Why do other online calculators give different results?
Understanding the difference between simplified and normative BS 5839 calculations
The Normative Method (Annex D)
The calculator on the previous page uses the full Normative Annex D method from BS 5839-1. This is the required method for formal compliance and system design.
Many "quick" online tools use a simplified formula that often omits the Derating Factor (D) or assumes a fixed alarm time, which can lead to significantly different (and sometimes non-compliant) results.
| Factor |
Simplified Tools |
This Calculator (Annex D) |
| Aging Factor |
1.25 (Common) |
1.25 (Standard) |
| Derating (D) |
Often 1.0 (Ignored) |
1.75 (Recommended) |
| Alarm Time |
Variable or 1hr |
Fixed 0.5hr (30 Mins) |
| Math Accuracy |
Linear/Estimate |
Full Efficiency Curve |
Why use 1.75 for Derating?
Batteries are less efficient when discharging quickly under heavy alarm loads. BS 5839-1 recommends using a factor of 1.75 unless specific manufacturer data proves the battery can handle the high load more efficiently.