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Old 07-22-2008, 10:08 PM   #1
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So many electrical questions

I am very new to skoolie and while I fear my questions are already answered on here I can't seem to efficiently find said answers so here are the queries on my mind:
Appliances: Has anyone had experience using solar fridges? If so, has the cost of the solar panel(s) truly proved energy saving compared to power generated from a shore line or generators?

I am by no means an electrician, I have learned some basics and applied them to wiring a houseboat with varying degrees of success but have yet to start from scratch on a school bus RV until now. My bus is intended to carry many people and so the conversions we are doing are in part to be useful in their own right but also to comply with the basics necessary to register as an RV and avoid the CDL question. Therefore what we're putting in place will most likely not be heavily taxed.

DC/AC wiring plan: I believe I will put in a battery bank of deep cell batteries, not sure yet if marine grade or golf cart are better, any suggestions? I assume this bank gets ganged and goes through an inverter of appropriate size to be most efficient at maximum load then into a circuit box that sends out several outlets. This is obviously a very basic sketch of what's to be done and I'm hoping someone on here can help me fill in the blanks. One thing I don't yet understand is the use of DC electronics and the position of the inverter. Do I set up a DC box that feeds the DC appliances and an AC box that is fed from the inverter, and does all the power come from the same battery bank or should I have separate banks for each system? Also it seems there is a grounding concern for when charging from shore power, is some kind of relay switch necessary to divert the ground from the chassis to the shore ground?
Finally if I'm not carrying a generator and plan only to charge up from shore power is it best to introduce an isolator to make sure we can maintain our fridge and basics at all times? All I'm planning to be powering is a small fridge (not yet acquired), interior lights, and several outlets for computers, hot pot, and cell phone charging. Our stove will be a propane camping stove. Any suggestions on how many batteries would be best for this plan?

Thanks so much, I'm getting excited about wiring things, up but feel ill prepared to do it right just yet.

Dolores
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Old 07-23-2008, 04:55 PM   #2
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Re: So many electrical questions

Hi-

Lots of good reading at the site Chuck mentioned.

I have a single battery bank that powers both the DC circuit and the AC circuit. I have 4 x Trojan L16H batteries, rated at 420 AH each @ 6 volts. They are wired in series/parallel to make a 12 volt system. A fat wire goes to a 600-watt inverter, another fat wire goes to a DC switch panel.

It's important to have some sort of instrumentation to keep an eye on your battery bank's health and state of charge. I use a Tri-metric 2020 but there are lots of options.

Do you know how often you'll be on shore power? That will help determine the size of your battery bank, and also the number of solar panels if you go that route. The first step down that road is to quantify your power needs. Make a list of DC and AC appliances and loads, and the amount of time they'll be going, and you should end up with a good idea of what your daily power requirements are. Be sure to account for different power usage on weekdays and week end days; take the higher of the two numbers. The final number should be in watt-hours per day. Depending on how much autonomy you want, this will help determine the size of your battery bank. The AC load should give you a starting point for sizing an inverter, but bear in mind that AC motors have a startup surge that requires more juice than normal running (food processors, power tools, etc.).

Using the max possible load you may encounter on either the AC circuit or the DC circuit, you can size the wires needed to run from the battery bank to the inverter and DC switch panel. The distance between the battery bank and inverter & DC switch panel also factors in to wire-sizing. There are tables on the web and elsewhere that will tell you the correct wire size to use for carrying x current over y feet.

If you do your power calcs using a spreadsheet program, you can easily see how power requirements are changed by removing certain things, like a DC fridge or a hot pot, or using a laptop vs. a desktop computer.

Hope this helps -- good luck.

Sean

PS: I'm not sure what a solar fridge is -- any electric fridge can be powered by a battery bank. There are efficient DC fridges that are made for off-grid use. They cost a lot more than a dorm fridge from Sears, but use a lot less power. If you are going to be using shore power, then there is no question that a standard fridge will cost less than a DC fridge and the requisite solar panels, battery power, wires, etc. If you need to be away from grid power for any period of time, then it may be time to start doing calculations about fridge efficiency, power consumption, and battery & solar panel requirements. If you are only going to be off-grid for a few days at a time, it may be better to simply buy bagged ice and keep your perishables in cooler(s). The price of an efficient DC fridge, panels, batteries, wires, etc can buy a lot of ice.
Here's the page on our fridge install: http://seanf.smugmug.com/gallery/1308951_2Ni8k
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Old 07-24-2008, 03:10 PM   #3
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Re: So many electrical questions

Thanks so much Chuck and Sean. I've poured over the wholesalesolar website and the poop sheets and think I have a better idea of what I'm going to do now.

A few more questions: The device you have to check battery levels and general health, where does that go? Is that a junction between the battery bank and the fat wires to the inverter/DC switch? This bus is not being converted for boondocking or dry camping or whatever it's called that keeps is away from a plug in very often. We'll most likely go no more than one night at a time without an AC hook up. Sean, your fridge looks ideal I think that's just the kind I'm going to look for, and that right there is going to be the only major appliance I need to power, so my needs aren't great.

Also I still don't know the way to go in terms of using the bus's alternator to recharge the batteries. Is this commonly done? Does it require an isolator? how does it work?

Thanks again for all your advice,
Dolores
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Old 07-25-2008, 09:11 AM   #4
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Re: So many electrical questions

IMO, if you're not going for more than a day without being connected to shore-power, you could probably do without any fancy instrumentation. A simple volt-meter would probably do it, along with just being electrically frugal. Just be sure to size your battery bank so that you don't discharge more than 50% of available capacity. Discharging even less than that will extend your battery life...so, all other things being equal, more batteries will last longer.

If you are interested, the TriMetric is installed with four wires connected to various parts of the battery bank; it is independent of the locaiton of the AC or DC circuits. The four sensing wires are small-guage wires, and they don't carry much current, so they can be fairly long. A shunt is used (and purchased separately) if you want to see amps in/out in real-time; the shunt IS installed at the battery bank (but it is small).
I recommend installing any battery instrumentation where you can easily see it to keep an eye on things. The installation instructions that come with the TriMetric are fairly comprehensive, once you're comfortable with the lingo involved. It's a USA-made product too.

Since I don't move very often, I don't charge my batteries from the bus alternator. Someone will chime in shortly.

Cheers
Sean

PS: Another way to search this site is to use Google. Type in the following on the Google page, excluding the quotes:
"charging alternator site:www.skoolie.net"
...should yield some results.
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Old 07-25-2008, 09:48 AM   #5
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Re: So many electrical questions

Thank you Chuck. The link to that web site is most excellent. Lots of usefull info.
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