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MG Midget and Sprite Technical - How do I find what's draining the battery

I have a voltage leak that is draining the battery, being a mechanical guy, electrics are somewhat of a mystery. However, I have a multi meter and am willing to give it a go. Can anyone give me a pointer on where to start?
P Ottewell

Disconnect the ground cable to the battery and hook up either a test light (best) or a volt meter between the ground and the battery terminal. This will tell you when voltage is flowing. Disconnect the fuses, one at a time, and see what happens. If the drain is on one of the circuits protected by the fuse being removed, the light will go out or dim significantly. An alternator, if your car has one, can put a very light drain on the battery--sufficient to register on a digital volt meter but not sufficient to illuminate a test light. A larger drain from the alternator would show up as a stronger illumination of the test light and either would go away when the plug to the alternator was removed. Older vehicles, having a generator (dynamo) and mechanical voltage regulator (control box) can have contacts in the voltage regulator stick, causing it to leave power on to the generator when the ignition is switched off and draining your battery.

It would help to know what your car's model year is and, if originally equipped with a generator, have you done an alternator conversion.

Les
Les Bengtson

Sorry Les I'm taking it for granted that everyone knows my car, however it is a 1275 1972.
P Ottewell

You need to switch evrything off including the courtesy light if you car has one.

Remove the positive battery lead from the battery and with the multimeter setup to ammeter, measure the current between the disconnected battery lead and the battery terminal and it should be about 25 mA.

Write the result down and replace the terminal on the battery.

Remove each fuse and again measure the amps across the fuse terminals. The totals of these readings should match the current across the battery already measured.

From this you should investigate the route of any current above 25 mA.

Report back your results.

eddie
Eddie Cairns

Les,
if you click on Pete's 'View vehicle profile' the ifo is there

Pete,
yeap process of elimination is best

causes that come to my mind;

perhaps it's charge not fully getting to battery
bad battery or condition
connections to/from charge unit not clean, secure and protected

loss of change;
bad battery or condition
connections to/from items not clean, secure and protected
boot light, courtesy lights/switches
constant supplies - immobiliser, alarm, clock, stereo unit
ignition switch wiring(?)
as Les has put
Nigel Atkins

Nigel, it's not a significant drain, battery charges OK, but will go flat over a couple of weeks if not driven, I thought it was the old stereo, but I took it out at the end of last summer, I wasn't using it. It's not a real problem but I am keen to investigate.
P Ottewell

Does it still have its original dynamo, or has it been converted to alternator?
Dave O'Neill2

something is wrong if a good fully charged battery can't hold it's charge for a couple of weeks
Nigel Atkins

It has an alternator..
P Ottewell

even more so then that somethings not right

check "water" level in each cell before (strictly speaking) removing battery from vehicle to slow charge battery to full charge

after fully charged you can test battery for holding charge out of the car or return to car and test drain of battery (see Archives or battery info on t'web)
Nigel Atkins

Nigel, it's sealed low maintenance battery that fully charges on trickle charge. It could be just the battery losing its charge. I'll try the ammeter tests first to see if its a voltage leak. If it is the battery ill change it at some point, as I said not a real problem if I keep driving it. Even more reason to drive her as much as possible. Going to start a new thread on idle speed now.
P Ottewell

quite rare to get a faulty battery but a mate had a similar problem with a varret less than 12 months old and gave up trying to convince supplier that it was the new battery at fault as he used the car little

much against his nature he bought and fitted another new battery and the fault dissappeared

course that wrankled even more :)
Nigel Atkins

Right, tried the ammeter test, zero reading between the disconnected +ve lead and the terminal on the battery. So switched to voltmeter. .78 volts on no 1 fuse, so disconnected the alternator plug and go zero volts on no 1 fuse. Therefore the alternator is dragging the voltage and that must be the voltage leak.
P Ottewell

I would suggest it is a faulty diode in the alternator allowing discharge through the alternator. Charge the battery, disconnect the alternator supply, reconnect the battery and see if it drains. If it doesn't I'd say it is the alternator.
Neil
Neil T

This was posted by Paul Hunt on the MGB BBS recently.

I saved it for future reference!...

To check for drains remove the battery earth strap and connect a voltmeter on it's 12v range in place of the strap. On a car with an alternator and no drains a typical analogue meter will show about 6v, which is the microscopic leakage of the diodes, and unplugging the alternator will drop that to zero. If you get that then you need do nothing more - there are no drains. Digital meters are much more sensitive and will probably show 12v from the alternator. If unplugging the alternator drops that to zero, then you have no other drains, but the alternator diodes could be leaky. So now you have to disconnect the meter, switch to the highest current range (if not auto-ranging), and reconnect. A few micro-amps is fine, milli-amps is too much.

If an analogue meter shows 12v which drops to zero when the alt is unplugged, the alt diodes are probably leaky. Slightly leaky will drain the battery, but if they go very bad they can effectively put a short on the battery which can burn the wiring (hence battery cut-off switches are a good idea anyway ...).

If either type of meter shows 12v, which is still there when the alt is unplugged, you have a drain. Disconnect any clock, alarm, radio etc. first to see if it is one of those. If not check a courtesy/boot/load space light isn't staying on. If not that remove the brown to purple fuse, which feeds the horns, headlamp flasher, courtesy lights. If still there remove the other three fuses, but with the ignition off they should make no difference. If still there you will then have to start disconnect brown wires from things like the ignition switch, main lighting switch, starter relay, and finally the starter solenoid until it does go. Whatever you disconnect that makes the meter drop to zero, indicates that circuit is feeding the drain.
Dave O'Neill2

Further to Dave O'Neill's post (quoting Paul Hunt's MGB BBS post). With respect, Paul's procedure for checking battery discharge is very confusing.

This is my take on what to do for a car that has a Lucas alternator.

(1) Because this work will be done with various doors/boot/bonnet open, disable all courtesy light switches by taping them in the 'off' position. Make sure the radio is 'off'

(1) Disconnect the battery earth lead and replace it with a meter (analogue or digital) set to a high current setting (say 5 amps or above).

(2) If there is significant current drain showing (ie 0.05 amps or more), then sequentially remove each fuse and watch for the current to drop. If current drops, investigate what the specific circuit(s)/load(s) that the fuse feeds.

If there is no drop:
(i)remove the alternator connector. If the current drain drops, then you have a faulty alternator (internal 'pack' - either diode pack or regulator pack). They are replaceable if one is confident with electronic repairs. A couple of milliamps (0.002 amps) is acceptable because of the alternator's internal circuitry.
(ii) you may have an ignition switch problem eg a constant current feed to the coil. Disconnect the low tension coil feed and watch for current drop on the meter.

(3) If you have removed the alternator connector, all fuses, low tension feed to the coil and you still have a current drain showing on the meter then you need to sequentially disconnect any other apparatus connected in the car until you see the current drop to zero (eg car stereo, car alarm). Also, check the headlight cct because that is not fused in midgets.
Andy Hock

Can someone please provide a similar procedure, to that kindly provided by Andy, for dynamo equipped cars.
Doug Plumb

Replying to Doug Plumb's request for a procedure to test battery drain on a car that has a dynamo . . . .

I have absolutely no experience with (or knowledge of) dynamo equipped cars. Having said that, the procedure (for checking battery drain) that I gave for alternator equipped cars would be pretty much the same as that carried out on a dynamo car.

The difference between an alternator and a dynamo is (to the best of my knowledge) as follows:
>> An alternator's internal windings produces an ac current that is converted to dc by its internal 'rectifier pack' (aka 'diode pack'); that converted dc voltage is then stabilised/regulated to the correct voltage for the car by the alternator's internal 'regulator' pack.
>>A dynamo's internal windings produce a dc current. That dc current is regulated/stabilised by the external, bulkhead mounted, 'regulator box'. I recall the regulator is a brown/paxolin box with several connectors on its bottom half.

When testing for battery drain on a car that has a dynamo, I would imagine that the control box should take even less dormant/quiescent current than that of an alternator (typically less than a couple of milliamps). Excluding its earth/chassis/bracket connection to the engine, I believe the dynamo is solely connected to the control box (but I could be wrong). If that is the case, to eliminate the control box/dynamo as a possible cause of battery drain, one would have to disconnect the control box's terminals from the rest of the car's wiring. If the battery current drain disappears after having disconnected the dynamo control box connections, I would imagine that the control box would be at fault (or need adjustment?), rather than the dynamo itself (because, electrically speaking, the control box sits between the dynamo and the rest of the car's circuitry.)

I do hope that I haven't caused confusion!
Andy Hock

I thought that the significant difference - in relation to current drain - is that an alternator takes a small quiescent drain when it is not running, whilst a dynamo does not. But a dynamo will draw a current if the Black box" voltage regulator is faulty and the contacts sticking on; - but then that is not a dynamo fault!
Guy

This thread was discussed between 22/04/2012 and 25/04/2012

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