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28V semantics

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itsmoked

itsmoked

Electrical
Feb 18, 2005
19,114
I am wondering why in the automotive world cars, with 12V batteries, are deemed 12V systems even though they are 13.2V to 14.7V typically but in the aviation world they call planes 28V even though they have 24V batteries and pushed in the charging realm may reach ~27V?

Keith Cress
kcress -
 
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MIL-STD-704 and MIL-STD-1275 specify 28V.

TTFN

FAQ731-376
 
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not sure what the whole story is; never tried to research it.

However, most aircraft rarely run on batteries for extended periods, and the power converters have to be sized for the alternator outputs, which, for MIL-STD-704 are quite ragged. Additionally, the overall voltage range of both mil specs cover 24V, since the standard voltage range is 22V to 29V.

Most airliners run on on-aircraft auxiliary power units (APUs) while on the ground, or directly off ground-based APUs.

On many aircraft, the batteries don't even go to all the electrical systems, thus making it imperative that they can reliably run on 28V from the APU as opposed to battery that might not even be there.

TTFN

FAQ731-376
 
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TRU's in many aircraft are designed to output 28V nominal when the input is 115VAC. This value fluctuates widely as the 115VAC input could be anywhere from 85VAC to 122VAC. Different battery types also affect the standard "28V". For example, lead-acid batteries are kept charged at over 28V. NiCad aircraft batteries can be 27V to 30V for 19 cell or 25v to 28.5 for 18 cell, depending on the system. Boeings new 787 uses Li-ion batteries and the nominal voltage is set to approx 28V.

 
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Even though automotive systems are called 12 volts, most of the components are specified at 14 volts. Also, when higher voltage systems are used, they are typically called 28 or 42 volts. 12 volts is just a historical inaccuracy, probably carried over from the transition from 6 volts.
 
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