Parasitic battery drain or bad new alternator

During the last hour of a nine hour trip, battery charging system light came on our 2009 Honda odyssey. We eked out the last 35 miles and got to a place that tested the 3.5 year old battery. It showed it all but dead and they put in a new one. Alternator in the car is less than one years old. Car started up fine and we drove it to our destination 5 miles away.
Next morning, the battery was completely dead. Had it towed to a local shop and they said it was just a loose connection. They charged it overnight and it started up again. We asked them to keep it for 24 hours to make sure, and when we picked it up this morning, again it started up without a problem.
Just drove it 2 miles down the road, let it sit for two hours and the battery is almost dead again.
If it’s a parasitic drain, why would a battery light come on while we were driving and the alternator was charging it? If it’s a faulty alternator, why would it be dead just sitting overnight. 10 hours from home and 2 days left before we have to head for home.

Just a thought…
Maybe the connection came loose again. if so, some connectors have a plastic tab to hold it together. if it breaks the connector can come loose from vibration. it might need to be replaced.

Not loose. Just fixed by the repair shop 24 hours ago. And I didn’t think loose connectors were a cause of battery drain while parked.

If the battery was fully charged (that’s a big if), I don’t know that driving the vehicle 2 miles would be enough to kill it. You can get an alternator tested for free at most parts stores. If the newer alternator is a cheap parts store brand (Duralast), it may be bad. Parts stores will test the battery too. Check the battery first, then check the alternator. Generally, it’s one or the other and the wrong one is often replaced

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That sounds like an alternator problem. A (good) shop wouldn’t automatically replace the battery w/that symptom. Instead they do a charging/battery system test first.

Here’s a diy’er version, requires knowledge how to work a DVM.

Before first start of the day (after car has been sitting unused overnight) the battery should measure about 12.6 volts. Then immediately after starting the engine 13.5 - 15.5 volts. I’m guessing your car won’t pass that test.

when you said it was a loose connection, my thinking was the connector that plugs into your alternator came loose or out. they plugged it back in, charged your battery a little. if the connector fell right out when you started the vehicle it was running on the battery all the way to your destination, draining it.
but like I said it was just a thought. bring it back to the shop who supposedly fixed it. best of luck.

WAG: bad diode(s) in the alternator.

A faulty diode bridge or regulator inside the alternator can cause the battery to drain, even if the car’s not running. It happened to my 1992 Buick many years ago.