Auto Battery Question

Moonlite37 · Mar 5, 2017

I have a battery that goes dead in two weeks but will start the engine after a couple days after a charge. I took the battery back after 3 months at the big battery place tested it with a modern hi tech tester and they declared it good and charged me $4 tor the test although I had bought the battery there. I van find no drain through the alternator. I called another location of the same store and the so-called technician said that it is entire possible , but very unlikely that a battery can test good with a modern load tester and still go dead. So far I have had the battery unhooked and waiting -----

Steve@Advance · Mar 5, 2017

Letting it sit disconnected will tell if the battery is self discharging, but it won't tell if it is getting a proper charge while running.

Not sure what you have the battery in, but anything with electronics needs to be tested for drain with a milliamp meter, in series between the battery cable and the post. The reading should level out to near zero once everything is off.

Might want to check the charging system to see if it is charging as it should. Test it under full load, lights and accessories on, engine up to speed. It should maintain around 14.5 volts. Things like loose or burned belt, bad diodes, high resistance electrical connections can cause an alternator to partially charge, be unable to carry the load under high demand conditions.

Stephen Newell · Mar 5, 2017

Disconnected a good battery will last for months before beginning to run down. If it doesn't run down disconnected then the problem is with the vehicle. I run all the electrical on my tractor through a fusible link. Left connected the battery will run down in a couple weeks so I pull the fuse when I'm not using the tractor.

notjustair · Mar 5, 2017

I had a radio in a car that suddenly went wacko. It did what you are describing. I don't know how, but it acquired a sudden parasitic draw. I removed the memory fuse and now it's fine. I have to tune in the station when I start the car, but I'm more inclined to listen to the engine music in my old cars anyway.

Paul from MI · Mar 5, 2017

Friend had a similar thing on a 1969 chevy. Turned out to be a mercury switch that came out of its bracket, left the trunk light on all the time. took a while to find.

Mike Groom · Mar 6, 2017

Have them do a load test on the battery.Had the same problem on a battery that was 2 months old with a 72 month warranty,took it back tested good with there two prong tester they had at the counter,told them to do a load test on it,had to get the manager involved at the Farm & Fleet store,but it failed the load test,and they replaced it at no charge.This happened to my neighbor also about a year later.I would find a different store if they charged me to test a battery I bought there!

JDEM · Mar 6, 2017

All lead-acid batteries have a self-discharge rate. Usually around 5-10% per month. Load testing will not show if that rate is excessively high.

Did you check milliamp draw between a battery cable and battery post? If so, what is it? Up to 100 milliamps on a newer car or truck can be normal. If higher, you have other issues besides the battery.

El Toro · Mar 6, 2017

I would install a master switch that will isolate the battery from being grounded. The military has had that for years. Hal

Auto Battery Question

Moonlite37

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Steve@Advance

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Stephen Newell

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notjustair

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Paul from MI

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Mike Groom

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JDEM

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El Toro

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