Question re charging the battery from the engine

[Aeolus]

Today, I started the engine to charge the battery and altho the engine was running fine and the alternator belt was correctly tensioned and not slipping, there was no charge getting through to the battery (at least as measured by the voltmeter - it showed 12.5v both before and after I started the engine). After perhaps 10 minutes, the voltmeter reading jumped to 13.5v, and a couple of minutes later to 14.5vish, i.e. what one would expect (I think).

Any ideas why this should have happened? Is it normal or does it indicate some component is on its last legs?

 

[ShipsWoofy]

If I am reading correctly

you measured the battery voltage 12.5

started engine, measured voltage with engine at tick-over 12.5v

engine warms up and probably running a smidgen faster 13.5v

then it jumped to 14.5v

yes / no

This sounds entirely normal. When I start up I will often have to rev to around 1/4 throttle 1800 rpm or so to kick the alternator into action. At engine tick over the alternator without a charge controller will just about hold it's own. You need to excite it a bit by throttling up to get it going.

Now this is guesswork, but I think the alternator speed i.e. the pulley sizes are chosen to stop the alternator putting too much load on the engine at tick-over. But this is based upon the only reason I can think of for the manufacturer not allowing the alternator to run fast enough to throw lots of charge to the battery.

 

[Searush]

Starting an engine puts a heavy load (amps) on a battery. This will lower the battery voltage. The low voltage you have is due to a high charging current (yes, I know the alternator is SUPPOSED to be a constant current device!) As the battery recharges so the current drawn will fall & voltage will rise. QED?

 

[sjohn_gibson]

It is worth checking all the connections - loose or corrosion. I had a problem earlier this year and I was in the process of removing the alternator when I found the negative cable connection to the alternator was poor. Remade the connection and all has been fine ever since.

 

[DaveS]

I'll second that. The same applies to starter motors /forums/images/graemlins/blush.gif. An apparently serious mechanical problem can actually be a very simple electrical fault.

 

[Aeolus]

I had revved the engine a bit so I don't think that's what it was, and it had never behaved that way before.

I'll check the connections.

Thanks for all your help.

 

[William_H]

An alternat6tor requires a current through the field coil (rotating coil) to generate a magnetic field and hence output current. Most alternators rely on current from the battery via the gen light and key switch to provide the initial excitation current. The resultant magnetic field provides power via separate diodes to generate its own field current. So the gen light goes out.
Now it is possible your alternator was gnerating after start up but a combination of low revs and low battery meant that the battery voltage did not rise for somme time. or
It is possible your alternator was not generating output possibly due to current from the gen light key switch not available or insufficient due to resitance/corrosion etc in the wiring. The later operation is poossible due to residual magnetism in the armature (field coil) eventually providing enough field to self generate into action. Alternators are usually regarded as unable to do this unlike an old generator.
if the alternator continues to act a bit doubtful you should check the brushes and slip rings. These are mounted either under the regulator or plate at the end opposite the pulley with terminals with the lighter wire attached attached.
The brushes are relatively small and should provide some pressure onn the slip rings. These are full circle copper rings which rotate with the armature. They can be faily grotty without giving too much trouble but ideally should be bright clean and smooth. You can press a bit of sand paper or similar in thhere and clean them. If they are really bad then you need a new armature or complete alternator. It is often easier to change the armature because it is the same as many car alternaors but the body may be different for your boat engine.

One last comment an ampmeter on the alternator output is far more usefull in checking the output of alternator. olewill

 

[Guest]

A simple visual check ....

I know it sounds obvious but this little visual check actually answers a few questions and may just indicate your problem ... (you've done the meter test ... now ...)

Before starting engine - switch on ... Red Ign light is bright ?

Start engine and leave on tick-over ... Red Ign light is pulsing or still reasonably bright for a number of seconds ?

Increase engine speed ... Red Ign light goes out ?

Depending on length of time an alternator stands stopped - the field decays and when restarted needs the exciter current to generate it again. Idling engine will often not be enough to excite field sufficiently and a slight increase in engine speed with cause a jump in excitation and alternator kicks in ... increase speed more and alternator will really push out.
You may have just experienced the above.

I know with my engine where it stands for long periods between use ... I often get the above .... a good burst of revs and it's fine again ....

I've said it before ... no harm repeating .... a very good cheap diy tool is the LED Battery / Alternator Tester available in most car acc's shops .... Draper make one (one I have) for 9.99 .... other places have them for as little as 4.99 .... May be a bit crude - but told me when alternator was not working ... told me also that battery was likely shot ...

 

[Aeolus]

Re: A simple visual check ....

Went down to the boat and found one of the battery connections was loose. Tightened it and all started working normally. Many thanks to everyone for your help.

 

[pampas]

]If they are really bad then you need a new armature or complete alternator. It is often easier to change the armature because it is the same as many car alternaors

This is frought with complications as the pole pitch/diameter may variey, the only alternators that are difficult to do are the Hitachi range as theirs are a sort of stainless steel all other types that I have come across. you can buy and fit new slip rings. I trust that this clarifies the situation better sorry about the method of quote /forums/images/graemlins/wink.gif