12 or 24 volt widget on 24V electrical system

[rmaddy]

I'm working out the nitty gritty details of a planned 24V electrical system in a cargo trailer conversion. Part of the system will have a DC-DC converter to give me a 12V fuse box for all of the typical 12V items such as lights, fans, and DC outlets.

With that in mind, I'm trying to figure out for items that can be either 12V or 24V, which is better? For example, the water pump. I can get a 12V water pump or I can get a 24V water pump. Also, the DC refrigerator I'm planning handles both 12 and 24 volts. In these cases, are there advantages either way to choosing to wire them as 12V or 24V?

The only advantage I know of off the top of my head is just like the rest of the system - smaller wiring due to lower amps on the 24V over the 12V.

I suppose choosing a 12V water pump has the advantage of being easier to find in a store in the future if I need to replace it quickly on the road.

 

[highdesertranger]

"I suppose choosing a 12V water pump has the advantage of being easier to find in a store in the future if I need to replace it quickly on the road."

bingo we have a winner. you answered your own question.

I would say that is probably true for all dedicated 12v items.

as anyone ever seen a 24v phone charger or laptop charger.

I have to ask why 24v. I realize the smaller wire lower amps thing but is that the only reason?

highdesertranger

 

[rmaddy]

I have to ask why 24v. I realize the smaller wire lower amps thing but is that the only reason?

See  from Will Prowse for a good (to me at least) explanation of 12V versus 24V solar electrical system.

as anyone ever seen a 24v phone charger or laptop charger.

Hence my use of standard 12V plugs in my system.

 

[Trebor English]

A 60 watt light bulb uses 60 watts.  At 120 volts that's 1/2 amp so a skinny wire will suffice.  At 12 volts it's 5 amps.  Still 60 watts.  Electric utilities use high voltage transmission lines to reduce loss.  They still use the local power line frequency.  Transformers efficiently convert low voltage to high and high to low.

The transmission line towers are huge with vast spacing between the wires.  Alternating currents cause losses other than just simple resistance.  For example, the skin effect causes the current to concentrate on the skin of the conductor.  Since less current flows in the center that portion of the copper is less utilized.  They could convert the high voltage AC to DC, transmit over long distance with less loss, then convert DC back to AC.  It is expensive to buy the equipment and losses in the equipment eat up the savings.  The result is that electric utilities, with rare exceptions, do not use high voltage direct current transmission lines. 

With MPPT solar charge controllers being expensive and having a limited maximum current a 24 volt solar battery charging system could save some money.  If the DC to DC converter to make 12 volts from 24 is less expensive than the higher price of lower voltage MPPT charge controllers then a 24 volt system might be preferable. 

24 volt components like cell phone chargers and laptop computer power supplies either don't exist or are expensive.  Sticking with 12 volts will make the purchase of devices easier and cheaper. 

If you mount the inverter close to the battery your high current transmission line might be only 2 feet and the high voltage AC can travel the long distance to the microwave, toaster, and coffee maker at 120 volts (US, 240 volts other places) rather than an inconvenient 24 volts or less efficient 12. 

Golf carts use 36 volts.  Battery chargers for them are available.  You could use 36 volts.  Some new golf carts and John Deer Gators are using 48 volts.  Each increase in voltage gives a decrease in transmission loss. 

The 24 volt DC to 12 volt DC converter will have the same roughly 20% loss as an inverter so the  efficiency improvement might be less than you expect.  Larger conductors could be better than a 20% conversion loss.  Fatter copper wire might be more reliable.

 

[rmaddy]

I'd appreciate it if we can please keep this thread on topic. I don't wish to start a 12V versus 24V electrical system discussion in this thread. Feel free to start a new thread if desired. Thanks.

Any thoughts on the pros/cons of wiring either a 12V or 24V version of a device (when given the option) to a 24V electrical system? And what about a device that supports both voltages such as a DC fridge?

So far it seems that wiring the fridge to the 24V bus has the advantage of needing smaller (slightly cheaper) wire and avoiding the efficiency losses that would be incurred if connected to the 12V fuse box which is a connected to the 24VDC-12VDC converter. This is important for something like the fridge which will run 24/7.

For something like the water pump, it may not make any difference since it will be used far less frequently. I would save a tiny bit (if any) in wiring cost but that would be greatly offset by the more expensive and less common 24V water pump.

 

[kygreg]

 

Any thoughts on the pros/cons of wiring either a 12V or 24V version of a device (when given the option) to a 24V electrical system? And what about a device that supports both voltages such as a DC fridge?
 

24v will always be more efficient than 12v especially on things like a frig that runs 24/7 or general ventilation fans that you want to run many hours at a time. These days you can get water pumps, lights, refrigerators and fans in 24v, just 2 days away from amazon.

 

[tx2sturgis]

Here we go again, Will Prowse said it so it MUST be true.

However, I recommend staying with 12v for your normal, standard, consumer level system unless you have a specific and very valid reason to go with 24v.

If you have that reason (or those reasons) please let us know and we will work with you on it.

 

[Trebor English]

I think it is obvious.  If you have a 24 volt battery and a 24 to 12 volt converter you want 24 volt loads. 

1) You can use smaller wire.

2) You avoid the 20% loss in the 24 to 12 volt converter.

3) The finite reliability of the 24 to 12 converter doesn't affect the availability of 24 volt components.

 

[rmaddy]

I think it is obvious.  If you have a 24 volt battery and a 24 to 12 volt converter you want 24 volt loads. 

Thank you. You make good points. It seems obvious once you learn it but not before.

24v will always be more efficient than 12v especially on things like a frig that runs 24/7 or general ventilation fans that you want to run many hours at a time.

Thank you.

Good info

 

[tx2sturgis]

It is only 'obvious' if your priorities are getting a half percent more efficiency at the expense of higher cost and little off-the-shelf convenience and don't mind buying and installing a lot more hardware that you will have to order, possibly keeping extras on board, they generally cost more so any savings on copper wiring is moot.

Yes, there are cases where it makes sense...the military has standardized on 28v for decades. And several decades ago, when batteries were less powerful and long-lasting than they are today, semi trucks often had 24 volt starters, and a series-parallel relay near the 4 batteries to feed 24v to the starter when the button was pushed. Even then, the entire rest of the truck was 12 volts: the lights, the radio, the heater and AC blower motors, etc. 

Again, a specific, non-debatable case is needed. Provide one or more, GOOD, cost-saving reasons, and I will not argue those. 

24volt systems are a boutique market for most modern mobile users in normal road going rigs...but if you just WANT 24 volts, well I wont argue that...but the bottom-line numbers rarely justify it in a van or small RV. 

There is probably no SOLID reason or true financial benefit for doing it in a common RV or van.

 

[tx2sturgis]

My above reply was referring to USA vehicles and situations, I realize other countries have different standards in some cases.

Its a bit like talking about 110v and 220v AC...in the USA we generally standardize on 110 (120) for household appliances but of course forum members posting here that are in other countries where 220 is standard household voltage will usually let us know when talking about those appliances, and the inverters to power them in a vehicle.

 

[kygreg]

Move ahead with 24v, every aspect of your system will be more efficient, easier and cheaper to wire. 24v items used to be harder to find, but now with amazon they are just 2 days away. The only thing I will need 12v for is a couple of usb charging outlets. My frig. main van exhaust, water pump, and all my LED lights are 24v, all off amazon, easy peasy.

 

[wayne49]

"frig that runs 24/7"
That would be a poorly insulated refrigerator.

 

[wayne49]

Make sure there is plenty of signage that the base DC voltage is 24V.

 

[rmaddy]

"frig that runs 24/7". That would be a poorly insulated refrigerator.

You mean I shouldn't be using it with the door open in AZ as an A/C?

Make sure there is plenty of signage that the base DC voltage is 24V.

Lots of labels. Good reminder.

 

[Trebor English]

The only thing I will need 12v for is a couple of usb charging outlets.

You don't need to convert 24 volts to 12 volts then 12 volts to 5 volts.  You can go 24 direct to 5 volts.

https://www.amazon.com/DROK-Converter-Adjustable-Transformer-Stabilizer/dp/B07KWX33X5/

Voltage Reducer, DROK DC-DC Buck Converter DC 8-35V to DC 1.5-24V 5A Power Supply Step Down Regulator Module 24V 12V 5V Volt Transformer Stabilizer

OR

a dual USB socket
https://www.amazon.com/USMEI-Waterproof-Voltmeter-Motorcycle-Tractors/dp/B07HVM8X1C/

USMEI Dual USB Charger Socket Waterproof Power Outlet 4.8A with 12V/24V Voltmeter LED Digital Display for Car Boat Marine Motorcycle ATV RV Campers Tractors