Plain-language answer: Battery internal resistance is the effective opposition inside a battery that causes voltage drop and heat when current flows.
Why it matters
It changes with chemistry, state of charge, temperature, age, and measurement method. Rising resistance can make a battery look charged at rest yet collapse under transmit load.
Use it correctly
Measure with a consistent method and temperature, record loaded voltage at known current, inspect connections, and compare trends rather than one isolated number.
The common mistake
Blaming the radio when undersized wiring or a high-resistance connector causes the same voltage sag.
Decision boundary
Current regulation, program rules, manufacturer limits, exact station configuration, and live observations control. Recheck changing rules, solar conditions, and operating status before acting.
