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Old 03-25-2014, 09:19 AM   #11
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This can be as simple or as complicated as you want. On my motor home I had 3 135 watt panels, charge controller and 4 6VDC deep cycle batteries along with a 2000 watt inverter. A fairly complicated system. And of course a generator if needed.

At home I have a 5 watt panel wired directly (no controller) to a very small battery that powers 160 milliamp led light string at night. It's been running for 3 years with no problems.

I your case I would first consider a simple system - A 135 watt panel wired directly to the batteries. The chance of "overcharging" is very slim. Undercharging would more than likely be the problem so you would need to check battery voltage occasionally to see where your "at".

This is very easy and cheap to setup. If it fulfills your needs you are done. If it doesn't then you can add an extra panel - again I would consider trying without a charge controller.

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Old 03-25-2014, 09:29 AM   #12
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Don't worry about it. It didn't come across that way to me.

Besides, I never went through with solar, just did the calculations. You at least have real world experience to prove book learning isn't always right.

It did make me think though, so this morning I tried to track down where I got my various information from. I think the majority of it came from these two threads, and the embedded links within them:

Charging Batteries With Solar Panels
Ontarians using solar to charge batteries

I remember I had started from those and was looking to figure out a formula to calculate what size solar I'd need.

I then ran through a variety trying to figure out a 30% efficiency rating (as per the info I was given) over 24 hours, compared to a constant current draw over 24 hours. When I put it all together, I got back 40W per amp rating averaged over the day. So, if the load on the TT averaged out over 24 hours was 2 amps per hour, then I'd need 80W of panel.

So, for panel output I think I used:
X W *.3 = 0.3X W / 12V = 0.025X A * 24H = 0.6X Amp replenished per day.

And for load I used:
Y amp rating * 24 = 24Y Amp used per day.

Assuming I wanted to replenish what I used, putting the two formula's together I got:

0.6X A = 24Y Amp

Solved that to be:

X = 24/0.6 = 40

So, 40W of 12V panel to replenish 1 Amp (averaged over 24 hours) at an assumed panel efficiency of 30%.

Problem is, I know I didn't just take Herk's word for it with that 30%, I do remember consulting some solar charts that seemed to confirm that was the right coefficient for a flat mounted, non tracking solar panel in my area (near Detroit), but for the life of me I can't find those now.

There's no use crying over spilled milk, unless it's on your keyboard.
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Old 03-25-2014, 09:40 AM   #13
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Posts: 1,330 your post #6 you mention minimum charge current. Where does this concept come from? Since trickle chargers clearly can there be ANY minimum charge current...let alone one as high as 13 amps? Or is this simply related to the working of your own solar charging system parameters? Not clear on what you meant.

(BTW...nice apology to K.... but I didn't think you were being a P! But Kaadk's point was valid. No matter where you are...if you've got flat can expect a maximum of the equivalent of 6 hours of full output a day because of incidence angles of the sun. You also lose output vs. rating since testing conditions for rated wattage are not real world...[altitude, temperature and air flow! ]You lose hours as you go north...but in a fixed structure you can reduce this loss by mounting at a fixed angle designed to compensate for the latitude.)

Interestingly...your 20 amps "regular output" using the bluesky mppt... times the 6 hours rule is 120amp hours a day on average. Exactly the 30% of 400W I said was possible with MPPT. So we all actually agree!!
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Old 03-25-2014, 09:55 AM   #14
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Thanks all for the great info and links. Lots to read, think about, and numbers to crunch.

I do have a voltage readout attached to the batteries. We can camp about 3 days (without using the heater) and draw the batteries down to around 70%. I'm not looking to boondock for weeks at a time, just be able to stretch our stay to 4 or 5 days.

I do plan to move the panel(s) several times a day to keep them efficient. I will do more homework and let you know what I decide. Thanks again!
2013 GMC Sierra 2500HD 6.0 Vortec
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Old 03-25-2014, 10:11 AM   #15
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OC...note that I said that you need a way to measure CURRENT, not voltage which CANNOT tell you what you need to know about state of charge while IN USE.
Having lived on solar and wind (+ some diesel!) for a few years, I can tell you that a TRue battery monitor is crucial to understanding your system, your use and proper charging regimens. Here's a link to one that is excellent.

They go for about $160 bucks and if you are doing a lot of boondocking... you'll wonder how you ever got along without one...whether you charge the batts with a generator or panels or by riding your exercise bike! LOL
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Old 03-25-2014, 01:26 PM   #16
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" your post #6 you mention minimum charge current. Where does this concept come from? Since trickle chargers clearly can there be ANY minimum charge current...let alone one as high as 13 amps? Or is this simply related to the working of your own solar charging system parameters? Not clear on what you meant."

I lost the second reply to this message..I'm having a rough time. Here's a summary of what I wrote then lost....twice:

I remember reading somewhere during my research to build a rv/camper solar system that you need a minimum of C/20 when recharging a flooded cell battery to maximize battery life. This rate will create bubbles that mix the solution. This will help prevent stratification and sulfation that will reduce battery life. Since I can't find that link, I have attached two links that have good information on battery maintanence. Trojan states the recommended charge rate of C/8 -C/10 as the preferred rates and Blue Sky says C/3 as a maximum.

Blue Sky Energy Inc. | Frequently Asked Questions & Technical Tips

Battery Maintenance | Trojan Battery Company

Lower rates will work of course, and if you are just maintaining a charge, they should be lower or you will overcharge the battery and "boil" off the solution.

One very important thing to consider is that there are only so many hours of direct, brite sunlight in any given day. The higher the charge rate, the sooner you'll get those batteries charged up. A battery charges to 90% quickly but that last 10% takes some time.

BTW, my charge controller max's out at 25A. When my T145's are down by 30%, the solar will have them at 100% around lunchtime on a sunny, clear day.
2000 Cherokee 29BH with 6V batteries, LED lights & 400 watts of solar power, flipped axles and raised. 2007 Tundra 5.7L DC-LB with lots of mods. C-co, 8/158th AVN Maint.
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Old 03-25-2014, 03:48 PM   #17
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Recommended charge rates are quite a different thing from minimum charge rates. A wet cell will generally not accept a rate in excess of C/4...and most sources suggest C/10 as the point at which you may wish to increase the size of the charger. But there is no minimum rate...provided you can provide equalization charging occasionally to " stir things up" and prevent stratification and loss of capacity. The higher rates of charge make EQ necessary less often...but don't eliminate the need, especially in RV's where most batts spend a lot of time on float charge or in storage.
I'd definitely want enough solar panel to push that C/4 rate if room and budget permitted.
Thanks for clearing up the terminology.

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