Understanding your internal resistance result
Where this power loss actually goes
Power lost to internal resistance doesn't disappear, it converts to heat inside the cells and connections, which is part of why battery packs need adequate airflow or cooling under sustained high loads. That heat also feeds back into faster aging, covered in more detail in the capacity over time calculator, a heat-to-aging feedback loop NREL's energy storage research studies at the cell level across chemistries.
Why the loss grows faster than the load
Because power loss follows current squared, a load that doubles doesn't just double the resistance loss, it quadruples it. This is one reason C-rate ratings exist in the first place, since pushing well past a battery's continuous rating creates disproportionate heat and voltage sag rather than a simple linear tradeoff, a heat-driven derating problem that shows up on the inverter side too, covered in the inverter noise and heat derate calculator.
Why bigger banks tend to run cooler and more efficiently
Adding parallel capacity spreads the same current across more paths, lowering the pack's overall resistance and cutting both voltage sag and heat generation at a given load. The parallel configuration calculator covers the wiring side of building that kind of larger bank, and the backup runtime estimator can show how much extra runtime that lower-loss setup actually buys you during an outage.
What to check next
If your efficiency loss looks high, the voltage sag under load calculator shows the practical effect on your equipment, and the round trip efficiency comparison puts this loss in context against a chemistry's overall efficiency. This same resistance-driven loss is part of how the DOE's Office of Electricity evaluates real-world round trip efficiency for grid-scale storage systems.