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Discussion Starter #1
Could those of us that habitually turn off our Auto Stop Start expect to get a much longer life span from their battery?
I would imagine a A.S.S. system in constant use would require a battery of much higher spec to handle all those extra starts.
So, would it be fair to figure that NOT subjecting the battery to those stresses should result in a much longer battery life?
 

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2013/14 2.5i-L CVT
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The battery in my Forester was made in late 2012 and despite using S/S, freezing overnight temps and only driving once a week 20 km/12 miles each way to the nearest town it is still holding a good charge. Warm engines usually start quickly so the draw on the battery isn't that great, plus S/S deactivates if the battery is significantly discharged.
 

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2017 Forester XT
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My expertise isn't solid, but my recollection is that lead-acid battery aging and failure relates to elapsed time spent at high ambient temperatures, not the amount of charging and discharging. The worst possible scenario would be a "reverse snowbird" migratory lifestyle where you and your car spend summers in torrid heat, followed by winters in the snow belt where you need the healthiest battery. Non-migratory lifestyles, and the regular kind of snowbird, would all do better.

Don't be overly impressed by my profile picture; my car spends most of its time in the temperate lowland US pacific NW, where winter is usually +5-+10C and summer hardly ever above +30C. the picture was on one trip to Montana in February, and the car did briefly see minus 28C on the trip home, causing a triangle of frost on the inside of windshield, and halos of frost on the back bumper from exhaust condensation. Our OEM battery croaked at 3.5 years (in a car without auto-stop-start).
 

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2019 Forester Touring
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As a rule of thumb, vehicle batteries dislike extreme temps, both hot and cold. Both ends of the spectrum will have a much higher impact than, say, driving year-round in a place like Aruba, when it’s usually in the 70’s Fahrenheit...

Asking whether or not Start/Stop usage reduces battery life depends substantially upon the frequency of activation. If you drive a lot in a highly urbanized areas with repetitive stops every minute or so, then I would say disabling the feature may lengthen battery life. OTOH, if it only happens a few times over several miles, then the impact on the battery will be greatly reduced.
 

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I think the battery's "attitude" is subtly different between hot and cold -- in hot weather, the battery itself is unhappy and degrading faster, chemically, the car running or not. In cold weather the battery would be perfectly happy just sitting there unused; it's the car that is unhappy, and needs much greater battery capacity to get its engine started.
 

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Discussion Starter #7 (Edited by Moderator)
In the 'old' days batteries seemed to fail over a period of time, giving you some warning before total failure. The dreaded 'click', 'click', 'click'.
And that total failure always seemed to be on a cold morning, first start.
Modern batteries seem to go just fine and then give a hint of trouble before carking it.
Well, in my experience anyway.
Older batteries were also more inclined to take a charge up, to allow you to eke a bit more out of them. Modern batteries seem opposite.
Again, my experience.
 

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The only thing that shortens battery life substantially is keeping it at a low state of charge, this causes sulfation of the plates in the battery. A cold battery cannot deliver as much current but this won’t harm it, you just don’t get as much power until it reaches operational temp.

Lessening the draw on the battery slightly when the car is off and not charging and then asking for current when the car starts will not impact its lifespan at all as long as the SOC of the battery doesn’t drop to less than 70%. Even then you’d need to keep it a low SOC for a long time to damage it. The car will disable the on off of the battery is low enough so that drawing off it will damage it. You are avoiding contributing to a massive amount of pollution due to idling vehicles by using the system.
 

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2020 Forester base
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Personally I turn mine off every time, mainly because it annoys me. But from a technical perspective the system is supposedly designed to handle the extra work. That being said, using it less should increase the life not only of the battery but also every other component in the the starting system ( solenoid, starter, fuel pump, injectors, alternator , spark plugs ect ). Coming from an aviation background, many many items on aircraft get repaired, replaced due to cycle limitations if they are reached before duty time. Also, manufacturers are allowed to use a certain amount of stop/start data in there advertising for mileage numbers. Lengthening the life of parts reduces the manufacturing of replacement parts over the life of your vehicle, thus reducing pollution as well. And that argument can be hard to quantify. But like most things, it’s about personal opinion I suspect. Wow, sorry for that long winded reply,, I could have just said I agree,, LOL
 

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Not going to effect battery longevity in any meaningful way at all, period. Current draw would need to be high enough and long enough to bring your battery to about 70% state of charge (assuming it’s old school flooded but most likely it’s an AGM battery in which case you can drop to 60% without concern) to be considered a single discharge cycle and to introduce any concerns about sulfation and or plate degradation.

At a light, running the fan and all electrics in 5 minutes you are basically going from 100% to 99.9% SOC. Asking a battery to deliver current when it will receive its maximum acceptance charge about 1 second later does not harm it in any way, dropping it to a low SOC does. Starter motors on a car with this feature are designed and built to do constant starts. Your car will bypass the feature if your battery isn’t charged enough to handle it.

Starting the car with a modern engine management system uses about the same amount of fuel as 5 seconds of idling. Every 10 minutes of idling releases about one pound of C0 into the air. There’s a reason why car manufacturers are doing this, and it’s not to compromise the reliability of the vehicle.
 

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2019 Forester Limited
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Small discharges do impact battery longevity. See the below chart of cycle life vs depth-of-discharge for a deep cycle AGM battery. Discharging to 30% DOD (70% state-of-charge) reduces battery life, but small discharges also significantly impact battery life.

Also, with start/stop engaged, the battery spends more of its time in a partially charged state, further reducing life. It's these conditions that caused manufacturers to develop EFB batteries for start/stop systems, and the conditions are mitigated if one chooses to disable star/stop.

542806
 

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2012 SH Manual Diesel
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So, would it be fair to figure that NOT subjecting the battery to those stresses should result in a much longer battery life?
If all other things were equal, I’d agree that, that is a very fair and plausible assumption.

Auto stop/start batteries (Enhanced Flooded Batteries), are marketed as being able to accept charge more readily, and handle a deeper discharge. However, their CCA capacity and weight is also higher.

The more cycles the SLA battery goes through, the more the lead plates are slowly consumed. When the battery is recharged, there is not always 100% conversion in the chemical process. Also temperature, agitation, and stratification will also effect the chemistry process. So a slow depletion of the plates occur.

Sulphation of the plates also occur, when the battery is left undercharged. Since an alternator won’t recharge a car battery as well as a smart battery charger, they never get fully charged.

Add on top of that, all the added electronics in a car (especially electric power steering, eyesight, blind spot detection, driver detection, infotainment systems, then consider, smartphones, dashcams, GPS etc etc...). You couldn’t simply Guarantee that the battery was as fully charged as it could be, at the end of every trip.

So, the higher the state of charge the battery maintains, the better. If A.S.S. Isn’t necessary, or being used as it is intended, it makes sense to switch it off.

I have an EFB, in a non-A.S.S. car, with the anticipation that; it will be easier to maintain a higher state of charge; and will have a longer service life.
 

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Side note : The stock battery in my 2020 forester base model is an EFB type which is designed to better handle Stop/Start compared to traditional lead acid but can be charged the same way. However if you replace it with an AGM battery which handles it even better, the charger should have a specific AGM mode as the requirements are different ! This leads me to believe that the alternator would need tweaked or the AGM battery will never ever get fully charged. Unless of course it’s a “smart” alternator 😩
 

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2019 Forester Limited
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Some manufacturers provide different battery settings for the car's charge system (Ford?). Subaru does not, perhaps because the charge algorithm is insensitive to minor differences?? That may be wishful thinking. Charge voltage in my car is typically 14.2 volts, with short term jumps to the 15 volt range during braking (depending on state of charge). That's within the range of acceptability for an AGM, so I'll go that route when my EFB finally dies.
 

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I turn mine off whenever I remember. I have been told though that battery life depends on having your battery fully charged. Even a long run will not fully charge the battery so every so often I charge my battery with a CTec charger which I think is a seven stage charger which fully charges the battery. This also saves the alternator working harder.
 

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longer life span from their battery
If longevity is something your very interested in, you could consider fitting a Capacitor jumpstarter, to your starting circuit. Not only can you reduce the draw down on your battery; but you’ve always got a jumpstart, if ever you need one.

You could have it come on line, when ignition is turned on; or just have a switch on the dash to turn it on. I got one cheap from Jaycar, for about $80 on sale.
 
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