2008 S60- Trouble starting in cold temperatures

[Matt2008S60]

Hey guys. I'm new to the forum and have only had my S60 for about a year now. I live in Grand Forks, ND where temps get down to -20 F every year. Recently my car will not start if it is below 10°F. It will crank at a slower RPM than in warm temperatures, but will only sputter occasionally. I took it in to be serviced and they told me that the battery had 700 cranking amps at the battery, but this dropped to 350 when tested at the starter. The battery is an interstate battery and is only a year old. I suspect that there is some sort of corrosion that is causing this resistance somewhere between the battery in the trunk and the starter, but I all of the easily visible points are clean and no sign of corrosion. Any ideas?

[oragex]

I would be surprised on a 2008, but there can be a power cable cracked at the alternator.

As for the battery, there are two numbers on it. Ampers on warm weather and ampers on cold weather. It's the amp on cold that matter, I believe it has to be at least 650, the more the better. If the battery cannot crank at a certain speed, it won't fire up. A 0W40 or 0W30 oil helps greatly in these conditions.

You can do a test bring the battery inside house over night, try next morning. Always have the ignition key in Position II when disconnecting/connecting the battery.

[Matt2008S60]

I believe they told me it was cold cranking amps that was measured at 700 at the battery.
I forgot to mention that the S60 has 91,000 miles on it. Let me know if there is any other information that would be helpful!

thanks for the response