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Battery wont hold a charge

1987 bmw 528e. i have a battery that is 1 1/2 years old. for the last month, if i dont drive the car a good distance (10 miles +) the next day the battery is totally dead. i had it jumpstarted by aaa and they tested the alternator and said it was fine. a) should i get a new battery that is heavy duty because of my electrical system has a lot going on, ie…anti theft lite is always on etc… i have a mega-tronII interstate battery (9/07) or just a regular battery? b) does it sound like a bad battery to you? c.) should it be an actual bmw battery? thx for your help…i have written before on advise about a timing belt. i love this site.

First, go get the battery tested, preferably from where you bought it from.

If it fails, you should be able to get a free or prorated replacement, especially if you have your receipt, or see Interstate’s commercial about warranty replacement, even without a receipt-never hurts to ask).

If you have to buy a new one (and I think the Megatron comes with a 24 or 36 month free replacement warranty), I also try to go for a battery with at least two to three years’ free replacement warranty, so that if I get a bad or old stock battery that I will at least get a free replacement for that time period.

Secondly, consider getting a battery tender if this situation continues. However, I can tell you that I have vehicles sit for weeks at a time, and even my seven year old battery starts up my truck, so you may have an additiional electric drain issue to deal with.

thank you for your response jayhawkroy. my husband took the battery to a battery place and they said it was good. i thought because he had driven it, it had charged it, therefore showing a good charged battery. i read that cars with alot of electrical components need a heavy duty battery, rather than just a regular battery. i think this is a 2 year battery…the receipt says mt-48 interstate battery (?). is that what that means 48=2years in battery lingo? you mentioned an additional electrical drain issue, what would that be for instance?

Have a “parasitic load test” done on your car to find out what is draining the battery. This is not rocket science…

thk you caddyman. never heard of a parasitic load test. i will have it checked out. seems to me if the battery is fully charged though, will i still see a the same load that is draining the battery as i will when the car is dormant?

MT 48 is the Interstate battery type number, which relates to the standard battery group size 48, for your car. From the Interstate website, it appears that the Megatron has 30 month free replacement and 85 month total prorated warranty.

For all intents and purposes, the Megatron is considered a heavy duty battery. I agree with Caddyman that a parasitic load test should be next. It is possible that this battery could pass a load test, so it is not considered defective at this time.

Now you need to find out if there are parasitic electric issues that is causing the battery to deplete while parked.

thanks for your help, jayhawkroy and caddyman. i will follow your advise.