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Old 06-04-2013, 11:39 AM   #1
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Question for you solar "Gurus"

Actually several questions....assuming one group 27 battery being used at night down to maybe 60%, could a 45 watt solar panel be used during the day with no controller, and is my math right that it would charge at a max rate of 3.5 Amps?

Same battery, what size solar panel could be used continuously without a controller?

Thanks for your input.
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Old 06-04-2013, 12:59 PM   #2
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You should only expect peak output from panels 2-3 hours per day, and that will decrease due to season, cloud cover, whether any shade is falling on the panels, and temperature. As well, with no controller, the rate would decrease as the battery's voltage rises due to state of charge.

Replacing 40 A-Hr's even at at a max rate of 3.5 A would require over 11 hours of full-on sun.
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Old 06-04-2013, 01:15 PM   #3
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If you are in direct sun, a quality 40w panel will be capable of boiling your battery if there is no draw on it and it gets repeated days of sun. You can check the battery regularly for the level of the acid solution to determine how much effect it has. You must keep the battery plates wet.

A controller for that size panel is cheap insurance if you want unattended service.

I have no controller on two systems, but those two systems will overload onto my main system if they exceed 13.2v at the battery. I use SurePower 1315-200 Separators for this.

Dave
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Old 06-04-2013, 01:45 PM   #4
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You really do need a controller.
Unless you are will to manually monitor your battery all day long and turn off the solar supply when the battery is topped off.
For a 45 watt unit and 12 volt system, they are cheap.
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Old 06-04-2013, 01:48 PM   #5
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I seems that no one has understood my questions, so again: assuming one group 27 battery being used at night down to maybe 60%, could a 45 watt solar panel be used during the day with no controller?

Same battery, what size solar panel could be used continuously without a controller?

Your input is appreciated.
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Old 06-04-2013, 02:40 PM   #6
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Your question is a catch22. If you have an adequate size panel to recharge your battery every day, then it will require a controller to avoid damage over a fairly short period.You are in Texas and I assume the sun shines there quit a bit.

An 11W panel would be about the limit to avoid unattended damage but would not recharge that battery in one day.

The other thing that happens is, that the battery will boil without a controller or you to monitor it, and the acid spill will cause further parasitic drain and lower your overall capacity of the system. (makes a mess also)

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Originally Posted by SteveH View Post
I seems that no one has understood my questions, so again: assuming one group 27 battery being used at night down to maybe 60%, could a 45 watt solar panel be used during the day with no controller?

Same battery, what size solar panel could be used continuously without a controller?

Your input is appreciated.
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Old 06-04-2013, 03:08 PM   #7
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Originally Posted by masseyfarm View Post
Your question is a catch22. If you have an adequate size panel to recharge your battery every day, then it will require a controller to avoid damage over a fairly short period.You are in Texas and I assume the sun shines there quit a bit.

An 11W panel would be about the limit to avoid unattended damage but would not recharge that battery in one day.

The other thing that happens is, that the battery will boil without a controller or you to monitor it, and the acid spill will cause further parasitic drain and lower your overall capacity of the system. (makes a mess also)

Dave
All of these things I understand, and do not argue. At this point, I am not concerned with recharging the battery in a day, but NOT OVERcharging it. What I want to do is keep the battery charged with a solar panel during times of storage/not being used.

The other thing I want to do is use the 45 watt panels without a controller. This is because the trailer I wish to use it with (17' Casita) does not have space for the controller to be installed internally, and I do not want to leave it setting on the ground outside.

Perhaps I should have asked how much current from solar panels my battery will take without overheating, daily from say 11 AM to 6PM (7 hours)?

It's a simple matter to figure out how much current the panels will put out during peak sunlight, but I don't know how long during the day that will happen, and what the battery will tolerate.
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Old 06-04-2013, 03:13 PM   #8
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He is asking a "yes or no" question, and no one has answered it yet! Lots of good info, but no answer.
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Old 06-04-2013, 05:11 PM   #9
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i would think that once the battery is charged, more that a trickle charge (think battey maintainer) can boil the battery.
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Old 06-04-2013, 05:23 PM   #10
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Sunforce sells a 5 watt panel for trickle charging. So I think 45 watts is probably too strong for 24/7 uncontrolled charging.
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Old 06-04-2013, 05:52 PM   #11
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BatteryMINDer has a nice 15 watt panel combined with a 3 stage controller:
BatteryMINDer 12 Volt 15 Watt Solar Battery Charger: BatteryMart.com

This is a maintainer for a fully charged battery, not a charger. You certainly need more than 15 watts to charge a battery
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Old 06-04-2013, 06:18 PM   #12
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Steve,

I think I understand you, but calculating how much charge you get depends on the clearness of the day (clouds, haze) and the direction of the sun. To get maximum charge, you have to move the panel(s) to face the sun all day. Your results may vary. Our panels work very differently depending on those factors.

It sounds like you have no way to plug the Casita into shore power or you wouldn't be asking this.

If the converter is a multistage, I guess there is a way to control the input to the batteries, but I don't know how. It seems to me the part that controls the charge may be in the 120 v. circuit. But if it were wired into the input from the truck battery, maybe it would work. I think you would have to look at the circuit diagram for the Casita converter to see how to wire it. Or, just attach it to the battery and monitor the battery voltage until it is charged. That would require you to sit by it all day drinking beer and eating chips and salsa. That may make it worth it. I'd be glad to sit with you and watch the meter every once in a while, but the danger is we will both fall asleep and forget about the charge.

Our solar panel controller is not all that big, but neither is a Casita. Yet, it could be mounted in a box about 8" x 6" and a couple of inches deep and placed somewhere near the converter. Then you need wires to the panel(s) and the converter. But…

I think you want to connect directly to the battery. That is simple, but has drawbacks because you can't control the charge except by watching it or determining how long you can leave it alone. Even then, you'll have to check it periodically. And are you going to mount the panel(s) on the Casita, or trust that no one will take them?

If this is simply to keep the battery charged while stored, you'll only need to do this once a month or so. Then a voltmeter will tell you when the battery is charged and you do this on a day when you can check it every couple of hours. But I do like the beer and chips method.

I don't think I've answered your question at all, but it was fun to speculate on possible solutions and make it more complicated than it need be. Maybe a windmill is another approach….

Gene
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Old 06-04-2013, 06:30 PM   #13
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A tidbit from Handy Bob: The RV Battery Charging Puzzle « HandyBob's Blog

"Converters are set to maintain the batteries at the “float” voltage, about 13.5 volts, and supply a trickle charge (2-3 amps, or 25-35 watts) to keep the batteries full, while not over charging them and boiling them dry"
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Old 06-04-2013, 06:32 PM   #14
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Yes you can use a 45 W panel without a controller to do that but it would not be the preferred way to charge your batteries.

Your math is probably OK but solar cells do not output a constant voltage. It varies due to amount of light and what the load on them is. A 45 watt panel will produce 2.5 to 2. 7 amps during peak exposure. So you might get 15 -20 Amp Hr per day into your battery.

Charging Information For Lead Acid Batteries

If you follow the link you will see that lead acid batteries do not like to be charged quickly. So take advantage of the Solar charge controllers that have been developed to manage your battery charging for you.

Low current charge controllers are very small and can easily be mounted inside your trailer frame near the battery box.

I am currently building a solar battery maintainer using a 15 watt panel and a charge controller I got from Northern Supply. I have had good luck with this system on my sailboat. It kept the batteries well charged and prolonged their life. I typically used the boat once a week and left the solar panel in place the rest of the week. Cost is less than $150.

What I have read suggests the maximum wattage without a charge controller is in the 10 to 12 watt range. This works out to less than one amp into your battery,

A group 27 deep cycle battery may have a capacity of 100 amp hr. If you need to replace 60% of that with a 12 Watt solar panel at 0.6 Amps x 10 hr per day, it will take 10 days to recharge in the summer. Your results will take longer due to temperature, amount of sunlight, efficiency of the panel, age of the battery etc.
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Old 06-04-2013, 07:35 PM   #15
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Quote:
Originally Posted by SteveH View Post
All of these things I understand, and do not argue. At this point, I am not concerned with recharging the battery in a day, but NOT OVERcharging it. What I want to do is keep the battery charged with a solar panel during times of storage/not being used.

The other thing I want to do is use the 45 watt panels without a controller. This is because the trailer I wish to use it with (17' Casita) does not have space for the controller to be installed internally, and I do not want to leave it setting on the ground outside.

Perhaps I should have asked how much current from solar panels my battery will take without overheating, daily from say 11 AM to 6PM (7 hours)?

It's a simple matter to figure out how much current the panels will put out during peak sunlight, but I don't know how long during the day that will happen, and what the battery will tolerate.
It's not the amperage from the panel, but the VOLTAGE that your 45 watt panel will be sending to the battery that is important in your situation. Different types of solar panels have very different specifications regarding open circuit voltage (V-oc) and voltage at the maximum power point (V-mpp).

Panels can range from 15 V O-c to 22.5 V O-c, depending on their design and construction. Any voltage over 15.5 hitting your batteries will in effect become an equalizing charge and WILL boil the batteries, ANY batteries in short order. The optimum voltage for a float or maintenance charge is dependent on the battery type, but typically runs between 13.2-13.6 VDC.

VOLTAGE is the critical measure that no one is addressing, not amperage and the one factor that will determine if you kill or properly maintain your battery. I would NOT connect a solar panel directly to a battery and expect it to magically send a float charge to that battery....without a solar charge controller.
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Old 06-05-2013, 06:32 AM   #16
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OK, I asked these question specifically because I have many options that I did not state at the beginning of this thread. Really, all I want to know is how much current would a group 27 battery need for a purely maintenance charge.

What I did not tell you is I do have a controller, but don't want to install it because of the space limitations of the trailer. If I had enough room in the trailer without having the thing laying on the floor, or sitting on the table, I would simply install the thing and be done with it. But, laying it on the floor, or setting it on the table, both consume space that is very much needed for the activities Gene suggested, sitting watching the voltage while drinking beer and eating chips and salsa....space and an activity that I'm not willing to give up. However, recently I have taken up eating Pistachios with my beer. I actually like them a lot, they are supposed to be good for you (who in their right mind would not like health food with their other health food, beer?), and lets face it, salsa takes up room in the fridge that could be utilized for more beer.

Also, and getting back on the subject, I did not tell you that I in fact have six solar panels to work with. The 45 watt that I mentioned is really three 15 watt panels wired thru connectors in parallel for a total of 45. AND I also have three 3.3 watt panels wired the same way, in parallel to make what I call 10 watts.

So, I actually have the option of using 3.3, 6.6, 9.9 (10), 15, 30, or 45 watts of solar energy to maintain this one stupid battery!

Which one(s) can I use long term without damaging the battery?
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Old 06-05-2013, 06:43 AM   #17
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wow steve, that's a lot of cool stuff to work with.

some answers might come from the specifications of the goods you're working with. i'd look up the maximum output specs on the panels and the charging specs for the brand/type of batteries used. this way you'll know the maximum your solar can deliver and if the battery(s) can take it.
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Old 06-05-2013, 07:49 AM   #18
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I can go with pistachios Steve. But when Lew says volts are the important number so far as cooking the batteries, I believe him. You can put your feet up on the controller.

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Old 06-05-2013, 01:26 PM   #19
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put the controller on the ceiling :-)
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Old 06-05-2013, 01:35 PM   #20
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put the controller on the ceiling :-)
Oh sure, so I can hit my head on the damn thing! Obviously you've never stood up in a 17' Casita?

In the mean time, I've come up with sort of a solution that I think might work. The trailer has a Fantastic fan with a weather cover over it, and it's thermostatically controlled. I've got the fan set to come on, low speed, at about the same time the sun gets to the solar panel location. Dual purpose....keep the trailer cool inside, a good thing, and maintain the battery at the same time. I love it when a plan comes together.

Now all I have to do is select the amount of panels that will run the fan, and also put a little power back into the battery, but not too much.

Pistachios and Shiner Bock in hand and going out to watch the voltmeter.
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