As winter, and cold temps, arrive batteries that have worked just fine for the past 6 months will start to be put to the test
THIS is when they need to be up to par. The only to know is to LOAD TEST them (separately). Forget the static voltage, it doesn't mean much unless you're down by 1 cell (2v)
To be done properly, you put like a 200Amp Load on each battery and watch the voltage drop. You can get a kinda heads-up
yourself by putting a voltmeter on the system (you can use the "Aux" plug in the cab) and watch the volts drop as the GPs come on and you start cranking (this is using fully charged batteries with clean terminals)
If you've definitely got a problem you'll see it go under say 9v.
If you definitely don't
have a problem it will stay above say 10-1/2v.
In between that, go have it checked
This is kinda a half-assed test of course and doesn't take into account the cranking time. If it you cranked even the best batteries for like 15seconds it could lower the voltage to bad levels. BUT, as I said, it would give you some indication.