Rheostats, battery chargers, AC/DC converters and Solar panels or wind generator

[rickt300]

I work for several commercial greenhouses where we set up automatic watering/ fertylizing. Depending on how many plants we were covering we either used AC sump pumps or the little DC pumps used for bailing out boats. In both cases we used Rheostats to control water pressure. With the DC pumps we put the rheostat between the outlet and a battery charger. Never a problem arose. That is until the electricity was shut down for 11 days.  How many of the right kind of batteries (Golf cart batteries) would you need to run these pumps? They don't have to run long, no more than 15 minutes at a time 2 times a day and I can easily combine wind generation with solar. Also do rheostats waste energy like resistors do?

[efremtags]

A rheostat is a variable resistor, so yes it does burn power.


You need to determine your pump/rheostat current draw in amps and multiply that by the hours of run time a day. A battery is rated in am hours. You do not want to draw more than 80% of the battery's ratings ever.

So for example sake, a 1AH load for 12hrs a day draws 12AH. If you wanted a weeks backup you would need 7 x 12AH or 84AH minimum. Oversize by 120% so you avoid going below the 80% mark and you wind up with about 100AH battery.

[bilmac]

I am learning as I go keeping a camper trailer on my place with no 110. It's a modern trl. so uses lots of 12V power. It even has to have electricity to run the frig on propane. What I am discovering is that motors don't draw a lot of juice. The furnace has two blowers to operate, and even on cold nights when it comes on a lot, a charged deep cycle battery will keep it going for a couple of nights.

Another thing I'm finding out is that it takes way more solar wattage to keep up than I imagined. I started with a little foldable 5 watt job thinking that would keep me topped off. Not even close. Then I added a couple of 15 watters. Even if I constantly adjusted them to keep them working at max they fell behind. Now I have about 100 watts and I think if the sun shines strong most of the day, and I use a minimum amount of power, it would keep up. But a few days of clouds and I get behind.

100 watts sounds like a lot, but you have to divide that by 12 to get amps. That's only 8 amps. An 8 amp battery charger is on the weak side. Then there is the fact that it is only 8 amps at peak power. Less than that if not aimed right, and none at night.

There is an article in the current Backwoods Home magazine. A couple has been living with 700 watts for some time. They are minimilists. You hear from some who think they have to have in the 1000s of watts.

If you are just looking for a backup I would suggest a tiny generator. Harbor Freight sells one for 90 bucks that would probably keep your 12v stuff running as long as the gas holds out.

[blind ear]

If you have the resources you can build an envelope over your trailer. It is like putting a green house over the trailer. You have solar heat when you need it and you also have louvers on the envelope to provide shade when needed. This is a very effective "passive" approach that decreases the demand for commercial power generators.

[bilmac]

Trl will only be there for a few years till I can build a passive solar cabin.

[dpe.ahoy]

Well, I'm pullin the plug and gonna get 3 or 4- 45 watt solar panels to use at my cabin.  Mostly gonna be for a small frig, couple of lights and some music or dvd player for the kids.  Plan on havin 3 or 4 deep cycle batteries to keep things going and have 2 generators to use for backup if things need charged up again.  Not there more than a week at a time, usually only 2 days at a time.  I think that will keep things charged up between visits.  Now do I hook a panel to each battery or do I hook all the panels to all the batteries?  DP

[bilmac]

Three or four 45s is a lot of juice, I would think if you have generators as backup you wouldn't need that much, the panels would just need to be big enough to have the batteries topped off when you return. You might consider a propane fridge, they are expensive, but last forever.

[efremtags]

Solar power is an opportunity charge, so you need to do the work a 24HR power draw consumes in a few peak hours of available sun. As an example, a 100W light (not much of a load) left on for 12hrs a day wold need 500W of solar to keep it running!.

Rule of thumb is take your load, calculate the average load in a day in watts, then multiply by x10, thats how much solar you need.
Batteries require minimum of 3days of backup, otherwise battery life will be pretty poor.

For cabin systems, a good start is about 1k for a vacation home, but for a 24/7/365 home 3kw would be a reasonable minimum system. Either wold use a generator to supplement the system as needed.

Solar modules can be wired in series/parallel combinations to achieve the desired configuration.  Surf the web, there's a ton of examples out there. Solar home is a good magazine for novice tinkerers. Same goes for batteries.

[efremtags]

also check out DC refriderator by Steca, they will run on <200w of solar.

[dpe.ahoy]

Thanks for the info!  I'm pretty un-educated when it comes to wiring ect.  I'm trying to keep this as simple as possible, for me it's fine as is now.  For wife and 7 kids, not so much.   DP