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Car stereo parasitic battery draw

I had a problem with my '99 Miata needing frequent jump starts even though the battery is only about 18 months old. My mechanic did a parasitic load test and found that my aftermarket stereo was responsible. He pulled the fuse and since then the car starts like a champ.

So my question is, how can I know if this is going to be a problem with a stereo? I wouldn’t mind replacing this one, but how do I know if a new one would do the same thing? I looked over the manual for the one I have and it doesn’t give any useful clues.

What’s probably happening is the stereo isn’t going to sleep once the vehicle is shut off causing the current draw.

A new stereo fill fix that.


Tester, how would I know that a new one won’t also fail to sleep properly?

It could also be that the current stereo isn’t wired in properly. Most stereos have two power sources, one constant and one that’s only live when the key is on. At any rate, a new stereo wired in correctly should solve your problems.

Because that’s controlled by the electronics within the stereo.

A new stereo, new electronics.


Tester, so you think it’s my specific unit that’s the problem rather than a problem with the way the model is designed? That wasn’t clear at first.

asemaster, I thought of that, but if it were wired wrong wouldn’t it stay on even when the car was off? It seems to work normally.

Here’s what you have to look at.

The aftermarket stereo was installed and it worked fine?

Now after some time, the stereo is now drawing parasitic current draw?

It tells me the stereo isn’t going to sleep.


You need to connect the stereo to a fuse that does not have power when the ignition is off.


How is the stereo suppose to remember the preset stations programmed into the stereo if the power is totally cut off?


Barkydog, if I do that, it’ll lose its settings every time I turn the car off.

Tune it to NPR before you turn the car off and there would be no problem sleeping :smile:

Now, is there anything else on the stereo circuit? With the car off, is the stereo doing anything? Is there an additional amplifier etc hooked up?

I agree that all failing a new stereo might be the easier way to go. But you might want to make sure that this will fix the problem. I would first check the wiring for the stereo for anything unusual, shorts.

Tester, I’m not sure it was ever OK, but it was only recently I thought to ask my mechanic about it. The previous battery only lasted three years. At the time I just kind of scratched my head but now I wonder if the stereo was dragging it down then too.

Galant, with the car off the stereo doesn’t do anything. I also didn’t add an amplifier when I put the stereo in. I don’t think there’s anything else on the circuit, at least, pulling the fuse doesn’t seem to have killed anything else. And, it’s tuned to NPR anyway. :smile:

@tester good point, but if the radio is killing the battery there does not appear to be an alternative.

How many Milliamperes is the stereo pulling? Since the odds of a stereo pulling enough to kill a battery quickly are not very high I might ask if the car has an aftermarket amplifier also?

Any chance of a power antenna lead being incorrectly connected?

Make of stereo?

If it’s a Half Tone or Mute Master, there may be a problem with the stereo.

Sorry! Couldn’t resist!


I think when the OP wired in this stereo it was wired wrong and is always drawing power.

I hope you still have the wiring instruction for the stereo. If you wire the new one the same way…you’ll have the same problem.


As ok4450 says check the current draw of the stereo. The easiest way to do this is pull the stereo fuse and connect the ammeter probes directly to the fuse socket terminals.
By the way how long does it take for the battery to run down?

If you want to buy a replacement stereo with a known maximum off-state quiescent current you’ll probably have to contact the manufacturer and ask for that spec on the one you are thinking of buying. They should have it, but might not publish it since few people would know what it means. I don’t see it published in the typical sales brochure specs very often if ever.

Have you tried turning the existing stereo completely off before turning off the ignition? That might fix the problem for you and you won’t have to buy a new stereo. As long as you can remember to turn it off every time.

Replace the stereo…your old one has an internal switch fault…the new one will be NEW without the internal issue. This is the solution…stop second guessing the obvious. I installed stereos and remote starts/alarms professionally for over 12yrs. This is a semi common failure in stereos…has to do with relays on the circuit board that get stuck…they are so tiny that sometimes they corrode closed and or just fail and cause current draw/don’t shut off.

New Radio…No Problemos


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