Why the "Safety Margin" Isn't Optional Padding
Estimated runtime from battery capacity and current draw looks like simple division (capacity divided by current draw equals hours), but using the full rated capacity in that calculation overstates real-world usable runtime for two compounding reasons. First, batteries — especially lithium-based chemistries — shouldn't be discharged all the way to zero regularly, since doing so accelerates degradation and shortens overall battery lifespan; a safety margin (commonly 20%, leaving 80% usable) reserves capacity specifically to avoid this deep-discharge damage.
Second, a battery's effective capacity under load is generally lower than its rated capacity, especially at high discharge currents — internal resistance causes voltage sag under load, and the usable energy actually delivered before the system's low-voltage cutoff is reached is measurably less than the nameplate rating would suggest, particularly for older or lower-quality cells.
This is exactly why a runtime estimate using full rated capacity with no safety margin tends to be optimistic compared to real-world operating time — building in the margin isn't overly cautious padding, it's compensating for two real, separate effects (deep-discharge avoidance and under-load capacity reduction) that both push real usable runtime below the naive calculation.