I had a feeling that the batteries were limiting the performance of the motor because they couldn't deliver that much current. Batteries are normally rated in Ampere-hours (Ah), not in Amperes. An Ampere-hour is a measure of the energy stored in the battery, and is not directly related to the current that the battery can deliver.
The batteries' rated current is 48 A. I have four loads that each require 12 V and 13 A of electricity. Would there be any voltage drop, would the load have to be significantly higher to actually see a decrease, or would the voltage stay the same? How is this not a duplicate?
As we have learned, battery voltage is the missing link that allows us for direct comparison between a set of battery systems. But the most important specification for your application will always be the rated energy. Jolien Despeghel Jeroen Tant
Yes, the output voltage of a battery decreases (roughly) in proportion to the current drawn from it. This behavior is usually characterized as an internal resistance of the battery, although its actual physical origin may be related more to the chemical properties of the battery than to the actual resistance of any conductive part of the battery.
Would there be any voltage drop, would the load have to be significantly higher to actually see a decrease, or would the voltage stay the same? How is this not a duplicate? Yes, the output voltage of a battery decreases (roughly) in proportion to the current drawn from it.
If there isn't enough current and you add a battery, you can expect increase in torque because the voltage supplied by the batteries will be higher. This was a fantastic answer. Thank you for correcting my misconception -- I did not know that the resistance of a motor is constant.