I want a circuit that works as a duel charger(600-800 watt)Inverter AC & Solar Parallel And If possible Need Separate Circuit of Solar Charging with battery full charging Controller with auto cut and if need after solar charging battery itself can get charge form AC.Basically Add on Circuit that be integrated with existing 600-800 watt Inverter used 12v 180Ah Inverter battery.Its Need if possible solution.
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My real concern(unfounded at the moment) is the lm's v drop +the shottkey diode drop would have my underpowered panels falling below 13.8v a lot.

Add to that, no longer tracking the sun will pretty severely limit the panels full exposure time.

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help to maintain a charging voltage even when the panel isn't at it's prime?
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Panel undervoltage is not an issue, they will generate full voltage in quite low light, but the CURRENT is very light dependent. An LM317 etc will work fine, just in overcast conditions etc you will get much less current into the battery.

The amount of energy you can harvest during bad light conditions (especially dusk and dawn etc) is very small compared to the panel working in direct sunlight so don't worry about it too much. Your main goal should be to get the panels in FULL sunlight (ie no shading) for 4 to 6 hours a day and forget the rest of the time.