# What does "28 volts DC" mean, really?

The common battery voltage on aircraft is commonly referred to as "28 volts". What does that really mean in terms of a lead-acid battery?

In many uses, such as cars ("12V"), trucks ("24V") forklifts and golf carts ("36V" or "48V"), locomotives ("64V"), the nameplate number is exactly twice the number of lead-acid "cells". But in reality, the fully charged/charging/float voltage of the system with alternator running is higher - cars 14V, trucks 28V, locos 75V.

Battery units are generally sized at 3, 6 or sometimes 4 units (6, 12 or 8 volts nominal).

So, when aircraft power is specced at 28 volts -- do they actually mean a 14-cell lead-acid battery or two 7-cells (which is almost unheard of)... or do they actually mean a 12-cell battery with a "float" voltage around 28V, (built out of two common-as-dirt "12V to everyone else") batteries?

I hope this isn't overloading this question too much, but when nickel-cadmium packs are used, does that have 20 cells, or 19 or some other number?

It's the second option. In any other context, a small aircraft's electrical system would be called "24V". But the alternator regularly puts out 28V, so that's what's typically expected if you hook up your multimeter while the engine's running. Of course, the on-board equipment can operate over a wide range of voltages, often down to as little as 20V.

• Actually a lot of TSO'd equipment covers a 12-28 voltage range because some aircraft have 12V (14V) systems and they typically make them to be compatible. Jan 12, 2020 at 23:35
• This makes it clear enough, just a differnet convention: it's like if a niche of electrical engineers decided to call the electron source +, and for legacy reasons they stuck with it. Or the way (for reasons) aviators say their speed is 350kias when it's actually 500mph. Jan 13, 2020 at 4:03
• @Mindwin Your last sentence is absolutely false. Undervoltage can and has caused damage to computer equipment. That is, after all, why undervoltage protection devices exist. As an aside, I'm really surprised that your computer was sluggish at a lower voltage. After all, computers aren't like motors where the speed depends on power. Computer speed is regulated by a crystal oscillator. The only explanations I can think of is either it took longer to charge the power supply capacitors to boot, or it was overheating and auto throttling the clock speed to protect itself. Jan 13, 2020 at 17:13
• @Mindwin I have a ton of experience with PCs, and it doesn't work that way. SMPS will produce completely consistent output voltage at any input voltage. It's possible a 100-250V SMPS would work with a 55V input (usually they want at least 70-ish - but since 220V is really 230-250V, you likely had ~60V), but if it can't give enough power, it will fail to produce any. The PC would frequently power down, NOT work sluggishly. ATX power supplies do not inform the load about how much power they can supply. The load will draw everything until it goes off. Jan 13, 2020 at 20:34
• @jamesqf I said kias not kts. You're in the right place to ask what the difference is :) Jan 14, 2020 at 0:28

Multiple sources indicate that the 28 VDC bus on aircraft powered by 24 VDC batteries, and the 24 VDC systems on truck powered by 28 VDC alternators, are basically the same thing, and that is is just a naming convention.

The number of cells is a function of the nominal voltage and the cell type. From this site:

• Nickel based for aviation: 1.25V per cell
• LiIon: 3.6 V per cell.

For instance, from this site on the B737:

Batteries

Battery - Is a 36 ampere-hour, 24 volt, 20 cell, Nickel-cadmium battery and should provide 30 minutes (20 mins 1/200's) of standby power if all other generators fail.

Aux Battery - This is a reserve battery on the NG/MAX which is normally isolated unless the main battery is powering the standby system when it operates in parallel with the main battery. The aux battery combined with the main battery will provide 60 minutes of standby power.

The 28 VDC bus on aeroplanes is normally powered by a Transformer Rectifier Unit (TRU), which is powered by the 115 VAC bus. Power is transformed first from 115 VAC to 28 VAC, then rectified from 28 VAC to 28 VDC. The charging voltage must always be higher than the nominal battery voltage.

From the same B737 site, indicating that the average voltage is more in the order of 27 VDC:

The purists may like to know that the DC voltages are measured at the following points:

• True Blue Power's Li-Ion batteries have a nominal voltage of 26.4 VDC (3.4 V/cell) with a 28 - 28.8 VDC charging voltage. truebluepowerusa.com/products/aviation/… Jan 15, 2020 at 14:50

Let me start of with two facts that pretty much answer your question:

• A 24V battery is a battery that can output at least 24V over the majority of its capacity.
• To charge a rechargeable battery you need to push charge into it by providing it with a higher voltage than what the battery is currently at.

To put a bit more meat on, here is a typical discharge graph for different types of 48V batteries (I could not find one for 24V, but divide the y-axis voltages by 2 and it will be pretty spot on):

So, you design your circuit so that it can stay alive on the minimum-rated voltage from your battery (24V, in your case), but also so that it can handle the voltage that the alternator has to put out to keep the battery charged (for a 24V battery: roughly 28V), which is what it will be at most of the time.

So, when aircraft power is specced at 28 volts do they actually mean a 12-cell battery with a "float" voltage around 28V, (built out of two common-as-dirt "12V to everyone else") batteries?

Yes, two common-as-dirt 12V batteries with a float voltage of 28V.

• Dividing the 13 LiMnO₂ cells by half might be problematic. Jan 13, 2020 at 13:39
• @Pete, dividing by half is easy; it's dividing by two that you'll have problems with. ;-) Jan 13, 2020 at 14:06

But in reality, the fully charged/charging/float voltage of the system with alternator running is higher

Yes. If a car battery states 12V that's kind-of the lower limit. As in, it really shouldn't drop below 11.8 when fully discharged, although depending on the type of battery and vehicle it may still function (for some values of function). A fully charged lead-acid battery goes 10-20% above what's on the label (in case of 12V accu, up to 14.4V is not uncommon).

While the battery is being charged, the voltage is going to be even higher. An alternator for a 12V system will probably output 14.5V. External chargers can go above that. After all, charging tends to go faster if there's a higher-than-target voltage connected. I've seen 12V chargers charging at 15V, but that's probably not very good for the lifespan for the battery involved.

When a voltage is specified on a battery, consider that voltage to be what's left when the battery is almost drained. So everything connected to it should be able to handle voltages that are higher or switched-off during charging (the latter is uncommon in my experience).