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My 2002 Honda Odyssey has a weak battery. It is at the end of its expected life and I will be replacing as soon as I can.

The ignition switch has the following positions:

  • 0 - Off
  • I - Accessories
  • II - Run
  • III - Start

I notice that if I leave the key in position II with the engine NOT running, the battery drains rapidly - I can no longer start the car after about 5-10 minutes.

I have a hall-effect DC clamp meter, so I measured the drain in this situation. The meter is not very accurate, but its probably good enough for a ball-park. With the key in position II, the engine not running, and as many systems (lights, dome lights, radio, fan, etc) switched off, the current drain is about 8A.

Is this normal?

Is this drain due to alternator field coil energization? Or what else would cause it?

To clarify - normally I switch the car off and take the keys out right away, but it turns out that I am human and sometimes get distracted. Sometimes the keys get left in this position by mistake.

My question still stands - What is the expected current drain in this position (when all accessories are off), and what causes it?

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  • The bigger question is why you would leave the car with the ignition in the Run position for ten minutes.
    – user16128
    Commented Mar 29, 2020 at 4:30
  • 1
    Why do you want to know ? Leaving your ignition switch in the run position with the engine not running instead of in the accessory position is not necessary. that’s why the accessory position is there. What is the problem you’re trying to solve. None of these tests make a difference once you get a new battery.
    – Alaska Man
    Commented Mar 29, 2020 at 16:44
  • @Jeeped Normally I switch off and take the key out. Unfortunately I am human and sometimes forget. Or I put the keys in, about to set off, then get distracted b y something else for a few minutes. It is those distracted moments for which I get punished with a dead battery.
    – Digital Trauma
    Commented Mar 29, 2020 at 16:55
  • Come on now. It doesn't matter WHY someone has the key on for extended period of time. The only problem is the battery dying so quickly. If it will make you feel better to have a reason for the ignition being on then one example of requiring the key to be ON and engine OFF would be while your running diagnostics.. Honda systems take a longer time to run full diagnostics then most for some reason so 10 minutes is not unreasonable.
    – narkeleptk
    Commented Mar 29, 2020 at 18:11
  • @DigitalTrauma You are getting "punished" because you have a failing battery and you are a forgetful human, both are things within your control to change. If you can remember the results of your test and not leave the key in the run position for longer then that amount of time then you can remember to not leave the key in the run position at all until you buy a good battery. And if you need to run the diagnostics ( as narkeleptk points out ) you should have a good battery.
    – Alaska Man
    Commented Mar 29, 2020 at 18:57

1 Answer 1

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8A comes to 96W, that seems high but certainly not enough to drain your battery in a few minutes. Most auto batteries have a capacity of about 40-45AH, which means it should be able to supply your 8A for 5 hours before the battery is drained completely.

I'm guessing that your real issue is the bad battery, get it replaced and then get a better meter to read the current drain with the car OFF and the key OUT.

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  • Thanks. The drain when the car is OFF and the key is OUT is zero or close enough. I can leave the car off for a week or two and not have any battery problems. The issue comes when I leave the keys in, e.g. sometimes I forget to take them out until i've unloaded groceries, for example, then the battery is dead.
    – Digital Trauma
    Commented Mar 29, 2020 at 1:41
  • Why do you stop the engine then turn the key back to run while unliading groceries? Leave it off... @DigitalTrauma
    – Solar Mike
    Commented Mar 29, 2020 at 5:38
  • @SolarMike See the clarification at the bottom of my question
    – Digital Trauma
    Commented Mar 29, 2020 at 17:02

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