Using a Ryobi 36v battery to power an electric kart
I had the same issue right now, trying to use a Ryobu BPL3640D (36V x 4Ah) for a hackish project. The output of the battery appears to be extremely high impedance, it floats around the actual battery voltage of 40V (pretty full) but already starts dropping when measured with a high-impedance meter.

My circuit wouldn't draw any current when powered with less than ~22V so the voltage was just floating around that point for me without actually making the circuit work. However, connecting a 100kOhm resistor in parallel to the battery made it work for me, with the battery now putting out the full voltage. So maybe the output needs to be pulled below a certain threshold voltage to tell the battery something is connected before the battery's circuitry "starts up" and permits actual current to flow. This is just guesswork though.