Flat Battery

[Richard Grove]

My battery keeps going flat. How do I check if its the Alternator not charging as opposed to a duff battery?

 

Thanks

 

Richard

[Guest Mr Pid]

Hi Richard,

 

If the alternator is not charging the little battery light on the dash will not go out when the engine is running, it the alternator will not be sending power back through to extinguish it.

 

It may be that the alternator is charging, but only very weak, so when you run the car and everything else it cant keep up.

 

If you charge the battery fully then measure the voltage, then go back every half hour or so and keep checking, if it goes down it may not be holding the charge or you could have some sort of drain on the battery ( i am assuming its got distilled water in it).

 

A mutimeter is a useful help in this regard

 

hope you have some success.

 

Stu

[Longboarder]

Charge the battery out of the car fully or to at least 70%. Use a Hydrometer to test each cell in the battery. Should be 1.28ish full or 1.24ish for 70% charge. About 1.13 for flat. All cells should be roughly the same. Start and run the engine at about 2500-3000rpm for a couple of minutes and connect a voltmeter across the battery terminals. If your alternator is doing its thing you should get a reading of 14ish volts give or take 0.3 roughly.

 

Nigel

 

( Cost of toys! Hydrometers are about £5 and multimeters about £10.)

[Guest Ant]

With the battery getting towards flat get the engine running with a jump start, run it for 10 minutes at a resonable mid-ish speed, switch off, and see if it will start on its own. If it does the the alternator is fine as it has just charged the battery for you, therefore the battery is failing to hold that charge when left standing over night. If it doesn't start then it's probably the alternator, though in this case I'd want to do the test again with a flat-ish battery and put an ammeter in the alternator circuit to see what it's actually doing.

 

Hope that helps.

Ant

[Guest Stuart Ainslie]

Could be the diodes in the alternator causing the problem.

 

Do not run the car for a day or so and then put your hand on the alternator(engine off, of course) - If you can feel any warmth in the alternator, bingo - there's you problem.......

 

Stuart

[Guest boggie]

First get a multimeter and measure the battery voltage. If your battery is flat the measured voltage will obviously be rather low.

 

Now (jump?) start the car and keeping the revs above 1500 check the reading again, it should rise to around 13.8 volts. If you don't have a meter then a crude alternative is to get the car running at a low (sub800) RPM and turn on the headlights. Rev the engine up and they should get brighter (do this in a dark environment or you might miss it), if they do then the alternator is charging (at about 13.8 volts, hence the lights getting brighter). This would then point to either a dying battery or a drain on it.

 

To test for a drain make sure all electrics are switched off as though it were parked, remove one of the battery terminals and tap it lightly on the battery post. If you get sparks then something is draining the battery which may be hard to find. Try removing each fuse individually and tap the battery terminal each time, this way you will be able to work out which group of compoments is the source of your drain but it may be difficult to pin point which one as it may be simply a wire partially shorted on an earth so an auto electrician would be a good investment at this point.

 

Here's hoping you have a battery with a dead cell or two as a £40 trip to halfords will sort the problem. Good Luck!

Boggie

[Guest 2BBoulton]

Just a point of interest, I had a man in a van tune my car quite recently and when he measured the charge voltage it was around 16V. This is due to the fan bellt slipping (acording to the man) and is not harmful if the car only gets run at the weekends but if its used daily then it can bugger the battery.

 

Bruce

[Big Jim]

That's rubbish! one of the diodes in the alternator is knackered, carry on at this rate and your battery will boil and the plates will warp.

[Richard Grove]

Thanks for all the input.

 

Trip to Halfords tomorrow for some new toys (Hydometer etc.).

 

Richard

[Longboarder]

Agree its bull about the belt slipping but don't you mean the voltage regulator not a diode Jim?

 

Nigel

[Big Jim]

could be either, i've had a couple od alt's with diodes go, they showed high voltage ie 18v plus.

[Longboarder]

OK. Haven't come across that before. One funny one I have seen is when the ignition light is off or very faint at idle but glows brighter the faster you drive but not full brightnesss and that is usualy the voltage regulator.

 

Nigel

[Guest Bernie]

Bruce, I hope your van man is a better tuner than he is an electrician. That' total rubbish about the belt and the 16V being nothing to worry about.

 

I had a similar problems with a shorted diode on an escort. Once started the car would continue to run but if stopped for as little as ten minutes had to be jump started. In addition the alternator got bl**dy hot when the car was just left standing. Voltage across the battery when engine running was measured as 16V.

[Guest 2BBoulton]

Okay.... So if the belt slipping isn't my problem how do I locate what the problem is.

 

The tuning however was godd and the car goes a hell of a lot better since having it done.

 

Bruce

[Guest 2BBoulton]

Forgot to add that I have no problem in starting the car and the battery does not go flat when left (I think nearly one week is the longest between drives since buying it last September)

 

Bruce