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Some things are just bad design. Earlier this year, after a few months of lockdown, I discovered that even car batteries that were in reasonable condition can

Some things are just bad design. Earlier this year, after a few months of lockdown, I discovered that even car batteries that were in reasonable condition can go completely flat without much warning if the vehicle is only being driven rarely over an extended period. I was aware of the possibility of the small power draw draining the battery over time, but hadn’t expected such a strong effect so soon given I was still driving the car a few miles a couple of times a week. That was annoying enough, but then it turned out that the alarm system in my car has a backup battery to continue powering the siren in the event that the main battery is disconnected, presumably someone’s idea to stop a thief from silencing the alarm quickly. That backup battery can run the siren for several hours, obviously without reference to where the vehicle is or what time of day or night it is. Moreover, if you’re unlucky, when you try to unlock the car the failing main battery can have just enough juice in it to trigger whatever condition makes the alarm system think the main battery is being disconnected but without actually disarming the alarm system, thus setting off the siren. Which you can’t stop without power from the main battery. Which has just run out.

ehnto 6y ago
It's usually a voltage drop detection that sets of the alarm. If your battery is going flat, or is tired, it can drop below the voltage threshold set in the alarm it off. You may still be able to start the car. You may have a bad alternator though, if it's not the battery. It really depends on the car configuration, but it is surprising that even that short trip doesn't recoup the cost of starting. You can get alternators and batteries that are better at your use case too, work site utes and shuttles often get used like you describe.
- Chris_Newton 6y ago
  Yes, the voltage drop was the likely trigger for actually setting the alarm off. When I had the battery replaced, they did a quick test on the old one, and it was performing very far below normal. With the wisdom of hindsight, I should probably have left a trickle charger on the old battery now and then while it wasn’t getting much long distance driving to keep it charged up. I was just a bit surprised that there was so little warning given the dire state it had apparently reached before the alarm incident. But of course, the real problem here isn’t that a battery with unusually low usage drained faster than I incorrectly anticipated, it’s that my own car had an alarm going off that I had no reliable way to stop even standing next to it with the key in my hand. Clearly this should never be possible. In fact, I read that a lot of vehicle security systems do have some sort of emergency override for this sort of situation, but I found nothing relevant in the documentation that came with the car, and unfortunately the various suggestions I found online for similar models didn’t work on mine.
  - ehnto 6y ago
    My opinion and experience with car alarms is that they go off so frequently in error, that no one pays any attention to them. I have heard countless alarms go off, and not once seen a car broken into. I think it's better just to have a comprehensive immobiliser. I have also had my car stolen. They had the alarm disabled in seconds, and then had all the time in the world to get it running. The only reason I still have that car is because they stalled it in the middle of the street, and the immobiliser kicked back on, so they fled. That car is now mostly a track car, so it's actually got a pin-locked ECU that I pin in before driving it. No amount of hotwiring would get the ECU to work without that pin. Even still, it would just need to be dragged onto a trailer and I'd never see it again.
aasasd 6y ago
Btw, aside from the old battery, you might have something in the car that drains it excessively. IIRC I've had the audio system plugged in a wrong way and it put a small but considerable constant discharge on the battery, even when the car was off. Alarms themselves can do this too, I think.
KineticLensman 6y ago
Yes. I spent a month in lockdown not able to access the things I'd left in my car's boot (trunk) which can only be opened when the battery-powered central locking is available. Anecdotally, once some modern car batteries die in this way, they fail to hold a charge at all, so classic jump-starting doesn't work. So I only replaced the battery when lockdown was over, to avoid the same problem happening to a new battery.

ehnto 6y ago

It's usually a voltage drop detection that sets of the alarm. If your battery is going flat, or is tired, it can drop below the voltage threshold set in the alarm it off. You may still be able to start the car. You may have a bad alternator though, if it's not the battery. It really depends on the car configuration, but it is surprising that even that short trip doesn't recoup the cost of starting. You can get alternators and batteries that are better at your use case too, work site utes and shuttles often get used like you describe.

Chris_Newton 6y ago
Yes, the voltage drop was the likely trigger for actually setting the alarm off. When I had the battery replaced, they did a quick test on the old one, and it was performing very far below normal. With the wisdom of hindsight, I should probably have left a trickle charger on the old battery now and then while it wasn’t getting much long distance driving to keep it charged up. I was just a bit surprised that there was so little warning given the dire state it had apparently reached before the alarm incident. But of course, the real problem here isn’t that a battery with unusually low usage drained faster than I incorrectly anticipated, it’s that my own car had an alarm going off that I had no reliable way to stop even standing next to it with the key in my hand. Clearly this should never be possible. In fact, I read that a lot of vehicle security systems do have some sort of emergency override for this sort of situation, but I found nothing relevant in the documentation that came with the car, and unfortunately the various suggestions I found online for similar models didn’t work on mine.
- ehnto 6y ago
  My opinion and experience with car alarms is that they go off so frequently in error, that no one pays any attention to them. I have heard countless alarms go off, and not once seen a car broken into. I think it's better just to have a comprehensive immobiliser. I have also had my car stolen. They had the alarm disabled in seconds, and then had all the time in the world to get it running. The only reason I still have that car is because they stalled it in the middle of the street, and the immobiliser kicked back on, so they fled. That car is now mostly a track car, so it's actually got a pin-locked ECU that I pin in before driving it. No amount of hotwiring would get the ECU to work without that pin. Even still, it would just need to be dragged onto a trailer and I'd never see it again.

aasasd 6y ago

Btw, aside from the old battery, you might have something in the car that drains it excessively. IIRC I've had the audio system plugged in a wrong way and it put a small but considerable constant discharge on the battery, even when the car was off. Alarms themselves can do this too, I think.

KineticLensman 6y ago

Yes. I spent a month in lockdown not able to access the things I'd left in my car's boot (trunk) which can only be opened when the battery-powered central locking is available. Anecdotally, once some modern car batteries die in this way, they fail to hold a charge at all, so classic jump-starting doesn't work. So I only replaced the battery when lockdown was over, to avoid the same problem happening to a new battery.