The spec sheet says 65 A max for 5 sec.
750W @ 36 V is equivalent to 21 A, way less than the max. spec.
Assuming the batteries have a useable capacity of 80 % out of 5 Ah means 4 Ah are really available. Drawing 21 A means an uptime of max. 4 Ah / 21 A = 11 minutes -> 10 minutes with some reserve.
Only 5 minutes according to the datasheet which takes into account more factors than the above simple calculation.
However, the interbnal resistance of these bateries is <= 27 mΩ which will cause a voltage drop of 27 mΩ*21 A = 0.56 V per battery or 1.7 V per stack of 3. Therefore the power available to the motor drops by approx. 35 W.
It is well known that motors can take from 4 to 6 times their rated full load current at startup. Under this loading, the battery will esentially be looking at a near dead short for all intents and purposes. High discharge causes high gassing and heat within the battery which, when and if it breaks down will have possible explosive results.