ QUOTE ]

OOOOH! CAREFUL! I have a 5 LED system which does battery and alternator voltage (courtesy Maplins), rigged in my boat. Brilliant! Even the the least technically minded crew member can sing out if the little lights change colour.
If they do, I get my DVM out to find out why.

So why be careful? Cos when I suggested it on this forum a few months ago - I was howled down by all the purists who tell you its no substitute for a 'proper' voltmeter...... which i find is usually inaccurate and hard to read, is quite incomprehensible to certain crew members and tells me nothing my LED display has not already warned of. /forums/images/graemlins/smirk.gif

 

[davidpbo]

Re 2 Previous postings in thread.

Small Boat Champ.
I have used a similar indicator to the Draper on my 23ft Tonic for several years to test the state of charge of the battery or whether it is any charge. As a rough guide it is fine.

Aztec.
Does the clip on device you describe work with D.C. Currents?

David

 

[oldharry]

Alternator output can easily be tested by checking the voltage at the battery terminals. However it is not quite as simple as has been suggested.

The 'proper' test, other than by actually measuring the amperage which is not easy with an alternator if there is no permanent ammeter installed and risks damage to the alternator (momentary disconnection on load will blow the diodes instantly) is to load a charged battery at about 10 amps for 5 minutes (5 amps for 10 minutes will be the same!) A good alternator should restore the voltage to between 13.8 - 14.2 volts again within 5 minutes.

The alternator should normally hold the battery voltage within that range while running - but only if a) the battery was fully charged to start with, and b) other electrical gear is all switched off. My 'fully charged' voltage stands at 13.9 volts when the nav lights are all on for example.

Any appliance in circuit will cause a drop in voltage directly proportional to the current draw, and it is easy to mistake this voltage drop for a problem in the alternator.

Having established that output voltage is down, take it to an auto electrician for further diagnosis, as messing about can do more harm than good, and damage it. An alternator unable to sustain more than around 13.7 volts is definitely suspect.

 

[William_H]

You want to test the output of your alternator? It is not hard to check to see if your alternator is operating as described above. It is best of course to test using any method when the alternator is working fine so you know what to expect when you have doubts.
To actually get a performance in amps result of your alternator this is difficult.
The correct way is to put on a test bench running it at speed where a large field current is provided and the output loaded to get a max current. Note here that the max rated current will probably only be at 12 volts or slightly less. Now your regulator is likely built in so in this case you would load the regulator and alternator combination and you should get the rated current again the voltage output will be falling at that point. NB the output of an alternator is rated for feeding power to lights etc not battyery charging so lights are OK at 12 volts but battery charging generally needs over 14v so you could never get the rated current in to a battery.

To test on the boat you would need to put all your lights and other electrical load on and maybe add extra lights. Of course you need an ampmeter rated for the kind of current you expect ie 50 or 100 amps mx reading.
An ampmeter is adevice which is inserted in the wiring by removing a terminal or cutting the wire so is not very convenient.

Unless you have a fancy inductive pickup ampmeter a small resistance rated for large current is inserted in the circuit. This is usually mounted in the meter but this means that the meter wire has to carry the high current which it usualkly can't so a separate resistor is fitted in the wiring near the alternator or battery (known as shunt) and small wires lead to the meter which actually reads volts dropped across the shunt. Note just to confuse things the shunt drops some voltage (for a standard cheap multimeter it is 200millivolts .2Volt for the max meter reading and proportionally less depending on what you actually read. This volt drop can actually reduce the real current from the alternator while it is in circuit. Current will be more when you remove the meter or shunt.
You can actually measure the voltage drop across the wiring ie from Alt O/P terminal to battery +ve terminal but this is uncalibrated and so is only useful if you do it when you know everything is working fine then you have a reference for when you have doubts.

Just be aware that alternator have 3 phase AC output which then goes to 6 rectifiers to produce DC. If one of these rectifiers or winding s die then you get a limited output from the alternator. (This is often accompanied by high interference screaming sound on radios) If this happens you may not know there is a problem as with light loads and limited battery charging it will still do the job. (and you still get a voltage rise with alternator charging)

Sorry I seem to have waffled but it might give people some ideas. olewill

 

[ranga]

Google "Clamp Multimeter". Works very accurately.

 

[ShipsWoofy]

Do these things give a reading in amps, I guess they must. We used to have current probes in work that you clipped over the wire, the results fed to an oscilloscope near impossible to get a decent result.

Are they DC only?

 

[ranga]

Yes, gives a reading in amps, and the one I have says it is a 2000A AC/DC + Multimeter. It was a lot more than a regular multimeter (maybe AUD$200?) but has been very useful.

For example, I tested how many amps the alternator produces at different revs so that I could more efficiently charge the batteries at sea.

 

[Guest]

Oldharry ....

I have had my Draper for maybe 4 - 5 years now and it sits on the boat ready to do the biz ... I have posted at various times about this little tool whenever battery checking comes up .... so you and I both have been laughed down for it ...

That is not OUR problem - we know they work, we only spent a few quid on ours .... others can scoff, doubt and spend what they like ..... we are OK !!

With more thought - I think I will permanent rig mine .....

/forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif

 

[halcyon]

Re: Oldharry ....

Funny market the marine one. We were making LED battery status meters in the 1970's, which included a split charge relay, and low voltage power lockout. Stopped selling them in 1990 as people were fitting blocking diodes!!.
Why because that was the media norm, or had the highest profile, or that was what the chap in the next boat said.

Brian

 

[Guest]

KDPower ...

I remember you sent me an example for my boat ... and I thought it good ...

Must get sorted on it ... diodes were waste of two screws fitting the block ... so now mounted but disconnected .... VSR's ..... yep !