Lucas,
Try also to use only a car battery, without the glider battery.
Disconnect the wires from the glider battery and connect the wires to the car battery using the emergency cables.
See what happens.
Dan
On Tuesday, July 14, 2020 at 7:11:34 PM UTC-4, wrote:
Dan, many times I connected car battery to glider battery with car battery emergency wires and the difference is very big. The starter "flies" and the engine starts very easily. I will try directly to the starter as you say. Thank you!