battery question

[jamesnmlaw]

2002 BMW. A 2002 model year, not model. 325Xi. I don't drive it but maybe once a week or so, really a back-up car. that I bought for $900 and have spent about $5,000 to make it nice enough for a 2nd or 3rd car. Puts the BM in BMW.

Was experiencing an issue with the car dying when driving, going soft in limp mode, soon unable to restart, would crank but not fire. Eventually towed to BMW wrench that, after a few weeks, announced the cylinder heads were crack and needed replacing, and that I had a turn signal light out. Another couple of thousand later, the BMW people pronounced the car fixed but for a bad battery. BMW wrench charged up the battery so I could get home.

Went to FLAPS, bought a Duralast Gold in October, battery is stickered as Jan, 2023 creation. Installed (does not need BMW coding). Car starts and drives fine. Checked alternator, OK. Checked for parasitic draw using an amp-clamp, OK.

Battery drained to 0% charged within a week of no driving. Took battery back to FLAPS, charged up. Clerk said battery is fine.

Re-installed. Drains to 70% charged. Brought the battery inside the house, took about 15 minutes to charge from 70% to 100% using my Duralast charger. Left battery in house, disconnected. Battery drops to 70% overnight.

Question: do new batteries, charged to 100% (whatever that means) usually drop to 70% charged within 24 hours of sitting inside, unconnected to anything? Volts say 12.4 pretty consistently when at 70% charged. 13.4 when charged to 100%.

The car hates me. I'm beginning to hate the car.

[908/930]

I think 12.6 would be considered fully charged, yes likely higher when sitting on a charger. Is this a lead acid or AGM battery?

[masraum]

My contribution is that a car battery is 6 cells at 2.1V per cell, so a fully charged battery should be 12.6V. If you use a fast charge, then yes, you can over charge, but it'll settle down to normal. When you charge slow and low is better than high and fast. It's not unusual for a high/fast charge to be considered "surface" and then drop back down to where it really is in a fairly short period.

Still, 12.4 isn't technically fully charged. Other than that, I'm not sure what you're dealing with. I wouldn't expect a battery sitting to discharge quickly. Batteries sit on a rack at an auto parts store while fully charged, sometimes for quite a while and are usually still fully charged when you buy them.

[Scott Douglas]

I had a situation with my Honda battery going dead over a couple of weeks from sitting. It was due for replacement so I put in a new one. It died too after sitting for a while. I don't drive my car much being retired now and using my wife's car for day to day stuff.
I resorted to putting in a battery cutoff switch after replacing the 'new' battery on warranty after one year. I did some investigating into parasitic draws. Found I had an old Ipod hooked up inside that apparently wasn't shutting off when I thought it was off. Using the cutoff switch proved to be a pain so I've shut everything down now, including the dome lights, and haven't had a problem with starting the car since.
I'd charge that battery on a slow charge and see if it comes up higher than the readings you're getting. If it doesn't then I'd guess you have a bad cell.
How did they 'test' the battery after they charged it?

[1990C4S]

How are you measuring the '70%' number?

[jamesnmlaw]

The battery is a lead battery.

I have two (2) AGMs in the old Porsche, damn things hold a charge for years.

I'm going to try m Duralast home charger again today. Battery will be in the house, nice and warm. I'll test the battery tonight with the Duralast charger and with a multimeter.

Thank you for all the replies. I'll report findings tomorrow.

[908/930]

Keep in mind your multimeter can easily be off by a couple percent.

[jamesnmlaw]

So, battery is in the warm living room, charged the battery using my Duralast charger to 100% charged, 12.4 volts. The lovely Mrs. is a car nut so it's okay - her first words were, "nice car". 3 hours later, battery reads 75% charged, 12.4 volts, according to the Duralast charger.

Does that seem like a good result for a new battery?

Am repeating the experiment, scientific method and all that.

I like the battery cut-off/out switch as the BMW isn't driven but maybe once a week.

I am at the point where I want to park it down in crack-town with the window down and the key on the seat.

Will report in with the results of the 2nd round in the controlled setting.

Thank you to all Pelicans - the brain trust is huge!

[908/930]

Well, be really careful disconnecting that charger inside a house, unplug 120v first. I would not trust the reading from the charger for accurate voltage. it is likely fine, if in doubt get it load tested.

[john70t]

I recently experienced a junk $200+ OEM battery from the dealership. It does happen. It should not.

If a 2nd battery does the same thing, tend to suspect the car wiring.

But isolate the battery first.
Should stay 12.2-12.6V with lights and fan on.
Below 12V under load equals a bad battery.

[jamesnmlaw]

Following up - overnight, unconnected and in the house, battery says 80% charged and 12.4 volts.

I will do a load test tomorrow, weather permitting.

[Rusty Heap]

I have a digital load tester. Simple to use and cheap.


https://www.amazon.com/Alternator-Voltmeter-Indicator-Alligator-Charging/dp/B0BK7X2GSK/ref=sr_1_48?adgrpid=1343603774125692&hvadid=83975474815077&hvbmt=be&hvdev=c&hvlocphy=67734&hvnetw=o&hvqmt=e&hvtargid=kwd-83975582916350%3Aloc-190&hydadcr=7501_13211763&keywords=digital+battery+load+tester&qid=1701380028&sr=8-48