2 6v batteries with solar panel

Airbum

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Joined
Jul 4, 2021
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Hello I am new to solar and the DC world. I have 15 years experience with AC systems. I recently purchased a teardrop trailer. It came with a 100w solar panel on the roof. I believe it needs to be replaced. When it is in full sun checking voltage at the charger box ( at the pv terminal) I'm getting 10.8 volts between positive to negative. First I check to make sure the solar was clean. There is very light dust. The only way to see it is dirty is if I run my fingers on the panel. I didnt not clean it. I figure this is an off road trailer and there is no way to keep it 100 percent dust free. I doubt homeowners are cleaning their roof top solar panels on a daily basis either. Writing this out made me think to follow out all the wiring to make sure it hasn't been damaged some how. There are connectors about two feet from the panel. I need to take those apart as well to check voltage there. That would help eliminate bad wiring. I was thinking about changing the current 12v deep cycle out for 2 6v deep cycle (golf cart) batteries and buying a generator. The only big power draw I will have is a 800 watt coffee maker. I will need a pure sine wave inverter to run this. Other load will be lights radio and small fan. I dont have the owner manual to check the wattage for these items. The light are LED should be an issue. The fan is tiny about 6in. The radio shouldn't be bad either. I cant seem to find any systems where people are using 2 6v 180 Ah batteries in series as their battery bank with a solar panel to charge said bank. First question how come this isnt a configuration in common use? 2 flooded 6v batteries are cheaper then I lithium 12v battery. Would my 100w panel charge said battery bank? How long would it take to charge this bank? Am I better off sticking with a single 12v battery?

Thank you in advice for any help
 

rin67630

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Apr 29, 2020
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810
You should check the Panel voltage without the charger box and notice the difference. What is that "charger box"?
If it is a PWM charger and your battery is discharged, 10.8 V on the panel side is just normal.
You should fist test that battery and full charge it from AC. Then you can evaluate the panel input voltage, but more important: its current at full sun.
 

rin67630

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Apr 29, 2020
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Yes, but if your current battery has e.g. a cell shorted, and your charge controller is PWM, it explains the 10.4V on the panel side.
The panel must not be defective.

P.S. if your only heavy load is a coffee maker, you would be better off buying a 12V coffee maker (or better, a gas powered one).
 

rin67630

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Apr 29, 2020
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810
@rin67630 yes the charge controller is a renogy wanderer pwm charger controller.
First replace the defective battery with your 2 6V golf cart batteries and measure again when you battery got at 13V, possibly everything returns to normal.
 

chrisski

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Joined
Aug 14, 2020
Messages
2,677
You have a good plan to disconnect the panel and measure open circuit voltage with a multimeter. Not uncommon for Flexible Panels to die. If the sun is shining the power from the panel will be 18 or more, to get a better readout look at the spec on the panel, but if its a flexible panel, that's probably impossible. If you have a Multimeter, this is free.

Using 2 six volt batteries in series is real common in boondocking. If these batteries are Flodded Lead Acid, you can check with a Deka hydrometer from on Auto Parts store.
 

Airbum

New Member
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Jul 4, 2021
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@rin67630 thank you the help
@chrisski how long would it take to charge a 2 6v battery bank with a 100 watt solar panel? Also I was thinking today. Should I have a switch or fuse between the battery and the charge controller? I was wondering today if plugging in to shore power with the battery back feeding to the solar panel was bad for the panel?
 

chrisski

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Aug 14, 2020
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I to A site like PV Watts and enter in a 100 watt panel, the angle you’re going to put it, and that will give you how much you produce in any given month. Divide that by the days, and that’s the watts you get per day. Differs so much by area. I am in Central AZ and that is ideal for solar, Seattle with bad weather can take 4 times as long.

Although people use a 100 watt panel for two 6 volt batteries, that probably isn’t enough to charge it. All together, it will have around 2.2 kWh, of which 1.1 is useful, so when you enter the calculations, you’d probably find it get a total of two or three hundred watts. Also, 1.1 kWh shoiuld be spread over two days to account for cloudy weather every now and then, which comes out to about 650 watts per day of usable solar energy. Although 100 watts oof panel may replenish that in the summer, you’re not that likely to get that in the fall spring or winter. Also want to be able to push back about 1.1 kWh into the panel for those bad weather days.

The Boondocking rule of thumb people tell me is two 6 volt batteries and 300 watts of panels. That is a good rule of thumb, but there’s so much more to it.

Charging by shore power, like a converter won’t hurt the panel, its separated from the shore power by the charge controller. I use the solar to charge the batteries most of the time, but on cloudy days, I will fire up the generator, use the RV converter and solar to charge the batteries.
 

rin67630

Solar Addict
Joined
Apr 29, 2020
Messages
810
I to A site like PV Watts...
May I suggest to use the EU site in my signature? PW watts is for On-grid power. Off grid is VERY different.
It has an off-grid mode and can compute your configuration and which percentage of the time you can be with an empty battery.
 

Just John

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Joined
Aug 15, 2020
Messages
2,426
Hello I am new to solar and the DC world. I have 15 years experience with AC systems. I recently purchased a teardrop trailer. It came with a 100w solar panel on the roof. I believe it needs to be replaced. When it is in full sun checking voltage at the charger box ( at the pv terminal) I'm getting 10.8 volts between positive to negative. First I check to make sure the solar was clean. There is very light dust. The only way to see it is dirty is if I run my fingers on the panel. I didnt not clean it. I figure this is an off road trailer and there is no way to keep it 100 percent dust free. I doubt homeowners are cleaning their roof top solar panels on a daily basis either. Writing this out made me think to follow out all the wiring to make sure it hasn't been damaged some how. There are connectors about two feet from the panel. I need to take those apart as well to check voltage there. That would help eliminate bad wiring. I was thinking about changing the current 12v deep cycle out for 2 6v deep cycle (golf cart) batteries and buying a generator. The only big power draw I will have is a 800 watt coffee maker. I will need a pure sine wave inverter to run this. Other load will be lights radio and small fan. I dont have the owner manual to check the wattage for these items. The light are LED should be an issue. The fan is tiny about 6in. The radio shouldn't be bad either. I cant seem to find any systems where people are using 2 6v 180 Ah batteries in series as their battery bank with a solar panel to charge said bank. First question how come this isnt a configuration in common use? 2 flooded 6v batteries are cheaper then I lithium 12v battery. Would my 100w panel charge said battery bank? How long would it take to charge this bank? Am I better off sticking with a single 12v battery?

Thank you in advice for any help
Your system doesn't care if your power comes from two 3 cell batteries, or one 6 cell battery, it's 12v.

Can a 100 watt panel charge it? Given enough time, probably, might take a week if you have depleted the battery. The maximum you will get is 8 amps, probably more like 5 under good conditions.
Depending on location, you may get 6 hours of charge during a day during summer, or 2-3 in winter (assuming no clouds). It really depends.
 
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