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Mike;

Here is a general rule: For each 1AH of battery you design around 1W of solar( or 1:1, or 100%). There is quite an lot of underlying math in that simple 1:1 ratio.  A lot of it depends on your camping cycles, from weekender to full-timer.  I can do the math such as converting solar Watts into AH. For now, the K.I.S.S principle.

How many days you can dry-camp on your batteries(bank)?   Could you add batteries as a solution for extra days? Are you a weekender with an occasional week of vacation?

Weekend campers may only need to add to the bank with the recharging being postponed until after the trip.  Ex., if an 84AH bank is enough for a two day weekend, but is flat on the third day of a three day trip, then it would be economical to add a matching 84AH battery.

If you are already maxxed out on batteries, you need enough solar to supply 1/2 of a days use spread over the first two days. Now we need to do some solar math. Ex., if an 84AH bank works for two days, you need the equivalent of an extra half battery for the third day, or 84+42=126AH. This says that you need 42W solar to get by the third day.  Remember, your bank is flat and must be recharged within a week or it WILL be damaged with the result that it will NOT perform as an 84AH bank after that.  When promptly recharged, your batteries will last hundreds of cycles. 200 cycles would represent 50 weekends for four years.  The 42W solar is not enough to recharge inside a week's time.

The 1:1 rule says to use 84W solar for the 84AH battery.

Here are my stats: I am way over 1:1 because I want to recharge my bank 100% each day.  I have 3 batteries; 2 * 55AH and 1 * 79AH = 189AH.  With 300W solar I have 300/189=158%. Even with that, there are times when I must supplement with the alternator and/or with the genny powering the converter/charger to meet the weekly charging rule. The extra solar is to avoid the use of fuel as the energy for recharging. Inclement weather is one cause of solar shortfall.

Questions?

John

Post edited 1:33 am – September 21, 2012 by Suzy-Q

Mike; I can suggest a solution with what I know now.

Your worst case will be winter when you run the furnace.  I have read that the fan can draw 5A.  If it ran 3hrs overnight you would use 15AH.  Three nights of that ALONE and your battery is below 50% which you should regard as flat and needing a charge cycle.  You could postpone the full charge cycle for a few days, but you do need to begin replacing AH in earnest to maintain the 50%. 

Plus, if your bank is on the tongue, it will be sluggish and will have less available AH due to the cold.  Another factor working against you when the bank is cold is that it takes more Volts to charge.  This is where a temperature compensating charger helps.

OK, here we go:

1. Consider using a Sears Platinum AGM for your starting battery.  By design, AGM can work as starting and deep cycle. You will be able to use the 7-pin to help share the load.  Really!

2. Switch to LED for lighting.  You use less AH when you need lighting the most.

3. 130W Solar Panel pdf file: http://www.sunelec.com/x-specs…..W-130W.pdf

My panels are from this vendor.  This link is from an email from them and is/was priced at $1.08/W.  I don't see the panels on their website now, but you could call them.  It is hard to get just one from them.  Anyway, look at the specs for the 130W.  Those are good specs for the controller below.  It is important to match specs as there are a variety of voltages for panels.

4. Morningstar SS-10 12V: http://www.morningstarcorp.com…../sun-saver

This has the temp. comp. that I mentioned above.  I am able to mount one right inside my battery box in the van because AGM is a sealed battery. No hydrogen or acid mist to deal with.  If you want two panels you would need the SS-20 12V.  The determining factor is the Isc(a) 7.90. With two panels you need to add.

Now, you are ready for some more solar math?  At your latitude, solar is a real challenge in winter and really needs to be tilted south. 

HTH;

John