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I have a Honda Prelude 1998 with the following symptoms:

  • keep the fully charged battery connected for 2-3 days without driving and it goes almost flat (disconnect it and the charge stays for weeks at least)
  • when the battery is almost flat and I disarm then arm the alarm back, few seconds later the alarm starts ringing (starting with today it does this on a fully charged battery as-well)
  • when I test the ampers with a multi meter and the alarm is disarmed, I get a constant 0.02A - 0.03A, but right after I arm the alarm, it keeps fluctuating anywhere from 0.03A to 0.08A a couple of times per second

My questions:

  • is it normal for the ampers to fluctuate like that when the alarm is on?
  • could this fluctuation cause the battery drain?
  • could a faulty/almost empty battery alone cause the alarm to ring?
  • what can I try to help me identify the issue?

What I tried so far was removing the fuses from the engine fuse box (not the interior one) one by one, but the amperage was still fluctuating.

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  • Several things: You need to do the same with the interior fuse box ... if you didn't find it on this engine fuse panel, the draw should present itself on the inside fuse panel; An amperage draw of .03A to .08A really isn't much of a draw at all ... it's definitely not something I'd be worried about; How old is the battery? May 31 at 11:07
  • @Pᴀᴜʟsᴛᴇʀ2 I think it's about 2 years old, but we barely use that car. What worries me is that it goes almost flat and causes the alarm to ring.
    – Roco CTZ
    May 31 at 11:22
  • Have you considered getting a battery tender for when you're not using the vehicle? It'll prevent the battery from going flat, which will not allow the alarm to go off. I'm talking about a tender, not a trickle charger. May 31 at 18:15
  • @Pᴀᴜʟsᴛᴇʀ2 yes, I have considered that, thanks for the suggestion
    – Roco CTZ
    Jun 1 at 9:22
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+100

If I understand you properly, your battery drains dead with the alarm connected, but if not connected, the battery drains and dies. So it's obvious that the problem is the alarm. So the question is...how? Here you have to go back to the old Radio Shack electronics kit. (I'm showing my age lol) The alarm control unit uses the car battery. It sends voltage over wires that by using capacitors, diodes, etc creates an "easy path" and a "hard path" for electricity. When following the easy path(s) from the positive battery terminal to the negative, all is well. If you interrupt the path, the electricity has to go somewhere, so it goes to the siren. The siren usually has a backup battery in case the primary fails, but it doesn't run the alarm.

That's an oversimplification but if you want to learn more there's plenty of info already written you can search for.

So what can happen is if you have a wire or switch or something else partially grounding, it will constantly feed the ground with power from your car battery. Eventually it gets low enough that it can trigger what some car alarms have which is a tamper circuit using the second battery if power is removed.

To correct it, put your meter on and disconnect the various runs to the door sensors and such one at a time until you see the drain stop. Then you know what leg it is. From there you can follow the wire, visually inspect, and if you find a problem, use the meter to verify.

Hope this helps! Some useful links

https://hackaday.com/2020/03/10/solar-panel-keeps-car-battery-topped-off-through-obd-ii-port/

https://shoptoolreviews.com/automotive/diagnostic-testing/parasitic-battery-drain/34587/

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  • Thank you very much for the response. I might have phrased it unclear, but the battery is fine if it's disconnected from the car, but it's drained if it's connected to the car (I have no idea if the alarm is the one draining it or not). What I know for sure is that the alarm starts ringing seconds after you arm the alarm. I haven't managed to test what happens if I completely disable the car alarm, but I guess it's pretty difficult (which is the point of the alarm I guess).
    – Roco CTZ
    Jun 11 at 7:36

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