12v or 24v

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Lostranger
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12v or 24v

Post by Lostranger »

I've mentioned in the bus barn category that I'm getting ready to build a new rolling home that my wife and I will live in full-time. The bus conversion we've used for the past ten years had no solar power. The coach system was 24v and I used a 12v house system.

The new house will be built in a semi trailer and will be mostly solar powered. We'll have a generator and shore power options, but only as backup. We want to be able to live off grid for extended periods. We cook with propane, and we don't use a microwave, hair dryer, or many of the other power hungry "necessities". We'll have central air but will never attempt to use it off grid.

Now that I have a clean sheet, I need to decide which voltage to use. The only appliance I'm keeping that needs 12v is a Dometic refrigerator. It does not run on DC and only needs 12v for the control circuit. I figure that should be no problem either way. I have a couple of 12v pumps, but those are easy to replace.

I'd like to charge the house batteries from the truck engine when we're traveling. Most of the tractors I've considered are 12v, but maybe adding a 24v belt drive alternator would not be a big deal.

I'm drawn to 24v, but I have no solar experience, and I'd like to hear from those of you who have done it for a while. Most of the discussion I've found on the web is more theoretical than practical. What would you consider the ideal batteries for this rig? I'll have plenty of space and weight capacity, but not unlimited funds.

Also, does anyone have experience with solar film? I've just heard about it, and I wonder if it might be a good way to go on a large trailer roof.

Jim Huskins
Marion, NC
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12v or 24v

Post by Lostranger »

Follow up to my battery question. Would four Trojan L16 H batteries be a good starting place for our mobile off grid system? Could go either voltage.

Jim
Sharkey
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Post by Sharkey »

Jim, when I ordered new equipment for my system, I went with 24 volts. Unless you have a need for 12 volts for appliances, etc, there is no reason to stay at the lower voltage and several good reasons to pick the higher.

The first and most obvious is that conductors, fuses, etc all can be smaller as the current flowing at any given power level is half what it would be at 12 volts. Everything runs cooler and you spend less on wire, etc.

A bigger and more important reason is that it is easier to create higher battery capacity at the higher voltage. Taking your L16 batteries as an example, two in series would be 350 Ah @ 12 volts. In order to get 1,400Ah, you would need four paralleled series strings of two batteries each.

Let's convert that capacity to Watt-hours: 1,400 x 12 = 16,800 or 16.8 KwH.

To get the same capacity at 24 volts, would require two paralleled strings of four batteries each: 350Ah x 2 = 700 x 24 = 16,800.

Why does this matter? An absolute rule of smart solar installers is never more than two paralleled strings of batteries. Once you go beyond two strings, the ability to discharge and recharge each and every cell (cell, not battery) in the system equally becomes unmanageable. The batteries deliver less useable power and fail sooner. Many installers refuse to build even two paralleled strings, opting for even higher voltages: 350 x 48 = 16,800.

One series string of 8 six-volt cells (48 volts) is going to have a lot less wiring and many fewer connections than four strings of two cells (12 volts) connected in parallel. Fewer connections means fewer places for corrosion to grow, fewer points of failure.

Maybe you don't need that much capacity (yet), but it illustrates the advantages of higher voltage systems.

As for batteries, you should consider Trojan L16-type batteries to be your "training wheels" set, and plan on installing a true industrial battery after the first set dies, probably in 3-5 years. Many installers specify L16 batteries because they are cheap, and relatively expendable, they make a good lesson for a new off-grid owner who will probably murder them in a short time while learning what mistakes not to make while living with a finite amount of power. Once the owner figures out how to check the electrolyte, not run them dead dead, not letting them sit discharged for long periods, make sure they are equalized periodically, keep them warm in the winter and cool in the summer, clean and tighten the connections frequently, etc etc etc, then a second set of more expensive batteries like HUP or Rolls Surrette are specified. These batteries come with a 10 -20 year warranty, and really will last that long with proper care.

12 volt power for your fridge or other small loads can come from a DC-to-DC converter or a load balancer like a Vanner.
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12v or 24v

Post by Lostranger »

Thanks Sharkey. 24v it is, and I'll take your battery advise to heart as well. I'm going to have lots more solar questions, but all in good time.

Have you had any personal experience with solar film? What little I know sounds like it might be the ticket for our needs.

Jim
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