AC drains battery while running

This is the first time i’ve posted here but I really don’t have any where to turn. Im stumped. Im driving a 2012 Honda Pilot. For some reason on this vehicle, when the battery gets low the transmission bucks and slips and acts up. So that being said, today it started acting up and of course the battery light came on telling me that the the voltage was low. I thought maybe alternator or loose battery terminal but no. After testing it several different times it was the AC. The battery light goes out and the car runs normal when the AC is off. But after I turn it on it takes minutes before the voltage light comes on and the transmission starts acting up. Any ideas? Im completely stuck.

There may be a problem with the Electric Load Detection in the charging system.


What does battery voltage measure before vs after actvating ac?

The voltage before I turn on ac is a steady 14.4. After I turn on AC it begins to drop about .5 volts a second. Until the voltage light shows and after the AC is turned off again it begins to go back to normal just as fast until its back to 14.4 volts again.

How old is the serpentine belt and have you checked tension to be sure tensioner is working? A bad idler pulley could be loading it down as well. The A/C load pushes it over the edge and it slips…

I just replaced the serpentine belt about 3 months ago. The tensioner look good

I suggest you have the charging system load checked at a shop to make sure the battery and the alternator are working like they should be. If those areas are okay then have the current draw checked for the AC system to see if that is within limits. I assume there is no slipping of the drive belt since you have already touched on that.

Ok thanks everybody for your help. Today I found that the blower without the AC is draining the same. So I guess its not the AC after all.

I was wondering about other large loads like that also causing the issue. I suspect you are going to have to replace the alternator, unless there is a problem with the wire connection between it and the battery. A high resistance in that connection would cause an issue.

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Thank you all for everything. I also would like to say that today I took the car to advanced autoparts to have it tested. Everything tested fine(with out taking the alternator out) however, now the car is fine. With a FULL load (defrosters, ac, heated seats, etc) the car idles at 13.1v and moving is right around 14v. I ran the AC for a full hour. Turned it off and on. Turned the car off and on. Played with switches, radios and wires. No matter what I do I cant get the voltage to drop below 13v. Uggg. Theres nothing worse then an intermittent problem

Some Hondas have had issues with a faulty AC relay. That’s all I know, but it gets blamed for killing batteries.

You may have had a partially discharged battery and now it is fully charged. Ive had a battery discharge bc the brake light got stuck on for example. Best of luck.

You are welcome for the help. My best guess for the problem is the main output lead from the alternator had a connection problem to the battery and there was an added resistance in the line. Things will be fine with a small current load so only a small voltage drop will occur across the bad connection. When more current is needed to charge the battery then a significant voltage drop will happen and the battery can’t the current it needs to keep the charge up on it. Look for signs of heat damage at the connections. If you find something then you kind of found a smoking gun.

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That happened with our 05 Odyssey…it was causing a huge drain (it was higher than 80 miliamps, probably way higher than that, but I don’t remember). Once we replaced the relay, no problems with the battery dying overnight.