Dual Ammeter
You'll never see the difference in current on analog ammeters of the type you mention - their resolution is too coarse. You'd need an accurate digital one that could resolve to 0.1A or at least 0.5A.
With a 75A alternator, you'll probably only see 30-40A for a few minutes after a long cranking session, then dropping down to 5-10A until the battery(s) are recharged. That's barely off-center for a 60-0-60 ammeter.
The imbalance of batteries is quite subtle unless there's something terribly wrong with one (like a shorted cell). A better method to check the comparitive health of two batteries is to charge them together (as wired in the truck), then disconnect both, from the truck and each other for a couple days, then take a voltage reading of each using an accurate digital voltmeter. Different voltages of more than approx. 200-300mV difference (0.2 to 0.3V) is an indicator that they're self discharge rates are different, an indicator that one battery is getting pretty tired. Fully charged batteries should settle to 12.6-12.8V after a few days "at rest" (without charging or load)
Another good method is to charge both, turn off the truck disconnect one battery and turn on the headlights for, say 20 minutes measuring the voltage at a couple time intervals. Then turn the light off, switch batteries and turn them back on making the same measurements for the other battery at the same time intervals. Both should discharge down to approx. the same voltage (within approx. 0.25V). If one discharges to a significantly lower voltage, that's the straggler and you might think about replacing it. If neither one is above, say, 12.2V after 20 minutes of headlights on, then think about replacing both. Keep in mind that the "working voltage" of a automotive battery is 10.2 (fully discharged) to 12.8V (fully charged). The alternator should put out 13.5V (minimum) to 14.5V (maximum) to properly charge a lead-acid battery
-Pete.
