It was cold yesterday... (cold is a relative thing) ... about 30, when I went out to start my 07 Dodge 5.9. The grid heater light came on the voltage dropped down and came up like normal when it's cold... turned the key and it cranked slow, didn't start. Turned the key again and it still cranked slow but started up.

Engine light stayed on. P2509 code pops up, which I figured was caused by the low batteries. I drove for about 6 hours and after a couple of coffee stops the check engine light went out.

Anyway, went to Autozone in Weatherford to see if they could test my batteries while they were installed. They asked which was primary. I told they were hardwired and there wasn't a primary, he didn't buy that. So he tested both while still hooked up and got the same results.

He said his machine tested each batteries and were good at 775 CCA. Is that what I should expect from an Autozone battery tester? Can they be individually tested while installed?