When to change batteries
 

I've been watching these batteries for a few months now. I'm not sure if they NEED to be changed now or if I can push them a little without damaging something.

After sitting all night (about 20dF here) both batteries are showing 12.5V
During a load test, both batteries quickly drop to about 10.5V, but hold that for 10-15 seconds.

Scangauge II
KOEO
FLP= 10.5V
FVP= 10.5V
FCM= 48.0V
These numbers do not drop during cranking and increase to 13.5V-14V directly after startup. I get a brief surging idle at startup for a few seconds and then all is good.

With the extreme cold weather coming (-5) I wouldn't chance it as cranking amps go down the colder it gets and it will take more amps to crank over the thicker oil. I hate getting stranded and you could take out the starter. Best to load test them.

Healthy batteries make a happy 6.0.

That's my issue I guess. I got it through a week of below zero temps without issues. I don't even plug in unless its 10 or below. It started fine without plugging in after work also. If it were any other engine I wouldn't hesitate to get them through this winter and change them before next, but I don't want to stress the FICM or anything else.

I pushed my luck and made it through that polar vortex nonsense a few weeks back, but one of the batteries was toast after that. Found out the hard way in my driveway but at least wasn't stranded someplace random. Sounds like you're about due.

With those kinds of voltages, I'd replace the batteries as they look like they're getting weak.

I'll take a look at mine during lunch, but I don't think they drop that low cranking.

I'd have to say I agree.

Thanks all that responded. I took the advice and went and picked up a couple of 875cca from NAPA. They have a $10 each rebate deal going, plus gave me a 10% AAA discount, and their base price was cheaper than the Diehards and Duralasts that I was considering.

I did not charge them so these numbers from this morning are after sitting on the store shelf, and running less than 5 minutes after install yesterday:

Scangauge II
KOEO
FLP= 12.5V
FVP= 12.0V
FCM= 48.0V

FLP and FVP did not jump right up to 13.5V like the old batteries did. When I started driving, they seemed to settle in about 12.5V-13V. Is that due to less demand on the ALT? The cranking effort was much better and ignition came about 50% quicker.

This link for FICMrepair.com answered a bunch of my questions. Like you guys recommended here, looks like I was riding the line. Looks like a good bunch of general 6.0 tips here.