Solar charging question

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Benevolus

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Hi everybody, I'm new here. I have a question for all you professionals!

My TT is located on some vacant land I own with no services. The unit has two group 27 deep discharge 12 volt batteries wired in parallel. I'd like to hook up the batteries to a roof mounted solar cell. My question is, what is the lowest/smallest amount of output in amps that I would need to keep those batteries charged while not in use? BTW, after using the TT, I would always leave the batteries fully charged via my generator.

Thanks for your input and help.

Steve
 

Karl

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A 6 to 10 watt, 15 volt panel should do the job nicely. You'll need to put a 3 amp. diode in the circuit to prevent reverse current flow, and it will also drop the maximum voltage down to about 14.4 volts. If you anticipate lots of long, sunny days, you may want to put a second diode in series with the first one to bring the voltage down to 13.8 volts which is just about right for long-term charge maintenance, and will prevent overcharging.
 

Gary RV_Wizard

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Karl gives good advice. A solar charging regulator will do the same job as the diode, if you aren't into do-it-ourself electronics.

The question is, do you have adequate sunlight for the panel to work near its peak efficiency. If sunlight is broken (coming through tree branches) or shining directly on the panel for only an hour or two a day, you have to compensate by using a larger wattage panel. That gives you more output for the charging time or sunlight available.

If you need more output, it may be cheaper to put in two small panels in than one large one. Two panels can also be used if the sun comes from different angles at different times of the day, e.g morning sun and afternoon sun.  The angle of the rays on the panel has a big effect on power generation. A 10 watt panel delivers 10 watts (0.8 amps) only when the sun is directly shining on it from right overhead.  Much of the time it is less, especially in northern regions where the sun is  always relatively low in the sky.
 

Benevolus

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Thanks guys!
My land is located in upstate NY. The spot is an opening in the middle of the forest, very secluded.
Would I need to use a controller and/or regulator with my simple setup?
(Oops, just saw I needed a regulator)

Thanks again.
 

Benevolus

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Last question, would a 4.5 amp controller suit my purposes?
After that, it's a choice of 1- 20 watt panel or 2- 10 watt panels.
 

Gary RV_Wizard

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The small panels are often installed without a regulator, since they can't overcharge anyway. They are a  trickle at best. A 4.5A controller will handle a 50 watt panel (4.5A x 12.6V = 50W), which should be plenty for your purposes.  A 20W panel is kind of at the border between "must have" a regulator and sneaking by without one. A regulator, or at least the diode, is better.  If it were me, I'd probably try a 15W and see how it does. You can always add another in the future.
Perhaps Karl can elaborate on why the diode is a good thing.
 

Benevolus

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I'd rather spend the small amount of money for a 4.5 amp epoxy encased controller than risk overcharging. I can get one for about $27.00

I like the idea of 2- 10watt panels to capture both times of sun-good thinking RVR
 

Karl

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As Gary correctly stated, when using low power solar panels, it is seldom necessary to use a charge controller. The diode, however, is something you want in the circuit. Electricity flows from a higher voltage source to a lower source. Let's say your panel is putting out 15 volts in full sun. It will send current into a battery at, say, 12.6 volts - higher to lower. Now the sun goes down and your panel is no longer putting out 15 volts; maybe nothing at all. Now current will flow from the battery into the panel. A regular diode allows current to flow in only one direction, so it will prevent this drain on the battery.

Having two panels wired in parallel and facing in different directions sounds good, but requires something additional. You guessed it - diodes. One for each panel. Same situation as above; the panel facing the sun may put out 15 volts while the other one puts out something less or even zero volts. High to low.  As an experiment, I hooked two different batteries in parallel. One was a standard 9V battery which actually measured 8.5V with no load, and a 1.5V AAA battery which measured 1.6V, no load. When connected together and left for 20 seconds, the output of the combined batteries was only 1.85V. The net voltage when connected together could be calculated, but it's not an easy formula and doesn't really serve a purpose here.

If you want to spend $27 for a controller, that's fine - or you could make your own with a couple of Zener diodes for about two bucks! 
 

WA9YSD

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Looks like you have some good answers.  Remember, the question really is how long do you want to take to bring those batteries back up to full charge?  If you used up 600 amp hours charging at 3 amps will take  over 300 hour of good sunlite to charge those batteries provided all the loads are shut off.

The other answer is what every your pocket book can afford.
 

WA9YSD

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Sorry bad math, if you used up 600 amp hours and are charging at 3 amps it would take over 200 hours with good sunlight to bring the batteries up to full charge.
 

WA9YSD

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After re reading everything that was posted there is another question.  24 batteries in parallel?  Hmmm  You may also need some type of battery tender.  That throttles down the current so it does not boil off the batteries.  what would be needed is some type of automatic current limiter that will allow more current to the battery bank as they drift down from not being used, and when they come up to charge throttle back on the charge current.

With the Bank like that you may need some type of equalizing resistors between each battery charging wires so that the first one does not take on the full charge current all the time cause the resistance between each battery add up.

Remember one 12 volt wet cell battery has 6 cells.  Each cell makes up 2 volts.  You want to charge at about 1.5 volts higher the 12 volts.  Remember when you check the levels in each cell one cell is usually lower than the others.  Well 24 batteries in parallel will charge in a similar manner and you will need to address this problem.

!5 volts at max sun light may be a problem too.  The Zener voltage regulating diode may be the right way to go.
 

Karl

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Jim,

I'm guessing your talking about Group 24 batteries and not 24 batteries in parallel? It's seldom a problem with the first battery in the bank taking the majority of the charge because the heavy interconnect cables (2/0 - 4/0) minimize that possibility. Besides, each battery will have a slightly different internal resistance which would make figuring out the proper dropping resistor for each one next to impossible. The zener across the solar panel should work fine.
 

WA9YSD

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I miss read the Group 24 batteries and not 24 batteries in parallel?    Oppp!

It's seldom a problem with the first battery in the bank taking the majority of the charge  ---- Not necessarily true! Depends on how well all the connections are maintained.

Heavy interconnect cables (2/0 - 4/0) minimize that possibility. --- True, however cable wire size was not stated.  Still the one in front will boil off more eather way.  Keep in mind each individual joint is a high resistance point no matter what the wire size.

Besides, each battery will have a slightly different internal resistance which would make figuring out the proper dropping resistor for each one next to impossible. ---Not true.  The battery is the resistor.  Simply run parallel charging wires say to every second battery if they are all in parallel.

The zener across the solar panel should work fine.----- True

Here is something. TWO 6 volts batteries in series will have more amp hour capacity than TWO 12 volt batteries in parallel.
 

Karl

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Jim,

When I reply to questions like this, my answers are usually of a general nature and make some assumptions such as clean, tight battery connections, cables of adequate size and in good condition, etc.
Still the one in front will boil off more eather way.  Keep in mind each individual joint is a high resistance point no matter what the wire size.
If the interconnect cables are of adequate size, the difference between the front and rear batteries wouldn't be more than a few millivolts, and I'm not sure what you mean by "high resistance"? 1 ohm, hundreds of ohms, milliohms?
The battery is the resistor.  Simply run parallel charging wires say to every second battery if they are all in parallel.
Batteries are not resistors; they have internal resistance which varies by temperature and level of charge. Can't see how running separate charging wires will help when you already have large interconnect wires between the batteries.
TWO 6 volts batteries in series will have more amp hour capacity than TWO 12 volt batteries in parallel.
Huh? Please explain that statement. It makes no sense at all.
 

Ned

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When connecting batteries in parallel, it's customary to tie the positive side of the charger to one battery and the negative to the other so the interconnection resistances will even out.
 

Gary RV_Wizard

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TWO 6 volts batteries in series will have more amp hour capacity than TWO 12 volt batteries in parallel.

That is not a generally true statement - it depends on what 6V and what 12v battery you are comparing. For example, two golf cart 6V's (GC1's) will have more amp-hours than two Group 24 12v's, but not more than two Group 31's.
 

WA9YSD

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Internal resistance yup.  Joints, have resistance too.  Charging at low current that joint looks high in resistance through time it get worse.  Thus the battery looks like a resistor.  Everything adds up when stringing 24 batteries in parallel.

Battery as a resistor?  Using resistors to equalize charging voltage and calculating the internal resistance?  There was an easyer way to do it.  Something wrong with that?  Just keep it simple and minimize your losses when charging.

Only place I used equalizing resistors is across diodes when stringing them in series to build up the total voltage rating of the string.

You would not want to put resistors across batteries when charging with solarcells cause it is a load.  Counter productive.

The cell in a 12 volt battery is smaller than the cell in a 6 volt battery.  I am talking about battery replacement and taking out two 12 volt batteries and putting two 6 volt batteries in the same physical confines so you can get more out of them.
 

WA9YSD

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Two 12 volt batteries in parallel or two 6 volt batteries in series, I had gotten into a habit to rotate the two batteries every year.  Kinda like tires on a car.  No battery is a like.  Each persons usage is different.  Each battery is different, so are the connections, the batteries is the major factor.  I feel that this is the best way to counter the interconnection resistance affect.

Not sure if I got your point straight.  Two 12 volt batteries in parallel to minimize interconnection resistance, interconnection resistance as in internal wiring of the RV?  Yes it would, your right also give you more amp hour draw, but not as much as two 6 volt batteries in series.

Two 12 volt batteries in parallel, I am not sure but should give you max current quicker like inrush when a large motor is first turned on.

As the batteries get older and degrade, things are turned on and off interconnection resistance is no longer a concern cause your voltage is dancing around more.  Then again the older the RV gets connections are oxidizing as well.  The older the RV the more things your going to have trouble shoot to resolve the problem.
 
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