Should I replace the battery? AGM, Worth the cost?

DW had a little trouble starting the car (2015 Forester) on some cold days so I got the battery checked at AutoZone. It rang up 12.7V. The guys commented the battery is no good.

OEM Forester battery is really smallest they could put, with just CCA of 390. So I am not actually surprised to see voltage down. After he checked the battery, when I went to start the car, it showed 11.5V but the car still started in one shot. So my questions are

  • Should I REALLY replace this battery? I don’t have jumper cables or a starter pack and I really don’t want to buy either of that.
  • If I should replace the battery, is it worth investing $270 for (Northstar Group 35) AGM batteries as opposed to $90 for lead acid? What advantages do they offer? Is it worth investing in these batteries that they will beyond 3X lead-acid batteries?

Thanks in advance.

More than voltage needs to be tested to determine condition of battery. A battery usually lasts longer than 3 years. OEM size is adequate. AGM not worth it for most applications.

Describe what the car does/doesn’t do when starting in cold weather.

I don’t understand how you’d draw the conclusion that the battery is bad based solely on the evidence of it putting out 12.7v, when that’s perfectly within the healthy battery range.

However, where I live, 390CCA simply wouldn’t cut it, so it’s important to know what you mean when you say “cold.” If we’re talking “winter is when you turn the AC off and roll the windows down,” that’s fine. But if we’re talking “Hey, did you know that -40 is the same temperature in both C and F?” then you probably need a more robust battery, just for those super-cold days.

Start by telling us how cold “cold” is, and then tell us what exactly happened when your wife had trouble starting it.

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The battery’s Cold Cranking Amps are measured to determine the real condition of the battery.

Here’s a battery for your Subaru that delivers 550 CCA’s.


No to AGM, unless you subject your vehicle to severe off-roading. As far as I can determine the Forester takes a Group 35 battery. You can get 640CCA in this group.
Even though I have roadside assistance available, I proactively change out my batteries at 3-4 years. Look at it this way $40/year.

Two things. Make sure that a battery analyzer is used that has an “AGM” setting, or your results will be suspect. Secondly, if your car’s charging system is set up for a particular TYPE of battery (AGM or flooded cell), you should stay with that type. You can switch brands if you like, but the charging system in many newer cars will not tolerate a replacement battery that does not have the characteristics of the OEM battery. AGM batteries charge low and slow so a traditional flooded cell battery in your car may not charge properly and therefore may die sooner.

Yeah, 12.7 volts is perfectly fine but you need to know CCA. I check mine a couple times a year to see when they are getting weaker. 600 plus CCA are really pretty standard, at least in a Delco battery. On my three year old battery, I lost a little CCA over the past 6 months so I’ll be watching it more closely. Suggest you just go get a new regular battery with a little more CCA before it gets really cold out. I think a jump start cost me around $50 on a Sunday morning so something to be avoided.

If your 2015 Subaru was built in 2014, the battery is now well over 4yrs old, so I’m not surprised it tested bad

That said, if the oem battery isn’t an agm, you can stick with a regular group 35. The agm batteries typically cost at least twice as much, but they don’t last twice as long, not in my experience, anyways

Tip . . . Costco has very good prices for batteries, a 42 month no questions asked free replacement period, and they definitely have group 35, because it’s quite common

If the battery should fail within the 42 months, they’ll simply swap it out for a fresh one, no questions asked, no games, unlike some other vendors


@shadowfax, cold is NYC cold. We have had lowest 24F this year when DW had an issue starting the vehicle. But nowadays, its upper 30s in my neighborhood many mornings.

I am not too sure of the healthy voltage. I thought it should be above 13V. Hence I took it to AutoZone to get it checked.

@bing How would you check CCA?
Jumpstart around here is $65 and I am in no mood to pay that.

If 12.7V is a standard voltage, why did AutoZone’s meter recommended ‘Replace the battery’? Any idea? From what I could see on the tiny screen on the gadget, t just showed the voltage, not CCA and Replace battery.

I could get a new battery from Costco (Have a Kirkland in another vehicle) and replace this one, no problems, but I would like to know how one would definitely figure out if the battery is really bad.

Thanks in advance.

That is all I would need to know . I don’t care about testing a battery for anything . I just want the silly vehicle to start.

It should be in the 13/14v range when the car is running, but when the car is off anything above 12.6-ish is good, voltagewise.

Keep in mind that as others have said you can have good voltage and still have a bad battery, but any time someone only takes a voltage reading and then tells you it’s bad solely because it’s at 12.7v, he’s full of it.

The way you determine if the battery is bad is to load test it. Ordinarily, the load tester at the battery store is sufficient, but given what they said to you, I’d at least want to have it tested at a different store on a different machine. Remember that the guy behind the counter might well have been hired last week, with no prior automotive experience, and still hasn’t learned everything he should know in order to give good advice.

Nobody load tests battery’s anymore. It’s an old/antiquated method to determine the actual condition of a battery.

Battery’s are now tested using a conductance tester to measure CCA’s.

Here’s the reading I got when I tested my own battery with a conductance tester.


@Tester Where can I get tested like this one? I could try my local mechanic but before I go to him, is there any other store, other than AutoZone that would test?

Walmart gave me that kind of readout on my battery a couple years ago. It seems to be a widely used device.

Or you can buy your own less sophisticated tester for under $100. AZ testing kind of reminds me of every battery must be replaced like AAMCO transmissions where every transmission must come out.

If their test equipment is faulty then they would have a lot unnecessary warranty claims.

You want to test the battery again? If you had trouble starting the car and the battery failed the performance test you should have replaced the battery yesterday.

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You guys must live in the south. I’ve never had the need to replace a battery that was under 7 years old. Most last to 9…a few lasted past 10 years.

southwest, technically

But yeah, batteries don’t last terribly long here

Batteries that make it to 7 years are outliers here

I live in MInnesota. And that battery I tested and replaced was a little over 5 years-old