Future Vehicle Voltages, 12 or 24?

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August 24, 2008

Many North American automobiles up until the mid ‘50’s were equipped with a 6 volt electrical system.   This changed to a 12 volt system over a couple of years on pretty well all makes of vehicles.   This still stands today, with even many large class A motor coaches using 12 volt direct current as their main operating electrical power.  This is, however, in contrast to many bus conversion type rigs which have adapted to a 24 volt D.C. system. 

So, which is better, 12 or 24 volt direct current?  Why did larger vehicles, including highway buses, opt for this higher operating voltage?   To understand that, we should look to what the driving forces were behind the automotive industry back when 6 volt systems were replaced with 12.  The higher voltage offered better reliability and ability to operate more accessories in a more efficient manner.  Cold weather starting was greatly improved by using higher voltage starter motors.  Additionally, wire gauge could be substantially reduced in size, so that the harness would be only half the size and weight that was required for a 6 volt system.  For battery cables and ground cables and straps, this would be a substantial weight and cost reduction.  For example, a six volt operating system uses battery cables about the diameter of your thumb, while a 12 volt system only has cables equal to the size of your pinky.  This is due to the amperage being only half that required by the lower voltage.   Inverters also increase in efficiency dramatically with higher input voltages.  

The normal push-back for this type of system revolves around availability of electronic accessories in that voltage range.  The fact is, many electronic devices currently accept 12 to 30 plus volts dc, as is.  Additionally there is no problem running a low load 12 volt feed off the battery bank running parallel on the 12 volt power side. 

So, we are paying somewhere between $100,000 and $800,000 for an option filled motor coach that employs a 12 volt electrical system.   Many other large or specialized vehicles are equipped with 24 volt systems.  Not only conversion buses are using the higher voltage, but so are aircraft.  Many airplanes also use higher d.c. voltages ranging from 24 to 48 volts.  For aircraft, weight is certainly the main issue supporting the higher d.c. voltages, however, reliability is probably a close second.    


So, what about it, let’s hear from you.  Is higher d.c. voltage the way to go for today’s motor homes?  Have we just been given a less efficient voltage because the manufacturer has a better selection of 12 volt accessories?  It has to make you wonder.

What do you, think? 


Wondering About A Higher Power   –    Lug_Nut    –     Peter Mercer

Leave a Reply


  1. Daniel, The 6 volt auto system of the 40’s and 50’s wasn’t broken either, but the upgrading to 12 volts was a great improvement. Likewise, electronic ignition was a great step in replacing the older, but good, working mechanical type. Future progress and upgraded systems must always be studied. Thanks for your input.

  2. If it’s not broke…..don’t fix it.

  3. boyd pettitt

    I think the manufactures have already answered this question. 42 VDC is the new voltage which will be appearing in the near future. This voltage has a lot to do with the mild hybrids and starter/generator systems. It reduces the size of all high current carrying conductors. It also makes a lot of money for the battery manufactures who will get to sell the new batteries.

  4. Bill Baxter

    Ah, the old question again.
    Here is my take. 24 V was specified for buses due to the ventilation systems. The blower motors and the air conditioning condenser fan are high amperage motors. At 24V the wire size was reasonable. We are still talking 0 gauge wire not 4/0 as would be the case with 12V. When a bus is using blower fans and condenser fan the load is about 180 amps at 28 volts (nominal 24V). That leaves less than 60 amps for cabin lighting, bus lights and driver ventilation fans for heat and defrost.
    As far as lighting goes stay with 12V but use LED for clearance lights as the 16 gauge wire used for supply is too small for the 12V amperage.
    For this conversion I left both house and coach battery banks as 24V. The inverter charger is happy, the solar controller is happy and the 24/12V equalizer is happy. Additionally I can cross tie the battery banks as needed for redundancy.
    Now for cars the model year is rapidly approaching when cars will be probably 32V. At least that is the info I’ve heard so far. The electrical consumption is getting higher than is feasible for 12V systems alternators. Yah, I know you can get a 300+ amp alternator but try stuffing one under the hood of a new car.
    Anyway my two pennies worth.

  5. Nick, That is interesting and surprising. So the 24 volt chassis only receives its charge from the engine altenator. I’m surprised a 24 volt charger/inverter is not used.

  6. Lug,
    Our 12 volt house and 24 volt coach systems are completely independent of each other, and the genset charges the 12 volt battery bank but not the 24 volt system.

  7. I’ve considered 24v and 48v systems for our next RV. It’s possible that I’ll end up with a 12v chassis system that also powers my basic 12v “house” needs for fridge spark, lights, etc. and a separate 24v or 48v house system that is dedicated to the inverter.

    Just thinking about things at this time, likely 2 years before we replace the old Southwind.


  8. John Malin

    It’s been my understanding that most large tractor trucks are started on 24v but the rest of the unit is on 12v power, so what would be the problem other than space for the bateries?

  9. Fred, great point. The European 220 volt house currant certainly reduces wire size within households, imagine the benefits associated with mobile applications. Thanks for your valued input.

  10. Fred

    Correction. The wire gage for 12 volts has to meet higher amperage requirements THAN for 24 volts. Sorry.

  11. Fred

    The U.S. has stubbornly stuck to the 120 volt electrical grid nationwide even though much of the rest of the world has been using 240 volts for half a century.

    Your point about wire gage is probably one of the most important points. Because a motorhome or any RV is larger than a car, the lengths of the wires become a more important factor. The same length wire for 12 volt has to meet higher amperage requirements for 24 volts.

    As an example, a 20 foot length of 12 gage wire using 12 volts would struggle with a 20 amp load. A 20 foot length of 10 gage wire at 12 volts could handle almost 30 amps. If the system was 24 volts, the same gage and length as above could carry twice as much current.

  12. Nick, Great input from someone that has 24 volt. I guess it would be easier if it were more popular. Perhaps that’s part of the hold up. Kind of the chicken and the egg thing.
    You say you needed boosting one time. Why could you not use your genset to recharge the chassis batteries?
    Again, thanks for your input.

  13. Twice we have had to have our 24 volt MCI bus conversion jump started. It can be done, but it’s a hassle.

    While the bus startiing system is 24 volt, my house battery bank is 12 volt, and I have converted the bus headlights, taillights, marker lights, etc to 12 volt. Finding 24 volt bulbs in places like Podunk, Arkansas is almost impossible. Even in Tucson, Arizona a couple of years ago we had to special order a headlgiht because nobody stocked them.

  14. Stefan, Granted a 24 volt jump would be difficult to find, but we are talking about a recreational vehicle with two separate battery banks. Generally a motor home should never be unable to start on one of its own banks and boosting such, would probably prove impractical given the amp hour size of the bank. Additionally, the genset may be 12 volt feed parallel off a 12 v. series 24 system.
    But, anyway, I can’t ever imagine getting a battery boost for my coach.
    Thanks for the input.

  15. Stefan

    Yes, 24 volts is more efficient but try to get a jump if you have a 24 volt system

  16. Higher voltage i cars was the buzz word a doze o so years ago. But with the success of the Prius and other Hybreds development of the 24 and 48 volt systems for cars appears to have been put on hold. Some of the Hybreds have far exceeded those dream voltages sporting as much as 400 volts stored under the back seat. It would appear that we do as a society allow ourselves to be steered by the press as to what we do. There is press for Hybreds butno press for increasing operating voltages.