Why 24V system

[Wyattearp]

Okay i for some odd reason got to wondering why a 24 volt system is used, and was there a main purpose, was it easier than 12volt or was it because 12 volt had not come to pass at that time.... Oh boy got them wheels a turning didn't i..

Thanks.

 

[Stretch44875]

Maybe because the wiring would be 1/2 the thickness of a 12v system for the same power capacity. I am sure there are other reasons...

 

[cranetruck]

You will see 36 VDC (42 V from charger) in cars within a few years. The higer voltage reduces the size of many components, conductors, power transistors etc. The military was just ahead of the commercial "sector" as so often when money is no object. One battery costs less than two and early automotive electrical systems were very basic. You will find transformers and motors rated at 400 Hz on aircraft for the same reason, size.

Bjorn

 

[lacoda56]

The way I understand it, 24v was set as the standard to keep parts inventory [bulbs, switches, etc.] to a minimum, and for compatability for slave starting. In the '50s and '60s all large vehicles were started with 24v because that was the technology at that time. Many civillian road tractors had series/parallel switches. 12v system switched to 24v for starting [battery cable nightmare].

 

[Wyattearp]

Thanks guys, i believe you all 3 hit on what i was getting at and wondering about... Lacoda that was my main thought was availability and having everything pretty much compatible.

 

[Armada]

You are right Bjorn. There are many benefits to increasing the voltage. Being an engineer in the auto industry, I can confirm automobile voltages will be increased, even up to 48vdc. Plans are already on the table. There is getting to be too much in the way of electronics being put into vehicles not to.

 

[Wyattearp]

I agree with you Armada an engine can't hardly be called mechanical anymore, just sensor and computerized, or else it won't run period.. crazy stuff

 

[Longhunter7]

24 Volts?

There is another reason! The voltage that is used for Military radios happens to be 24 volts!

 

[Low-Tech-Redneck]

I may be wrong, but I was always under the impression that a starter that ran off of 12v didn't have enough power to get the engine started. At least not in the 60's 70's And since the starter was wired for 24, they just wired everything else that way for ease of maintenence

 

[n1vbn]

In the 60's and 70's Heavy trucks and equipment used 24 volts starters with 12 volt series/parallel switches for two reasons. 12 volt electrical stystems are economical compared to 24 volt systems. But a 24 volt starter is better in winter due to the fact that the cranking speed is easier to maintain because of the higher volatage which reduces the amperage by half.

In the 60's most heavy commercial batteries were not rated as heavily as the are today generally 625 CCA was the high side versus 900 or better today. This means the average class 8 truck uses 3 12 volt group 31 batteries instead of four six volt batteries back in the 60's. A six volt battery was rated at 625 CCA back in the 60's giving the truck 2500 CCA to start with versus 1250 CCA. A Diesel engine in a truck needs a minimum of 1800 CCA to start in cold weather and 12 volt batteries were expensive compared to six volt batteries in the 60's. Six volt batteries were preferred by the trucking industry due to cost. One battery was a lot cheaper to replace than a 12 volt battery then.

Another reason the military chose 24 volts is reduced weight.

In the Summer you can get away with two batteries but in Winter you need 3, even 4 batteries up north in really cold areas of the country to have sufficient CCA to reliably start the truck.

There is an alternative starting system still in use today which is better than an electric starter. it is called an air starter which is a gear reduction starter, has it's own air reservoir. The Ingersal Rand air starter of the 60's would give you one 15 second attempt to start at -40 F' at 100 Lbs PSI. The latest version will give you three attempts to start at the same cold and temperature but will start with as little as 60 Lbs of air pressure unlike the old ones.

The major advantage to air starter systems is when you exhaust the air supply and the truck don't start what do you do? Most starter reservoir's usually have a glad hand connection you hook up to any tire to the reservoir wait for the pressure to equalize and then start the truck up. Once the engine is running just leave it idling until the tire has been aired back up.

Try doing that with an electric starter.

 

[jasonjc]

The 24Volt sysetem was use all the way back to WWII for radios so after WWII it was decided that all military vehicals would be 24 volt to permit the use of radios in any vehical as radio use was becoming wide spread.

4 6 Volt 625 CCA batterys wired in series will still only be 625CCA but will be 24 Volt.
To get 2500CCA you have to wire them parallel but at 6 Volt

 

[ajg6989]

another problem was parts stealing like with the old gas engines in the M114 they were the same as people had in there trucks easy parts supply not that it still isnt done but getting away from 6-12 vdc and less people can steal parts for there vehicle unless you have a deuce when you are getting out and want new tires and batteries

 

[ida34]

Jason,
What you say is true of the CCA available when hooked up in series but the draw of a 24 volt starter would be half of the comparable draw of the same starter load with a 12 volt starter. With the same power requirements you would need less amps if you had more volts. I hope this did not come out as gibberish.

 

[G744]

My experience with M-series trucks at the Proving Gorunds in Yuma was they did 24-Volt systems solely for the communications install (read radio) advantage. The mainline vehicular radio sets of the time were designed to run off a 24-volt system, with a few capable of backward compatability to 6- and 12-Volt systems.

Their criteria then wasn't the cranking ability as just about everything they had in the late 40's (when the entire early M-series fleet was designed) was gas powered and didn't have the high compression of Diesels.

Wire size didn't mean diddly to them, as they went to a uniform 14-gage size for most circuits when it was overkilll for a 12-Volt load.

Their engineering decided on something robust and uniform so as to eliminate the huge number of non-interchangeable electrical parts found in the WW2 vehicles. To wit, 90% of the electrical stuff on a 1/4 ton Jeep also fit the 3/4, 2.5, 5, 10 tons, and lots of armor as well.

A light switch was a light switch was a light switch. A generator was, well, you get it.

Only three sizes of batteries to stock; the 6TN, the 2HN, and the 4HN (not very popular, generator sets only). Just three types of light bulbs and sizes of sealed beams even!

It allowed the design of standardized radio installation kits throughout the fleet, again with a lessening of models from WW2.

Obviously true tactical trucks didn't resemble or follow the pattern of any civilian trucks at the time.

That legacy still survives today when you are silly and go to NAPA for dumb stares instead of something you should've gotten from a legitimate surplus (easier & cheaper) dealer.

dg

 

[gwalker]

Just a note on the 4hn it was also used in 715 (possibaly m-37also) truck in artic stations for the inproved cold performance.That is why the 715 has that huge battery box w the reversable lid.

 

[DDoyle]

Re: 24 Volts?

There is another reason! The voltage that is used for Military radios happens to be 24 volts!

DING DING DING

We have a winner! According to the mililtary documents from the 1940s (and this should be correct!), the reason for the 24 volt electrical system was to ensure radio compatibility. Note the date on the attached Detroit Arsenal image.

Regards,
David Doyle

 

[cranetruck]

Re: 24 Volts?

.....the reason for the 24 volt electrical system was to ensure radio compatibility......

Which leads to the next question, why/when were radios standardized for 24vdc?

 

[OSO]

RE: Re: 24 Volts?

.....the reason for the 24 volt electrical system was to ensure radio compatibility......

Which leads to the next question, why/when were radios standardized for 24vdc?

And yes, a very good question!!

 

[kinetichotshot]

There is an alternative starting system still in use today which is better than an electric starter. it is called an air starter...

Cant miss that distinctive sound!!! That'll wake you up in the morning!! (not to mention the neighbors, even the dead dog buried in the back yard!!!

It is very effective though, as long as you remember to drain the water from the reservoir often... otherwise that cold morning start still isn't going to happen!

I had often wondered about the 24v system too, thanks for clearing it up!

Laterz,
Gator

 

[obijohn]

RE: Re: 24 Volts?

And yes, a very good question!!

Because the radio systems of the '40s and '50s, up until the invention of the transistor and solid-state electronics, was all vacuum tube, and these things run at very high voltages and generally low current/amps to keep heat buildup to a minimum. It was easier to step up 24V with a transformer than to step up 12V... again, more efficient because of less loss due to heat. As modern radios were created, it was easier to make then run on 24V than to re-engineer all of the trucks, planes, tanks, etc. There is an electrical advantage to 24V as well, for the same reason it is better for radios. To get the same power (watts) you need half the amps, which means half the current, which means less resistance/heat buildup in wiring. So, 14 gauge wiring is fine for 24v but light for 12v high amperage systems.