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      10-19-2020, 12:33 PM   #1
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AGM Battery Lifespan

My 2014 M235i (Dec 2013 build) has a nearly 7 year old AGM Exide Battery. Anyone replaced theirs yet? Engine was long cranking cold starts a couple days this week when we got early winter. Curious how many years people are getting out of these batteries before replacement.

I haven't got the low battery warning on my dash yet, but it has shown up every winter when the temperature gets down around -25c, so it's not the best indicator for me.
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      10-19-2020, 01:02 PM   #2
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Mine is entering its 10th winter. I sense it will need replacing this winter.

Covid use (or rather lack of use) hasn't helped this year. Putting it on a CTEK charger has helped, but voltage is dropping a bit too fast, from a charge, to believe the battery will cope with another winter.
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      10-19-2020, 01:06 PM   #3
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Mine is six years old and shows no signs of needing replacement. However, when the temperature consistently gets below freezing every night I'm fastidious about plugging in my battery maintainer. if you get a low battery warning when it's cold you should be doing the same.
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      10-19-2020, 01:14 PM   #4
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Quote:
Originally Posted by Billfitz View Post
Mine is six years old and shows no signs of needing replacement. However, when the temperature consistently gets below freezing every night I'm fastidious about plugging in my battery maintainer. if you get a low battery warning when it's cold you should be doing the same.
Mine was giving the warning last winter after sitting on the charger all night. It would start effortlessly, drive the entire way to work, but when I shut it off it would chime low. After it sat in my warm work parkade all day it would be fine for the trip home with no warning on shutdown. The system seems really sensitive to cold battery showing low voltage.
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      10-19-2020, 01:40 PM   #5
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One problem is that the alternator isn't as large as it could be. When driving in cold weather the heater motor draws a lot of current, while the battery doesn't charge well because it's in the coldest spot in the car. I haven't had a warning, but when it's cold after returning home from a drive my tender never reads the battery as having a full charge. It does always show it fully charged the next morning, so I chalk it up to the engineer who decided to save five pounds of weight by using a too small alternator.
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      10-19-2020, 11:31 PM   #6
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Quote:
Originally Posted by Billfitz View Post
One problem is that the alternator isn't as large as it could be. When driving in cold weather the heater motor draws a lot of current, while the battery doesn't charge well because it's in the coldest spot in the car. I haven't had a warning, but when it's cold after returning home from a drive my tender never reads the battery as having a full charge. It does always show it fully charged the next morning, so I chalk it up to the engineer who decided to save five pounds of weight by using a too small alternator.
Isn't the alternator on the N55 200amps or more? That's a pretty sizable alternator for a passenger car.
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      10-20-2020, 12:03 AM   #7
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These cars are designed to keep the battery around 80% charged, at that level the battery will live the longest.

I think what happens with my car, the battery goes from 10c at work inside to -20 or -30c overnight. When I start the car the battery is cold and shows less voltage, so the car thinks the battery is low. The temperature change is enough of a difference that 80% ideal charge looks low enough to cause the warning.

It's never given me trouble other than the warning, and I always assumed it was slightly over engineered and sensitive. Problem is I don't know when this battery will be an issue and now it's 7 years old. If others have got 10 years I think I'll wait and see if it becomes a problem.
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      10-20-2020, 08:31 AM   #8
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Quote:
Originally Posted by Polo08816 View Post
Isn't the alternator on the N55 200amps or more? That's a pretty sizable alternator for a passenger car.
230 amps, but at what RPM? It won't put out 230A driving around town, let alone sitting at a stop light. Then consider what's electrically powered in the car, which is pretty much everything, including the steering and water pump. Also, while 230 amps is more than enough to power a house at 110v, that's because 230 amps at 110v is 25,300 watts. At 12v 230 amps is 2760 watts. The radio alone can eat up a substantial portion of that. From an engineering standpoint it's more efficient to power as many devices as possible electrically rather than mechanically, but if you do that the alternator should be sized to give adequate output under worst case conditions, not best case.
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      10-20-2020, 11:20 AM   #9
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I replaced mine at 4 years and I had not used a battery tender and it was 65-75%. It may have lasted another year or two but I was not taking any chances.
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      10-20-2020, 11:50 AM   #10
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Quote:
Originally Posted by Billfitz View Post
230 amps, but at what RPM? It won't put out 230A driving around town, let alone sitting at a stop light. Then consider what's electrically powered in the car, which is pretty much everything, including the steering and water pump. Also, while 230 amps is more than enough to power a house at 110v, that's because 230 amps at 110v is 25,300 watts. At 12v 230 amps is 2760 watts. The radio alone can eat up a substantial portion of that. From an engineering standpoint it's more efficient to power as many devices as possible electrically rather than mechanically, but if you do that the alternator should be sized to give adequate output under worst case conditions, not best case.
I'm not an electrical engineer, but I don't think the current is completely linear with RPMs. I would imagine the resistance would change depending on load (demand) on the alternator to an extent.
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      10-20-2020, 12:27 PM   #11
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No. I live in a mild climate so I fully expect to get 7 yrs or more out of an AGM. Especially since I'm easy on the brakes (i.e. coasting extends the duration that the alternator recharges the battery.). Remember our vehicles are mild-hybrids.
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      10-20-2020, 01:10 PM   #12
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Quote:
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I'm not an electrical engineer
I am. Alternator output isn't linear with RPMs. The output relative to speed is a curve, with 50% of the rated output usually reached around 2500 RPM, and 90% of the rated output by 3500 RPM. At highway speeds an alternator will produce a lot more current than the car needs, including charging the battery. In town and idling it will still produce enough to run the car, but depending on how much stuff is being powered it might not have enough left over to charge the battery. BMW addresses this concern with power management that diverts power away from non-critical devices and to what's considered the most critical device, the battery. This can cause of lots of error codes being tossed when a battery is too far gone to take a charge. But an OK battery can cause the same problems in very cold weather, because that reduces the battery's ability to take a charge. The same battery will act very different below zero than even at 40 degrees.
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      10-20-2020, 01:46 PM   #13
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My e46s AGM lasted almost 12 years. What finally killed it was the alternator failing and me having to drive some 25-30 minutes home on naught but the battery.
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      10-20-2020, 07:02 PM   #14
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Garage List
2014 BMW 335i  [0.00]
Mine died earlier this summer, at 7 years old.

Indications of it needing replacement were random codes of local voltage drops. Headlight module, steering module both consistently got codes for low voltage.
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Last edited by Eschmacher; 10-20-2020 at 07:07 PM..
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      10-21-2020, 04:31 PM   #15
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Mine’s just over 6 years now and I am getting signs of replacement. After 10 days of parking (very hot weather those days though) it was flat. And there are warnings in cars memory already about a weak battery. I was thinking to postpone replacement to next year, but today I ordered a new one, why waiting if you know that you have to do it soon. Winter is coming!

Last edited by Jimmert; 10-21-2020 at 04:36 PM..
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