[philso]

Check the health of your batteries with a load tester such as the Beak. It is possible that the output impedance of the battery has increased with age/abuse.

Check that all the connections between the battery and the PD Board are tight. A loose connection will have higher resistance leading to increased voltage drops.

The starting current for the motor can be as much as 5 X the running current so take Ryan's advice to ramp up the large motors especially if they have high inertia loads. A large percentage of the (3-phase AC) motor controlers that I work on are used for doing just this.