SLA batteries hooked up in series


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I have 3 12v 5 AH SLA batteries hooked up in series powering a 36v 750W motor, how long will they be able to power the motor?
Pick another much bigger system.
Doubt it will even get the motor up and running.
Check spec sheets "BP5-12"

Maybe 10 seconds. Be careful, batteries can explode.
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.