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I have a Honda Prelude 1998 and if the battery is connected (with car off), after 2 days it is mostly drained. If I recharge it and disconnect the battery, it has a good charge even after 10 days (I know it has a good charge because the charger says so when I try to charge it, and also the car starts smoothly).

The battery is around 2.5 years old.

I have followed some instructions on testing a parasitic battery drain and with everything off, the multimeter is showing a drain of 0.02 (with multimeter set on 20A; I guess that's 20mA) which seems normal.

Is there any advice I could get on what to look at next?

Could the battery have a fault that manifests only when the battery is connected (considering the charge stays there for a long time if completely disconnected)?

I will have to do more testing to confirm, but at some point it looked like the multimeter stayed at 0.02 without any change, but after 30-40 seconds it went up to 0.06 without anyone touching anything in the car and then it went back to 0.02

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  • How are you identifying that the disconnected battery has a good charge after 10 days? Will it start the car when reconnected?
    – HandyHowie
    Apr 26 at 9:07
  • @HandyHowie just updated my question with that info
    – Roco CTZ
    Apr 26 at 9:11
  • How old is the battery?
    – HandyHowie
    Apr 26 at 9:13
  • @HandyHowie about 2.5 years old
    – Roco CTZ
    Apr 26 at 9:16
  • D you have an alarm or security system?
    – mikes
    Apr 26 at 11:41
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Lead acid batteries can be very deceiving about the amount of charge the have stored.

A new battery may be able to provide the current to run a starter motor for several minutes, whereas an old or damaged battery may only be able to run a starter for 10 seconds. Because the engine starts within those 10 seconds you don’t realise just how poor the battery is.

Drawing those 20mA from the battery for a while can flatten the old battery, leaving it unable to start the engine.

I would suggest first replacing the battery, there may be nothing wrong with your car.

To identify a real drain, you need to start removing fuses one at a time to see if you can identify which circuit is causing the drain.

A drain can also be cause by the alternator, which may require disconnecting the large cable that goes to the alternator from the battery. Obviously make sure you reconnect everything before trying to use the car.

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  • I am not sure it's the battery since I had this kind of problem with the previous battery which turned out to have a dead cell inside (I don't know why). Would it be a relevant test to connect the battery, but leave the alternator disconnected for a couple of days? (without using the car at all, of course)
    – Roco CTZ
    Apr 26 at 9:35
  • Yes, as long as you don’t start the engine with the alternator disconnected you can disconnect it from the battery terminal. Just to be on the safe side, wrap some insulation tape around any disconnected wires.
    – HandyHowie
    Apr 26 at 9:40
  • You should be able to use your multimeter to measure the current being drawn by the alternator. Just don’t try to start the engine with your multimeter connected.
    – HandyHowie
    Apr 26 at 9:48
  • Thanks for all the info. Can you please tell me more about how you measure the current drawn by alternator, please?
    – Roco CTZ
    Apr 26 at 9:50
  • Just disconnect the large cable from the battery positive post that goes to the alternator, then put the 2 probes of your multimeter in between them with the multimeter set to the 10a current range like before.
    – HandyHowie
    Apr 26 at 9:57

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