Those batteries are all stuffed. From full charge, raise from ground to full lift height and measure each battery while under load. Should be 6.2-6.3 no load and minimum of 5.8v under load. Even 5.8v is pushing it to be honest, i'd prefer higher but 4v is a no-no. Your voltage drop to the controller BDI input sounds acceptable if you're measuring it correctly. Compare BV at the battery plug to voltage measured at terminal 2 on the right hand terminal strip beside the charger. Ideally should be 0.5v difference max (can be hard to achieve, relay mod cures this). Any idea of battery age?
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