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MG MGB Technical - Radio Draining Battery

'73 BGT, aftermarket radio/cassette. Started having battery drain problems, even after charging and driving. Had battery checked - no problem. Measured amperage with ignition off. There was a drain happening. Removed fuses one-by-one. The only thing that stopped the drain was disconnecting the unswitched power to the radio (clock, station memory).
Charged the battery again, let sit for two nights, and it started right up.
Don't understand how after over a year with this radio, it caused this problem, but I have removed the radio, and am looking for one without an unswitched power connection (back to retro, I guess).
Wanted to let anyone with newly noticed battery issues, that the radio could be the culprit, especially with new sound systems.

Pat
Patrick Callan

The drain of a clock and memory of a modern radio should be so small that it's barely noticeable. I don't start my '87 Chevy for months at at a time and the battery always starts the engine instantly. Make certain that the alternator or something else isn't the culprit. RAY
rjm RAY

Ray,
I pulled the ground strap from the battery, and put an ammeter between the negative battery terminal and the ground strap. It only registered a drain when the unswitched power wire to the radio was connected. Once I removed that, the ammeter showed no sign of drain.
I would have thought the radio drain would be insignificant, but there must be something wrong internal to the set.
At any rate, it's raining here for a couple of days, so that will be a good test - don't usually drive the BGT in bad weather. If the battery still starts the car after that, then the radio must be the problem.
I will, however, probably have the alternator checked anyway.
Thanks,
Pat
Patrick Callan

I recently had the same problem only it was with a CD/radio. With a CD, the eject mechanism is always hot with the memory & clock. In this case the Cd label on the RW disc had bubbled & was jamming in the player which was constantly trying to eject it. The battery ran down very quickly.It was drawing 480 milliamps. I discovered the source when putting an ammeter across the battery switch & thus closing the cct ,there was a distinct click sound from the CD/radio. Ejecting the CD stopped the drain on the battery. So, remembering never to leave a disc in the player again, I thought all was OK. Again the battery ran down only this time the drain was 240 milliamps with no disc inserted. Needless to say, the Cd/Radio has been replaced. Barrie E
B Egerton

Is the unit otherwise working OK? Ordinarily the drain should be very small and not cause a problem.

A friends radio recently packed up and one of the symptoms was draining the battery, at the moment he is removing the bottom fuse overnight until we can get time to look at it.
Paul Hunt 2010

Unit only has the radio, clock, and tape drive. All seemed to be working - so I really didn't expect it to be the problem.
I probably should have noted the amps being drawn, but the radio was the only thing that stopped the drain when disconnected.
I'll try to do a bit more diagnosis on the radio, but if the car starts after the next couple of days of rain, I think I'll take that as proof.
I did notice that the on/off button on the radio works correctly, so the issue must be in the clock/memory circuit.

Pat
Patrick Callan

Could there be a short in the wiring going to the radio? The wire could have rubbed against a piece of metal and caused a short circuit in the radio wiring harness. RAY
rjm RAY

Possibly a partial short (but then that's likely to be erratic) or it would have blown the fuse, unless there is enough of a bad connection upstream of the short to limit the current to below the fusing current!

We don't know what circuit is powering the radio memory (and clock), usually it would be the purple circuit as that is fused but always on. The horns and interior lights come off that fuse, if there was bad connection in that circuit as a whole the horns wouldn't sound, and the interior light would go out when you tried to.
Paul Hunt 2010

Well, I bench tested the radio "unswitched" circuit with a 12v source, and it was only drawing about 10 ma. Doesn't seem like that should drain a battery over night, but detaching that was the only way to stop any indication of drain.
Hopefully, better weather tomorrow, so I should find out if it starts after a 3 day sleep.

Pat
Patrick Callan

After sitting for 3 days without the radio connected, started right up.
I bench tested the radio with a 12v source, and there is only about a 10ma draw on the clock/memory power line. Seems normal, and very low to be causing battery problems. However, there is also about a 3ma draw to the positive output for each speaker. Could that indicate some sort of internal short or issue within the radio case?
Really stumped - 10ma should take forever to drain a fully charged battery.

Pat
Patrick Callan

I had a problem for years with my Midget. Did you have it wired so you could play the radio with the ignition key off? if so, that's your problem. wire it correctly so the key has to be on for the radio to work, and you'll be fine.

Don
don g

10mA still seems too high to me just for a clock and memory, that's what alarms and immobilisers settle down to. I'm sure I measured mine at about a tenth of that - clock, radio and flashing LED. However even 10mA shouldn't be enough to cause a problem overnight unless the batteries are very weak.

Is the 3mA through each speaker part of the 10mA? Or additional? As you bench-tested the radio I'm guessing it's additional, and in any case that definitely doesn't seem right to me. You must measure the standing current with the radio connected, as it could be quite different to
unplugged.
Paul Hunt 2010

Paul,
The 3mA was a separate measurement from the 10mA, and was the same at both the left and right speaker output.

Car started fine all day today (after a 3 day layoff). Right now, I'm pretty convinced that something is amiss with the radio. Will update if the battery shows any signs of drainage without the radio.

Pat
Patrick Callan

This thread was discussed between 25/04/2010 and 29/04/2010

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