Need help deciding 24V or 12V battery bank

[TechnoWeenie]

So, I'm running into an issue. I always overanalyze things...

The issue? 24V or 12V (or both?) battery bank.

I have an AM General M934 which is an Army 6x6 (Think deuce and a half, but WAYYYY bigger)

I have extensive experience with electrical and electronics in general.

My issue is.. I've always added additional battery capacity at the voltage of the host vehicle, as the host vehicle was set up to handle the equipment already installed (Mostly 12V systems).

I was originally planning a 12V battery bank, isolated from the vehicle electric system (24V), and recharged via solar... and very limited, if any, 120VAC devices.. 12V LED lighting, DC->DC for laptop instead of inefficient DC->AC->DC, etc.

I intend on eventually running close to 1kw in solar, as I live in an area with little sunlight in winter and want to maximize collection. I want the rig to be completely off grid, essentially. No backup generator, no gas (vehicle is diesel fueled), no shore power. I also know about energy conservation, ie doesn't make sense to run everything off 120VAC when so many devices use transformers and power regulators to bring them down to 12, 15, 19 or whatever lower DC voltage they operate at.

I'm really big on universality and commonality, in terms of electrical systems. 12V devices are EVERYWHERE and while there are 24V accy. options, like inverters for example, they're more expensive, so I'm hesitant to go with a 24V bank.

With a ton of solar, having 48V drop to charge a 24V battery bank means much smaller gauge wiring vs 12V... double voltage, half amperage....and this rings true for high current devices as well like a large inverter..

Then I thought, well, I can just add a small 24->12V converter...Then realized the 15-20% efficiency loss.

I don't wanna pull 12V off the battery bank if it's 24V because of load balancing....

I also need at least 50A @ 12V for comm gear.


So....

What I was thinking....

24V solar, 24V battery bank, battery cutoff switch to join front and rear batteries when vehicle is running to charge rear battery bank, if needed.....OR to jump vehicle from house bank if start batts go bad/dead....24V lighting, 24v inverter.... But how to get high current 12V?

Am I still overanalyzing things?


FYI. For size comparison, the truck behind the rig is a full size Chevy 5/4 ton (.mil doesn't say 1 1/4 ton, they call it 5/4 ton, hah) vehicle with a service body, called an M1031, that's towed behind the rig..

[nothermark]

FWIW I think you would be better off in the long run staying at 12 VDC for your systems. Cheaper and easier. Take the conversion hit once to get to 12 VDC but do your solar conversion at the battery bank to get to run higher voltages from the panels to the batteries. Put in a smallish 12 > 24 converter to maintain the engine batteries. I'd also plan on an inverter to supply 120 VAC for some things. I think you will also want a generator available.

[Soppy]

Go with the 24v system . The 24v inverters are slightly more efficient . Do not tap 12v off a 24v battery bank. You will destroy both batteries. DC to DC converters work great . You can get a large converter to supply 50 amps , or use multiple converters , one for each piece of com gear if the draw is not to much. You could also use a converter to charge a 12v battery . I am thinking the 50 amps is while using a transmitter and will be much less when not transmitting. Even though the converters are 80 -90% efficient , the losses in the 12v wiring will be 5 - 10% so the total loss is not that great . If you are using LED lighting , 2 - 12v bulbs can be connected in series .

[MSHappyCampers]

Hi ! Welcome to IRV2! We're sure glad you joined the gang!

You lost me from the start! Sorry I can't help!

Good luck, happy trails, and God bless!

[CountryFit]

i am in the camp of 12v. i have way bigger system as you described i am doing fine. not much benefit to use 24v but a lot more hassle to convert. be happy with what you have and enjoy it.

[nothermark]

Originally Posted by Soppy

Go with the 24v system . The 24v inverters are slightly more efficient . Do not tap 12v off a 24v battery bank. You will destroy both batteries. DC to DC converters work great . You can get a large converter to supply 50 amps , or use multiple converters , one for each piece of com gear if the draw is not to much. You could also use a converter to charge a 12v battery . I am thinking the 50 amps is while using a transmitter and will be much less when not transmitting. Even though the converters are 80 -90% efficient , the losses in the 12v wiring will be 5 - 10% so the total loss is not that great . If you are using LED lighting , 2 - 12v bulbs can be connected in series .

I'm curious why you think he should go to all 24 VDC. Almost everything in the rig will then be special wiring or need a 24 to 12 converter. Every converter adds cost and takes an efficiency hit. If he runs a 12 VDC bank his hardware is probably cheaper, he does not take much more of a hit on the solar efficiency and the only up converter he needs is to maintain his engine batteries. That should give him an overall better efficiency for the whole system.

[TechnoWeenie]

Currently, it is wired for a MEP genset. Which is three phase 240 and 120. The interior is being completely redone. Wiring is going to be completely redone. The existing breaker panel is going to be removed. Wall and ceiling panels will be removed. Insulation will be removed and replaced. This is a full retrofit. So, running wires is not an issue, however keeping things uniform is ideal. Having to run both 24 and 12 volt, and then 110 on top of that, would be a pain in the butt.

I have a blank slate, and can add any equipment that I want or need, within reason. So, it's not an issue of existing equipment having to be converted to one voltage or another, with the exception of the 12-volt radio equipment, which I'm kinda stuck with.



While the military has a tow behind 30 kilowatt generator that runs on diesel, that was normally used with this piece of equipment, I don't want to be tied down with a trailer, and spending $300 a month on fuel to have a 30 kilowatt generator running 24 hours a day for the 100 Watts I'm going to actually be using, is silly.



My expexted electrical load...

LED lighting, which is available in 12V or 24V, on PWM dimmers,which will probably me most of my energy usage.

TV or Projector. Most likely 120VAC but used sparingly.



Small roof mounted exhaust vent fans (mostly 12V, haven't researched 24V availability yet)

Possibly A/C in the future, most likely a 'portable'/floor model with outside vent (not window mount) which would mean 120VAC.. Rooftop is nice, and readily available in 24V but at 5x the cost.

12V radio equipment.

Stereo, which is available in both 12v and 24v

Pseudo-randomly placed 12v and 120 volt AC electrical outlets. Cig plug and std outlets. Minimal use.

Exterior motion sensing LED lighting. Available 24V but expensive.

POE camera system, DVR would probably be 12V.

The other option.... Is 200ah in 12V and feed 2 solar chargers, one 24v and one 12v.

The ability to charge the bank from the alternator without major hassle is appealing.

Again, not going genrator.

I also want to be able to start small and add as I need. Ie start off with maybe 200W of solar and 200AH of batteries.. As I expand, my usage may increase, such as adding a water pump (currently hand pumped from 5 gallon jugs), A/C as mentioned, fridge, etc.

I really don't like the idea of both voltages, for simplicity sake. But I'm open to options..

Like I said, I usually over-analyze things, and that kind of gets me bouncing back and forth between options and not able to make a decision.

Having fresh eyes on the situation is always appreciated, as a lot of times people will think of things that I have not, or be able to point out something that I thought of but I'm able to see it a new light with new input.

So, keep the opinions and ideas flowing. I appreciate it.

[nothermark]

You are making a good argument for 120 VAC and 12 VDC. Cheaper and easier to get parts. You don't want the big tow behind generator but you might find it worth looking into a built in smaller diesel generator if you can figure out how to tap the fuel tank. Otherwise a small gas generator that you can use if you get several days without sun.

I think you want more than 200 AH of batteries. I'd look at twice that if you want to be mostly solar. I'd also consider one smaller panel dedicated to keeping the engine battery charged and the rest feeding the 12 V bank. That eliminates a converter but adds a controller.

Part of what you need depends on what kind of usage you are planning. If you are building a base camp that can head out into the desert you will have the sun and not do a lot of main engine time so want some battery maintenance. If you are doing a tripping vehicle that will move every couple of days the engine battery will take care of itself. I'd include a 24 V charger than ran off 120 VAC and could be plugged into a portable generator or inverter output. That would be for emergency use if the engine battery ran down for some reason.

[Soppy]

Originally Posted by nothermark

I'm curious why you think he should go to all 24 VDC. Almost everything in the rig will then be special wiring or need a 24 to 12 converter. Every converter adds cost and takes an efficiency hit. If he runs a 12 VDC bank his hardware is probably cheaper, he does not take much more of a hit on the solar efficiency and the only up converter he needs is to maintain his engine batteries. That should give him an overall better efficiency for the whole system.

Voltage drop and wire size is the main reason to go to a 24 volt system. The wiring for 12 volts has to be twice the size of the wiring for a 24 volt system to deliver the same power. It's not much of a concern for low current , short runs and non voltage sensitive equipment. On 24v you could use 14ga wire to carry 15 amps . For the same power on 12v you would need 10ga wire to carry 30 amps. If the voltage drop is 2v on a 12v system , it works out to 17% . The same 2v drop on 24v is 8.3% .If the same size wire is used , the voltage drop would only be 1v on 24v making it 4.2%. Since the vehicle is 24v , the system will easily interface with the chassis system for charging .
The charge controller is another cost to consider . A charge controller for 720 watts of solar will have an output of 60 amps. The same charge controller at 24v will handle 1440 watts of solar . In the solar off grid industry , systems with less than 500 watts of solar were 12v . Anything larger , we would install a 24v system .

[powercat_ras]

Generally solar panels output a max of about 6 amps per 100 Watts of size. If you use a MPPT Type Solar Charge Controller it will convert a higher solar panel voltage down to 12 VDC charging output with better than 95% efficiency. You can wire solar panels in series to cut down the amps from the solar panels to the charge controller so there is less loss in the solar wiring.

All that said putting solar panels in parallel is better for RVs so that if parked in partial shade each panel can contribute its max possible. If in series one shaded panel in a series string will also cut down on the contribution of the rest.

8 Gauge Copper Cable is so inexpensive that the wire savings to go series in the RV application for solar cabling is minimal. 8 Gauge runs from the solar arrary to the charge controller can easily handle up to 600 watts of solar panels in parallel.

[Crankshaft]

Very interesting project. Keep us in the loop as it progresses.

Can't help much with the electrical issues. However, I do have an idea that may help with a generator/heat and cooling.

Look for an APU (auxiliary power unit) from a truck. New, they are very expensive but there are used ones out there. The APU will usually have at least a 6KW generator. These unit also provide heat and cooling. All powered by diesel fuel.