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Old 06-04-2013, 08:35 PM   #15
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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, 07:32 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, 07:43 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, 08:49 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, 02:26 PM   #19
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put the controller on the ceiling :-)
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Old 06-05-2013, 02:35 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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Old 06-05-2013, 04:53 PM   #21
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Oh sure, so I can hit my head on the damn thing!


Pistachios and Shiner Bock in hand and going out to watch the voltmeter.
A few beers and the controller on the roof won't bother you at all. Until then, wear a helmet (I know, I know, there's no place to put the helmet in the Casita—serves you right for not using the Airstream).

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Old 06-05-2013, 06:55 PM   #22
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Well, if it were aluminum colored, it would look just like a small Airstream, and besides, it has a battery so it must be maintained.

This whole exercise is so we can leave in the Airstream for the summer, and not have to worry about the Casita battery.

Or, maybe I could put a hitch on the back of the Airstream.......
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