Car Alarms and Flat Batteries

Most of us will not have been using our vehicles much over the past 4 weeks, so it should not be a surprise if you find your battery has gone flat when you come to use it again.

Statistics show that about 40% of AA call-outs are normally due to battery issues. Since the Covid-19 lockdown, this figure has risen to 60%.

Batteries don’t get much of a chance to recharge with short journeys, so unless you’ve got a battery charger or have been running your vehicle periodically you’ll probably want to make sure it’s good before you need to drive it once more.

Now, this may sound obvious, but if your battery is flat, then your alarm will not work either!

Here are some of the most frequent questions I get asked:

Will my alarm still be armed when the battery is recharged?

Most alarms have a none volatile memory. This means that the alarm settings will still be the same after the battery has been disconnected and reconnected. So if your alarm was armed before the battery went flat it will still be armed when you come to recharge it.

There are some alarms (for example Viper 3100) that will default to the armed state when the battery is reconnected. The old myth that disconnecting the battery to re-set the alarm simply does not work!

If my battery is flat why does my siren still go off?

If you have a battery back-up battery siren which is in better health than the car battery then it’s simply doing its job! (You can read more about which battery back-up sirens have decent batteries here). Note that the siren battery only powers the siren and not the alarm. This explains why your remote will not disarm the alarm and stop the siren from sounding.

Once the car battery is good again, you can disarm your alarm with the remote.

Why does my over-ride pin number not disarm the alarm?

The battery needs a good 12 volt supply before it will work. If the alarm does not have power then the pin number over-ride will not work either!

Will the car alarm drain the car battery?

All alarms will drain a car battery over time. However, some alarms will flatten your battery much quicker than others.

AVS alarms will flatten your battery over 3 times faster than a Vodafone Automotive alarm. Some popular GPS Trackers will kill your car battery in under week too so be warned!

Tips for looking after your battery

If you are leaving your vehicle unused for more than 2 to 3 weeks (make that one week if you have an AVS alarm!) then I suggest doing one of the following:

  1. Get a battery maintainer and leave it connected when the vehicle is not in use.
  2. Disconnect the car battery negative terminal (disarm your alarm before doing this).
  3. Run your car for over 30 minutes once per week.
car alarms and flat batteries

Car Alarm Flat Battery Issue

Your battery is already flat and you can’t use your car!

Either call the AA, try jump-starting the vehicle or get a battery charger. If your battery is old then it may be time to replace it too.

Some “smart” battery chargers will not start charging the battery if the voltage is already super low. To get around this you can jump-start the process by hooking up a 2nd battery so that the smart charger sees a voltage which then kicks it into life. 😉

Jump starting?

Make sure that you follow the vehicle manufacture instructions if you are going to jump-start your car.

Please get the leads the right way around. If you are in any doubt then simply don’t do it as you can do expensive damage to some cars if you cock it up!

I’ve lost count of how many times I’ve looked at a vehicle where some major fuses have been blown, yet the owner has denied that they got it wrong! 🙄

About Julian

Owner and Installer at Obsessive Vehicle Security Limited. More details here... Please keep Comments relevant to the post and use the Contact form for enquiry's
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2 Responses to Car Alarms and Flat Batteries

  1. Rick says:

    “If you have a battery back-up battery siren which is in better health than the car battery then it’s simply doing its job!” Is it? I respectfully disagree. If the battery discharges to the point where the main unit stops working and the siren thinks “oh, the battery line has been cut – noise time!” then that’s really a design fault. Cutting a battery line to break into a car is sudden event and the dying battery is very gradual. The truly smart system would be able to tell the difference and convey that information to the siren so it can go into an “alert” mode (say by chirping once in a while) so you can deal with the battery without the panic of a loud siren that can’t be disabled. Hell, the main unit itself could do that before the bettery actually dies completely.

  2. Julian says:

    Hi Rick,

    Thanks for the comment, you make a very valid point. I should have clarified that I was referring to triggering the alarm at the time you go to recharge it.

    Vodafone Automotive (Cobra) sirens are smart enough to know that the battery is slowly running flat and do not go off when this happens.

    The alarm going off actually happens when you go to charge the vehicle battery. The sudden change in voltage triggers the alarm.

    Once the battery has a good enough charge you can then disarm the alarm. The problem occurs when the vehicle battery is so low that it takes time to get a good enough charge to disarm the alarm.

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