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I’m a noob in electricity so pardon for my ignorance. Why the battery voltage is always lower than whole system and alternator voltage (considering light aircraft electrical system). E.g. In Cessna 150 system voltage is 14 V and battery voltage is 12 V, in C152 system and alternator voltage is 28 V and battery voltage is 24 V.

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This is more of an Electrical Engineering question than Aviation, indeed the exact same thing can be asked about any automobile.

Both of the figures that you quote are 'nominal': The battery cannot be expected to put out a much higher voltage than 12v, so in reality, all systems will work fine down to about 10volts or so, though 'normal' voltage will be 12.5v for a reasonably new, modern battery in good condition.

Conversely, the battery itself requires a significantly higher voltage to charge itself, so the alternator needs to put out 13 to 14 volts, which gets rounded off to 14v for simplicity when talking "in general".

Nearly forgot - for some heavy duty systems, a 12v system is insufficient, so the battery is simply doubled up, so 12v nominal becomes 24 volt, etc., but with the same tolerances.

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    $\begingroup$ You should express the last bit in terms of wattage. Going to 28v is mostly to allow a doubling of wattage capacity of the overall electrical system without having to make the wire sizes larger. Or if starting from scratch you can make the wires smaller. You also get a bit less sensitivity to poor connections, especially with voltage sensitive circuits like those with variable resistors, while having to live with more demanding insulation requirements. 28 v has been settled on by the aviation industry as the best compromise for DC systems taking all that into account. $\endgroup$
    – John K
    Jan 8 at 15:36
  • $\begingroup$ plus not all battery chemistries are 12 V / 24 V nominal. Ni-Cd and Li-Ion will have different nominal voltages but they are still designed to work on a 14 V / 28 V charging system $\endgroup$ Jan 8 at 19:40
  • $\begingroup$ Indeed, the alternator on my Piper Cherokee is exactly the same as used on '60s Chrysler products. If you have a more modern car and an OBDII reader, you can easily see the system voltages. On mine, it's 14.4 volts when the system is charging, 12.8 down to about 12.4 when running off battery. If it gets down below that, the alternator starts charging again. $\endgroup$
    – jamesqf
    Jan 8 at 20:04
  • $\begingroup$ @selectstriker2 Everything about aircraft design is extremely conservative for safety reasons. You are unlikely to find anything except Lead-Acid certified for a "old" design like a Piper or Cessna. (And of course Boeing learned about the consequences of battery problems the hard way, with the 787!) $\endgroup$
    – alephzero
    Jan 8 at 22:52
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    $\begingroup$ If you think about it, if you want to push the electrons the wrong way through the battery, you have to push them harder than the battery is pushing back. That's why the alternator voltage is higher than the battery voltage. $\endgroup$
    – user253751
    Jan 9 at 6:20

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