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Amp-Hours

Flight Engineering


 

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Photo: G450 Left Main Battery, from Eddie's aircraft.

Amp/hours, amp-hours, AH? What does it all mean? The G450 main batteries are "rated at 24 DC, 45 amp/hour." (G450 Aircraft Operating Manual, §2A024039, ¶2.D.) The emergency batteries are "rated at 24V and 9 amperes per hour." (G450 Aircraft Operating Manual, §2A-24-30, ¶2.E.) What does any of this mean to a pilot? The answer is, unfortunately, very little.

The amp-hour rating of a battery is a measure of how many amps a battery can supply for a period of time. It is calculated using the formula Amps X Hours. For instance, in the ideal world, this rating would mean that if you have a 45AH 24 Volt battery, you should be able to supply 45 Amps out of it for 1 hour, or 22.5 Amps for 2 hours, or 11.25 Amps for 4 hours, and so on. Unfortunately we don't live in the ideal world, so these values are just rough guesses. Electrical loads are rarely constant and tend to be rated at their initial surge or "spike" value and then decrease quite a bit. Manufacturers can rate their batteries based on short or long times which impacts how the equation really works. Most will rate their batteries down to zero but it will stop powering aircraft systems well before that. You need to know the numbers to pass your exams, but you are well advised not to base your survival on cockpit equations dealing with amp-hours, amps/hours, or AH.

References

Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.

Revision: 20141016
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