Figure 3-11 Battery Capacity Analysis Example; Battery Capacity Analysis - Garmin GTX 335 w GPS Installation Guidance

3.16.5 Battery Capacity Analysis

If it is determined that the modification results in an increase in electrical load then it must be further
verified that the aircraft electrical system remains in compliance which includes both electrical generation
capacity and if loads have been increased, that reserve battery capacity remains adequate to support loads
essential to continued safe flight and landing. If the existing battery does not meet the battery capacity
requirements, a battery that has sufficient capacity must be installed.
Refer to ASTM document F 2490 - 05, Standard Guide for Aircraft Electrical Load and Power Source
Capacity Analysis for more information.
1. Examine the nameplate capacity of the battery and assume 75% is available.
For example, 12A-h = 720A-min, therefore 720 A-min X 75% = 540 A-min.
2. Estimate the normal or pre-loadshed cruise condition (assume worst case cruise at night).
For example, 15 A X 5 min = 75 A-min. This assumes 5 minutes for the pilot to shed non-essential
loads. Any automatic load shedding can be assumed to be immediate and need not be considered
in the pre-loadshed calculations.
3. Determine the minimum cruise load necessary to maintain flight after the generator/alternator has
failed. For example, 10 A.
4. Determine the consumption required during the landing approach.
For example, 20 A for 5 min = 100 A-min. The cruise duration is therefore:
Battery Capacity – Pre-loadshed
190-00734-17
Rev. 1
+ Landing Load
Cruise Load
540 A-min – (75 A-min + 100 A-min)
=
=
=

Figure 3-11 Battery Capacity Analysis Example

(a) – ((b) + (d))
=
(c)
10 A
365 A-min
10 A
36.5 min
GTX 335 w/GPS Installation Guidance
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