Invest in a cheap (like $4.00 Harbor Freight) volt meter.
Disconnect the ground cable from your battery bank and let it sit for 3 or 6 hours. Then test the voltage of each battery. If one or more drop below 12.5 or 12.6 it is probably in need of replacement. A good battery will hold a solid charge for months when not connected to any parasitic load.
hershey - albuquerque, nm
Someday Finally Got Here
My wife does all the driving - I just get to hold the steering wheel.
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Expedition - Chevy Equinox