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I want to know if I can do this. I have 2 panels, 1.5 watt ea that put out a max of 110 milliamps. I bought a new AGM 14AH battery because my lead acid died early.

I plan to run both panels that fit in my small southern facing window, from the inside. I want to plug the panels’ 12v plugs into a splitter to run a single 12v extension cord to my ATV accessory plug to charge the battery. I don’t want to over charge, but I doubt these little panels would do it. I would lose some voltage with 16’ of cord I would imagine. I could check the battery with a meter on sunny days to see what the battery is measuring, but wondered if you all could run numbers and tell a novice. The panels are the small 1.5 watt at Northerntool. https://www.northerntool.com/shop/tools/product_200660262_200660262

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  • \$\begingroup\$ If you are just trying to keep the battery topped off, it may work OK. Make sure the accessory plug is live when the ATV is off. Otherwise you may have to connect directly to the battery. \$\endgroup\$ – mkeith Jan 8 '18 at 4:20
  • \$\begingroup\$ Consider a larger panel (say 10W) and a cheap solar charge controller. That way you are relatively well protected against over-discharge (because you charge faster) and overcharge (that's what the charge controller does). \$\endgroup\$ – Brian Drummond Jan 8 '18 at 11:20
  • \$\begingroup\$ Sounds like that’s the way to go. Any suggestions on a cheaper controller to work with a 10 watt panel? I assume 10 watt 12 volt panel then? \$\endgroup\$ – Magnumglide Jan 9 '18 at 5:01
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Your link doesn't provide an Open Circuit voltage for those panels, but we can guess: !.5W is probably the maximum power you can get out, 110 is the Short Circuit current. The SC current is close to the Maximum Power Point current, so calculating on that basis, the Maximum Power Point voltage is 1.5/110, approximately 13.5V. 13.5V is low for a 12V battery charging system, which in this case is deliberate: they are more concerned about over-voltage than they are concerned about optimum charging.

This is a compromise panel, sometimes called a "self regulating" panel. With a MPPV of 13.5V, the Open Circuit Voltage will be around 17V. That is too high. If you let your batteries sit at 17V, you will damage them. The system depends on the fact that the panels will charge to the batteries so slowly that, if you use the batteries, they will never become fully charged. If the battery is cold, and the panels hot, you'll get hardly any charging.

If the panels were optimized for charging a 12V system, they'd have a MMPV around 17V, and an OC voltage around 21V. The system would charge rapidly, it would work at any temperature, it would handle line drop and series diodes, and you'd have to use a charge controller to prevent overcharge.

These panels probably won't damage your batteries, if you don't leave it too long, and don't let the batteries get hot when the panels are cold when there is full sun when the battery is charged. These panels also won't keep your battery charged if you use it very much. Leaving your batteries uncharged is another way to damage them.

And if you want a better system, you have to do something different.

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14.2V*14AH = 200Wh means it can supply 2W for 100h so your charge time is expected to take weeks which could be longer if you have any load on the battery. Avoid a low voltage condition ( i.e. < 10% SoC ) for extended hours as this will cause rapid sulphation of the plates.

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