11 replies to this topic

[Dolly 89]

How do i check that my alternator is actually charging my battery? I have a multi metre i think all i do is put it onto the 20 volts setting and put it on live and neutral on the battery whilst the car is running, is this correct? and after that how do i check for a drain on my battery? I ask because i fitted a new battery and alternator about a month ago and now my car is just flat so i want to know what is the problem so i know who to go and moan at tomorrow, thanks

[Surfbluegarage]

use a multimeter set to volts dc. turn it to 20volts like you stated and use the red lead on the positive side of the battery and the black onto the negative side of the battery while the engine is running. you should be expecting anything fom 13 to 14.5 volts which is acceptable really. anything over 12 volts is acceptable as its overcoming the batteries internal resistance but les than 13 volts is low. you ca then use he multimeter again on the alternator using the red probe on the first big termnal coming off the alternator (if its an "amp plug" not an mpi) and the black lead to the alternators body. if the reading at the battery is lower than the reading at the alternator by more than 1.5volts you have a voltage drop and that should be investigated further.

if you want to test if somthing is draining the battery while the car is off remove the red lead from the volts input on the multimeter and put it into the "10amp" socket of the meter.remove the positive lead from the battery and connect the red lead on the multimeter to the positive terminal of the battery and the black to the positive battery cable you removed from the battery set the dial on the multimeter to "amps" and see what is drawn anything realistically over an amp is too much as a standard rover alarm/immobiliser only draws around 0.2mA.

[Dolly 89]

cheers for that, I take it when i test at the alternator i leave the car running but have the alternator cable disconnected so i can get access to the terminal right?

[strettman]

hi

if you stick your mutlimeter on your battery terminals whilst your car is running and you get around 14 volts or lower then your alternator is good. if it is around 12v or lower i'd imagine your alternator could have gone. from this point if the voltage is too low you should stick your multimeter onto the back of the alternator and see what reading you get.

the battery should be fine with it only being a month old, as long as you haven't bought a super duper cheap one

don't forget too if everything seams ok, you could just have something draining the battery, like a interior light left on all the time by accident and leaving side lights on.

thanks

EDIT: oh dear, looks like i was a little late, haha

i went away for an hour then forgot i typed this message out xD

Edited by strettman, 01 October 2011 - 09:48 PM.

[Dolly 89]

Cheers for that mate all information is good whether it is said once or a hundred times

[MR J]

I've had a charging problem lately, it turned out to be the engine earth strap had a dirty connection. I fitted a new lead and cleaned all the earthing points and now my battery charges at 14V. And the engine turns over better on the starter also!! Worth a check.

[dklawson]

cheers for that, I take it when i test at the alternator i leave the car running but have the alternator cable disconnected so i can get access to the terminal right?

No. NEVER disconnect the plug from an alternator while the engine is running. This can damage the circuitry inside the alternator. You should be able to access the terminals while the plug is in place. Depending on the plug, this may require pulling off the rear cover of the plug while the engine is "off", then connecting the plug again and starting the engine.

If I may, let me add a bit to what Mini_mad_daps has suggested. A fully charged "new" battery is likely to measure closer to 12.5 volts, not exactly 12. Measure across the battery terminals with the engine NOT running. Start the engine and repeat the test. You want to see an increase of 1 to 1.5 Volts from what you measured with the engine not running. If you see less than a volt increase then the alternator is not effectively charging the battery.

Incidentally, the charge warning light is not just an indicator, it is part of the charging circuit. The warning light must be:
1) Off when the engine is off and the key is out of the ignition.
2) On when the engine is off and the key is in the run position.
3) Off when the engine is running.
If the light is not behaving like this in all three circumstances the battery will not be charged.

[Dolly 89]

The light is working fine, that was the first thing i checked , my battery is flat so i am guessing it wont read hardly anything whilst the engine is off but i will check it anyway thanks

[Surfbluegarage]

yeah sorry forget to say if its an amp type plug that you can flick off the little wire clip and pul the back cover off of the plug without taking it out of the alternator.
as dklawson rightly said the charge warning light is more than just a light as the light completes the circuit and this inturn "adjitates" if ou will the 3 phases of the alternator to start the chargeing process. if the light is not lit before you turn the key and the bulb is blown it will not charge.

[Dolly 89]

right got it sorted, for some reason i posted on here before actually looking but the 2 top bolts on the alternator had fell out , I must have not double checked them when i put the alternator on and left them loose, but i was getting nearly 13v at the battery with engine running and 14v at the alternator, do i maybe need to change the cable to try to reduce the massive voltage drop in the system? and also maybe find a shorter route for the cable? it is working fine though and the battery is charging fine but i am a bit worried that, that is quite a low voltage to be ariving at the battery

[dklawson]

I cannot remember if you said you had already cleaned all the earth connections. If not, start in the boot. Clean the battery posts and the clamps that connect to them. Parts stores should have special male/female wire brushes just for this task. Moving down the wires, check the earth cable connection to the boot floor. Remove the bolt and use some sandpaper to clean both the contact point on the boot floor and the terminal on the end of the cable. Apply a smear of dielectric grease or Vaseline to the bare metal surfaces and bolt them securely together again. Now turn your attention to the front of the car. The alternator mounting hardware form its earth connection. Make sure all the points where the alternator contacts the brackets are clean and bare metal. As before, a smear of dielectric grease or Vaseline will protect the surfaces from corrosion. Now look carefully around your engine bay. There will be an earth cable between the car body (or other point) to the engine block or gearbox. The location of the earth connection changed a bit over the years so keep looking until you find an earth cable. As with the other connections, disassemble and clean the parts to bare metal, apply dielectric grease, then re-assemble. After all this, repeat your voltage drop measurements and let us know if the situation has improved.

[Dolly 89]

cheers for that, it will be next weekend before i do that now because of working and biking during the week, it works anyway so it will be fine till next weekend and then i will crack on with getting that done cheers