THe Battery on my Polo drains for no reason ..what could be

[1.4Polo]

The cause?

Here’s is the story.
The car had trouble turning over the starter motor

we measured the voltage at the battery terminals with the car running
and it was 13.3-13.8 volts

When the car was turned off the terminals were putting out 12.3-5 ish volts and the charge would drop over the 10 minute peroid we watched it

So we thought it was the battery
And replaced it with a new item
But this morning it would not start.

It seems for some reason all the charge has drained from the battery for some unknown reason.

The airbag light has also come on and will not go off
Is there some electrical problem that I should look for which is causing the battery to drain or is it just the alternator

[8080]

I'm not an expert on this, but I would have expected around 14.2v when the car was running, and drop to about 12.5v ish after being off a few minutes, but never go much below 12v. Could well be the alternator or the new battery is bad.

You could check the battery by fully charging with a battery charger it then watching for a big voltage drop after you disconnect it.

Other thing is, have you recently fitted another head unit or something that is drawing power when it shouldn't?

Some modules are sensitive to voltage drops - this might explain the airbag module fault

[paulzpolo]

is not something stupid like the interia light is on all the time (set in wrong position) or your parking light is left on

[Subtle Changes]

had same problem with mine, turned out the boot light was on constantly.. so i just took it off!

[DanOliver]

Do you have a subwoofer or speaker amplifiers anywhere in the car? It's possible the remote on/off cable has become disconnected and the amplifiers are running constantly.

My 800w sub was on and running while my car just had the ignition on and I was watching the voltage on the battery drop..

Just an idea

[PeterT]

I had the same problem with my daughters Polo; we too fitted a new battery. You need to find the drain I followed this procedure and found it was the stereo headunit. The Polo does not swith off the live feed the stereo had blue tooth we changed the unit for a simpler one. You need to follow the procedure and you need an AMP meter not a volt meter - Maplin for about a fiver. The current drain should be no more than 50-75mA or 0.05 - 0.075A. The procedure is not complex but requires patience.

How to Find a Parasitic Battery Drain
When your car battery goes dead overnight, usually either the battery is at the end of its life span, or you left something on, such as a light. Occasionally something is drawing power that’s not of your doing. This is a parasitic draw, and it can cause the same result as leaving the headlights on: a dead battery in the morning.
Steps
1. Remove the negative side battery cable from the negative battery terminal.
2. Attach an ammeter(this measures amperage) between the negative cable and the negative battery post. wait a few seconds to several minutes for the car to go into sleep mode. i.e. when you make the contact with the test light the cars computer systems "wake up" after a bit of time they will go to "sleep".
3. If the ammeter is reading over 50-75mA (milliamps), something is using too much battery power.
4. Go to the fuse panel(s) and remove fuses, one at a time. Pull the main fuses (higher amp ratings)last.
5. Watch for the ammeter to drop to acceptable drain. The fuse that reduces the drain is the draw. Consult the owners' manual or service manual to find what circuits are on that fuse.
6. Check each device (circuit) on that fuse. Stop each lamp, heater, etc. to find the drain.
7. Repeat steps 1&2 to test your repair. The ammeter will tell you exact numbers.
EDITERS NOTE: The original author listed this repair using a 12v test light. As an ASE certified mechanic, the best place for your test light is the trash. A test light doesnt measure voltage; it shows if enough voltage is present to light the lamp.
1. An ammeter meaures current (in amps), and that is what you are fighting against if you have a parasitic drain problem.
2. Harbor freight sells a $3 digital multimeter (dmm) that has an amps setting. I use a Fluke, but it's whatever your trying to spend your dollars on.

Tips
• A parasitic drain is when an electrical device is using battery power when the car is closed, and the ignition key removed. Therefore, when doing this test make sure that the dome light, under hood light, trunk light, etc. are off.

Warnings
• Don't forget to check inside cigarette lighter and power sockets. Sometimes coins can fall in and cause shorts.
• Some after-market alarm systems may make this test too long or loud to be worth the effort. If that's the case, seek professional help.
• Be careful working around the battery in a car.

Things You'll Need
• A digital multimeter or ammeter.
• A fuse puller.
• Any tools needed to access battery and fuse panel(s).
• An owners' manual or maintenance manual showing electrical circuits.

[1.4Polo]

Thank you to everyone who took the time to respond
i did not know anyone had replied as i was sent no emails to notify me

the problem was a rectifier on the Alternator
so it was replaced and the car seems to be ok
but if the problem comes back i will check the boot light and run through the detailed steps peterT kindly put up

Thanks again