Try this, fully charge the cart, go out for 7 miles, park the cart, and leave the switch on and the lights on, then measure the voltage across each individual battery, it is possible you have a single battery going bad. Having the load (headlights) on I believe will give you a better reading as I have seen automotive batteries give "normal" voltage reading immediately after not being able to start the car. The battery had a bad cell that had no capacity, but still had normal voltage.
Hope this helps.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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