Dying battery: 212k fix?

it's all too vague to know anything really. if you're going to keep it and replace the battery at some point, replace it at that point.
read up on rebuilt batteries, new is better.

 

Are you the diy type?

Done the egr circuit cleaning yet?

I have the Prolong equipment and it works to preventatively maintain our battery well.

Do the egr circuit and I bet you’ll see a performance improvement .

 

48 MPG for a Gen 3 is great from my experience. If you're used to driving a Gen 4, you'll definitely think the Gen 3 is a dog. Try using Power Mode for a better throttle response. Rapidly fluctuating State of Charge is a sign the traction battery is losing capacity, but not in danger of failure. I'd say you don't need to do a thing...

 

Try a fuel injector cleaner, the type you add to the gas tank. The mpg on ours (albeit a youngster with around 75K kms) seemed lackluster coming out of winter this year, not picking up, so with the last tank fill-up I add the suggested amount of an injector cleaner with PEA (PolyEtherAmine). The stuff I used was Motomaster Formula 1, fwiw.

Another family member ran through most of that tank; they needed a loaner. It wasn't that good for mpg.

HOWEVER, I tanked up again, and over 100 kms into this tank, the displayed mpg is dancing between 3.7~3.9 liters per 100 km (over 60 mpg US). I am trying hard, and I expect it'll fall off a bit by the end of the tank, but I'm pleasantly surprised by this uptick.

 

1. You're changing your oil every 10k - is this also how often you check it?
I'd start monitoring for oil use.
If you use a closed-hood maintenance philosophy - consider a 5K OCI.

2. +1 on the EGR cleaning!!
The head gasket you save might be your own!

3. Ignore the traction battery.
Your maintenance history suggests that the car is adult driven and if I pull off my combat boots to help with the higher math, I'm getting about 26k miles per year. 48MPG (real-world NOT MDF!!!) is probably fairly good given your usage since I'm presuming that you're on the highway a lot.
NE does not have many HOV lanes and the winters there are a little harsher.

If you pay attention to the oil and monitor the EGR gunkage you have the makings of a 300,000-500,000 mile G3.
When the traction battery does fail.....I'd determine how many more miles you want to put on the car and then either SELF INSTALL an OEM unit or get a re-manufactured traction battery.
While you're reading up on the EGR threads, you can start to do the noodle dance on this as well.

Good Luck!

 

That's a first....

Read some of the EGR cleaning posts on this car.
You're checking your oil regularly, so @ 212k the EGR represents the single biggest threat to your car's overall health.

Everybody always sweats the battery but they seem to be lasting most of 10 years in the G3 from what I'm reading and unless you want to go down the rabbit hole of individual cell troubleshooting and maintenance, it seems to be a 'wait for it to fail, and then replace it' kinda thing.
I'm an Electronics Technician and I have enough tools and training to do the job and I myself lean towards treating the battery pack like a light bulb.
Wait for it to fail, and then replace it.

So....if you KNOW that you're going to be keeping the car for another 200,000 miles and you're especially worried about breaking down in the middle of BFE, then you might want to tinker with the idea of replacing the traction battery...yourself.....with an OEM unit proactively but this is going to also be a dice roll because a brand new, factory fresh, OEM battery.......ISN'T.
Remember....they stopped punching out those battery packs back in 2015 back before they ruined....er....I mean switched to the new body style, and my finely honed Spidey senses leave me with the impression that the G4 battery pack is not transswappable with the G3 battery pack.

again.....YMMV.

If I woke up in your shoes, I'd close the loop on the EGR thing and maybe clean the Throttle Body and Mass Air Flow Sensor....MYSELF!!!
The symptoms that you describe actually approach those of a dirty Throttle Body butterfly valve or Mass Air Flow Sensor.
Never EVER EVER let any dealership clean either of these, because they will either not do it.....just pour a $5 bottle of cleaner in your tank and SAY they did it....or....they might actually use something that looks like it belongs in a gastroenterologist's office which might provide the same result as an attempt to use a power washer to clean a set of antique Ray Ban sunglasses in the case of the MAFS.

It's an easy DIY thing, or you can try out an independent mechanic if there's one in your area that can be vetted.

Good Luck!!

 

An owner of a car of this age should definitely own and configure some diagnostic equipment.

Will cost you less than $75.
You'll be able to read and clear error codes.
See live data like state of charge, battery voltage, etc.