car battery is it dead ? apologies nothing much to do with boats

[simonfraser]

yesterday drove 20 miles to boat.
parked, found boot to be by puddle, tried to move car, starter motor not turning over, lights on dash ok.
perfect breakdown, RAC took it's time, but no prob as with boat !
they diagnosed broken altenator and jump started the car.
drove home, charged up battery for 5hrs, had to be somewhere else.
whilst the car is running the voltage shows 13.5 V
with ignition on first on bit, battery 12.6V
what next ??!!

 

[savageseadog]

Does the ignition light go right out? check it in the dark. If it's on at all the alternator is US.

 

[VicS]

13.5 with engine running is too low, indicates a possible alternator problem but clearly not totally dead.

12.6 for the battery with a small load is not too bad.

If you can take the battery off and charge it. Check with hydrometer if you can. If not check its volts once off charge. It'll be quite high, 13 or more maybe at first, but after standing for 12hours it should be around 12.7 If its less than 12.5 it's probably had it, but worth charging some more.

With a properly charged battery see what the alternator will deliver you want at least 13.8 preferably around 14.2 but not more than 14.4. Actual readings will depend on spec.

Any doubts about the alternator, the warning light should come on initially and then go out when running, get it to a reputable auto electrician,

They will be able to check the alternator and diagnose any problem and repair it and also check the battery properly.

Dont go to a garage they will just put a new alternator on and probably sell you a battery whether you need it or not.

If the AA man said it was the alternator at fault it probably is.

There may be just some poor connections. Normally if the battery is down the dash lights go dim when you try to start so ??????? oh heavens knows without systematically checking.

BTW its a good idea to use a bulb and leads when checking for power as a digital meter can give a normal reading even through a dodgy connection (Been there got, the T shirt)

 

[simonfraser]

thanks vics, i shall get the alternator checked properly.
agree with the comment re garage alternator/battery sale !

 

[jonjo]

I have heard that RAC mechanics have a monthly battery sales target, so if a finger was pointed at the alternator it probably is the issue.

 

[William_H]

However if the battery is on the wrong side of 4 years old. Ave a new one anyway. You can spend the cost of a new battery just getting things checked. Batteries just naturally die. Yes quite commonly immediately after a period of charging. Alternators and wiring don't tend to give much trouble. good luck olewill

 

[VicS]

[ QUOTE ]
Alternators ....... don't tend to give much trouble.

[/ QUOTE ] Ive had a couple who obviously had not heard that. One in a previous car required, at different times, new brushes, new rectifier, new regulator and even new slip rings. Fortunately it was an easy one to work on.

A more recent car has had a new alternator, that one later repaired, and it's not charging properly again now.

 

[bedouin]

I would have thought that for a car alternator, 13.6V is about right, ot only a little low. I would not expect anything above 13.8V tops. How did you measure the voltage? Don't forget that most common multimeters have an accuracy of 2% +- 1 digit so can be up to 0.3V out

For most car batteries, once you have drained it to the stage were it won't start the engine then you have damaged it and reduced its life and many mechanics seem to suggest that you should replace it. I have one case where a brand new battery went flat and afterwards would not hold its charge, but most seem to be tolerant of being flattened once or twice.

As suggested if using a multimeter you need to ensure that the circuit you are testing is carrying some load. If checking a car battery I would tend to have the headlights on - thereby drawing about 10-15A from the battery. If that causes a significant drop in voltage then you know that you have a problem.

 

[simonfraser]

alternator voltage is 13.5 or so when driving, but drops to 12.3 when stationary with lights on etc. so i think alternator is indeed dud.

 

[ex-Gladys]

At idle with lights on, 12.3 is what I would expect. you need to observe on taking revs up gradually and holding steady at each "step" - 1300 rpm, 1800, 2300 etc. By the time you get to 2300 it should be kicking out full jolt.

Check you ignition light is comin on when you turn on. My car had that problem that I didn't notice because of all the other lights that come on, and so the alternator wasn't getting excitation until i'd revved it over 3k rpm. The fix? put the plug back in the back of the alternator....