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I want to charge a 12v Sealed Lead Acid car battery off of a solar panel (20v) and then draw about 10-20W of power from it via an inverter (the load).

I'm wondering if its okay to connect it together like this: Wiring diag

And what downsides, if any, are there and can I avoid them? Is it a problem hooking up the battery to solar panel without a load?

Thanks!

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    \$\begingroup\$ What has your research into battery charging revealed? \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 31 '13 at 5:56
  • \$\begingroup\$ A little bit... I'm not sure about the load being removed (as in what happens) but i assume that it simply charges the battery, i think you CAN charge the battery with higher than battery voltages but it sounds like it can reduce the life or in some cases cause it to explode?? Hence why i'm asking here (pretty new to EE stuff)... :) \$\endgroup\$ – NULLZ Aug 31 '13 at 7:46
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    \$\begingroup\$ Car batteries are not usually sealed lead acid batteries, I thought. \$\endgroup\$ – nekomatic Oct 11 '14 at 0:06
  • \$\begingroup\$ note, make sure that this setup you have, that the battery is NOT on the ground. For whatever reason, batteries on the ground will lose capacity over time. a simple wooden board, or other insulator, between the battery and the ground will do nicely. \$\endgroup\$ – user86234 Sep 7 '16 at 16:33
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    \$\begingroup\$ @tuskiomi, that is nonsense. Putting batteries on the ground does not affect their capacity. \$\endgroup\$ – mkeith Sep 26 '16 at 2:53
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Yes, it's okay, on condition that you never let the battery voltage exceed a maximum.

The maximum is typically around 14.4V to 14.8V at 21C temperature. If the maximum is exceeded for minutes to hours, the battery will be permanently damaged. The damage occurs around the same time that flammable gas is generated and vented, so there is a risk of gas explosion near the battery.

Your setup would require constant monitoring, especially if the load is disconnected or turned off. Check the voltage every hour, and disconnect the solar panel as the voltage approaches the maximum.

To find the maximum voltage, check the manufacturer's label or datasheet for the battery.

You can make your setup safer by adding a charge controller to protect the battery (so you don't have to monitor), by ensuring there is air flow, by removing any sources of ignition for the gas, and by keeping the battery away from things it might damage as a result of an explosion.

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    \$\begingroup\$ Re: "exceeded for minutes to hours" - I used to connect a 60W solar panel to my car battery every few months and let it charge to equalize it. I don't recall the peak voltage, but it went over 15.5 for hours. The liquid in the battery made bubbling sounds and "boiled" freely (the hood was open and no danger of a spark). I had a car battery last more than ten years that way. I also added EDTA early on and of course, made sure to add distilled water as needed. I suppose you were talking about a sealed or "gel-cell" battery though. \$\endgroup\$ – user56384 Sep 2 '15 at 12:48
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    \$\begingroup\$ Thanks. Yes, the question posed was for a sealed lead acid battery, where any venting reduces capacity. What you describe is more or less correct for a wet battery, and is certainly a good justification for the electrolyte active maintenance you did. \$\endgroup\$ – James Cameron Sep 2 '15 at 20:36
  • \$\begingroup\$ @nocomprende Did you add more electrolyte after certain amounts of time? \$\endgroup\$ – user86234 Sep 7 '16 at 16:12
  • \$\begingroup\$ @tuskiomi Yes, I did. To James: I have never seen a car battery that could not be opened somehow to add water. Maybe I just don't have much experience? I would not buy a battery I could not service, if I could avoid it. I guess the phrase "sealed car battery" seems like a contradiction in terms. Cars always have wet batteries. \$\endgroup\$ – user56384 Sep 7 '16 at 16:16
  • \$\begingroup\$ @nocomprende, thanks for the comment, yes, it depends on experience; sealed batteries for cars are available. Mine has one from the manufacturer. Manufacturers choose which type of battery to use in a model depending on the country to which it is being shipped; based on regulations, climate, service network, and other factors. A model isn't the same across all countries. There are also after-market sealed car batteries which have appeal, allowing a customer to compensate for end of warranty period and increasing service costs. \$\endgroup\$ – James Cameron Sep 8 '16 at 0:20
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If you keep the panel connected as shown in your diagram, it will absorb some power when it is not getting sunlight. Add a blocking diode on the positive rail between the battery and the panel, or, as previously suggested (and the best solution IMHO) use a small solar charge controller. Charge controllers are less than $15 USD on eBay or elsewhere on the Web.

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I think no. At least put diode from + solar to possitive battery then tap a limiter zener diode 12v to the battery + to red and black to -

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  • \$\begingroup\$ That would be a big zener diode. And also 12V isn't the right voltage. \$\endgroup\$ – immibis Feb 26 at 22:09

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