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12V versus 24V

OK, pls adjust to read “there exist no affordable 24V batteries…” though admittedly “affordable” is relative.

EBZH Kiewit, Belgium

Jan_Olieslagers wrote:

the internal resistance of the “secondary” winding does not change, so with double the voltage it will dissipate four times the power

The voltage drop, and hence the power dissipated across a resistor, depends only on the current, not on the voltage itself. That is why long distance electricity transmission grids use 110kV or higher voltages.

There IS a free lunch in electricity – double the voltage, halve the current, and you get the same power with less loss from the power line. Or use a thinner line to save a bit of money.

While it is a bit more complex if you want consant power output from an inductor, the same applies in an alternator. At the same RPM, you need a stronger magnetic field so the field coil needs a higher current, or a faster moving magnetic field (higher RPM).

Biggin Hill

I think most alternators have lots of leeway on the field current i.e. in typical use the field current is a small % of the maximum i.e. the voltage applied to the field coil is a small % of the full available 14V or 28V.

And in a 28V system you have 2x more voltage available to put across the field coil, so automatically you get 2x the output voltage from the alternator (the field current is 2x bigger then).

So generating 28V (at say 70A) is not harder than generating 12V (also at 70A), using exactly the same alternator.

Shoreham EGKA, United Kingdom

OK – so why not a 48v system? Even thinner wires etc….
I was told that the ‘safety’ limit for humans was around 42v, can anyone confirm this?

EGHI, United Kingdom

I thought it was higher the better, and AC even better, 3:phase perfecto? Otherwise it just what you can get away with. I.e. It is 12 volts because that’s what you can get back away with.

United Kingdom

so why not a 48v system?

Good Q.

Probably historical… there is a slight extra difficulty with electronics at 48V, plus the starter motor transient spec ( ISO-16750-2) scales with the system voltage and the 24V ones are bad enough, at 200V. And a 48V battery would have very small cells. And… all the 24V parts like alternators, light bulbs, relays etc are off the shelf commercial vehicle (truck) parts, and nobody is/was making 48V ones because there are no 48V road vehicles.

IIRC, the European safety regs start to come in at 50V; above that you get big problems. Actually, the Low Voltage Directive says 75V… so not sure whether that figure is relevant for some other reason. The LVD is the main reason why most consumer IT equipment uses the crappy chinese “power plug” power supply units, to avoid mains going into the equipment; it offloads the approval onto the power supply manufacturer

AC even better

True; 110V 400Hz is I believe widely (?) used in bigger planes. 400Hz to reduce the weight of all magnetic components, in proportion to the increasing frequency.

With my forthcoming TKS installation I am glad I have a 24V aircraft because there is no PMAd or STCd alternator for my plane which is bigger. It would probably be another field approval…

Shoreham EGKA, United Kingdom

Three phase; 110V 400Hz is the standard I believe. With some lower voltage DC backup, or some similar variation.

Last Edited by Ted at 26 Nov 16:33
United Kingdom

Cars are moving to 48V right now. GA aircraft can follow but you need to have enough compatible components available for this to make sense. Otherwise you waste a lot of money, space and useful load on voltage converters.

The first milestone is certified lithium aircraft batteries. This allowed cars to make the move to 48V. Until today they all keep a 12V starter/emergency lead acid battery AFAIK.

For electric cars 48V (or more) is much better, but I don’t see the advantage or relevance to light piston aircraft because the only thing which draws a lot of current is the starter motor, and 24V ones are plenty strong enough. Avionics don’t benefit because apart from LCD heaters and such stuff, you have to down-convert to 5V or 3.3V anyway…

Just thought of another benefit of 24V: with half the current flowing around the wires, you get half the effect on the compass system.

Shoreham EGKA, United Kingdom

There is a big benefit to 48V: cable size/weight. Being able to put the battery anywhere in the airframe is an advantage. 48V aircraft batteries will be very lightweight and allow for any form factor. The battery in my aircraft is very heavy, basically one suitcase of useable load or 40 minutes of endurance gone.

Peter wrote:

For electric cars 48V (or more) is much better

“Real” electric cars use 400V or so. The 48V is for light hybrids and is the new standard system in the car, replacing the old 12V system for everything. The car starter is also the car generator and the hybrid in new cars, built right into the transmission/gearbox (similar to turbine aircraft).

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