I found this article on Battery Emulators which is actually a good idea since batteries are NOT linear power devices. This is one of the biggest fallacies and something that everyone expects - that batteries provide consistent and constant power throughout its operating range = FALSE !

If you look at any battery discharge curve, you will see that a battery's power decays non-linear.; Meaning that the power it initially has (as voltage and current) is NOT as strong during and after usage. The voltage and current output decay over usage and time such that the power delivery actually diminishes. Furthermore, the power depletion (under constant load) will actually accelerate the consumption of its internal power; meaning that the power discharge actually accelerates even with a constant load.

You might be wondering why this is and part of the issue has to do with the operating temperature of the battery and the fact that a battery is actually self-heating due to its internal (current limiting) resistance. For excessive and fast discharging, a battery will heat up and actually reduce its output efficiency due to the fact that as more energy is dissipated, more internal heating and waste of energy occurs. This is why lower consumption of current is better and is typically fractional of the Capacity (C). So for achieving maximum efficiency and the lowest self-heating, lower fractional Current consumption as Capacity is recommended (e.g. 0.5C, 0.25C, 0.1C).

Lower Capacity consumption, efficiency, and determining how much power (and life) a battery can produce is a difficult because of the non-linear effects. So experimentation is necessary to understand the best 'sweet' spot for usage. ths will also help you engineer the best battery pack configuration for serial (S) and parallel (P) of cells (e.g. 13S4P = 13 series batteries in parallel with 4 strings).

Since experimentation of batteries can be very time consuming and costly, a better way to achieve this is thru the use of test equipment that will emulate the discharge curves of various battery chemistries and cell types (e.g. LiIon @ 4.2 fully charged at 100% down to 3.2 and 96% discharged).

This article talks about a Battery Emulator using a Keysight type :
https://www.rs-online.com/designspark/you-probably-need-a-battery-emulator-no-really

There are others such as Keithley that are sold thru the Tektronix website as well :
https://www.tek.com/en/products/keithley

A "Battery Emulator" is the latest marketing label for what was historically called a SOURCE-LOAD measurement type of test equipment, wherein the SOURCE test equipment can emulate the battery and the LOAD would emulate your product as the Device-Under-Test (DUT).

Any battery based device can benefit from understanding the discharge power curves and operation to understand the battery performance, life, and other reliability/safety concerns. However only real batteries can provide the actual data so when creating discharge curves, its important to make sure that the CORRELATION of real battery data to the emulator (SOURCE) is realized.