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I have a 130Ah AGM deep cycle 12V battery for camping connected to 2 x 100W solar panels. I have been testing the system out to see whether it will sustain what I require. I think I’ll require around 100Ah of charge per day which would approximately discharge the battery to 25% SOC.

How much charge can my solar panels theoretically replenish if I can access say 6 quality sun hours in a day?

Does the wattage of the solar panels represent the amount of charge that can be supplied to the battery per hour? If so, I imagine I could generate 1200 Watt Hours or 100 Amp Hours in a day which would be enough to replenish the amount of charge that I wish to consume. Is my math correct?

If I didn’t use more than 100Ah of charge per day, would there still be an advantage in terms of battery longevity by chaining another identical battery to my existing one to double the capacity?

If I wanted to use more than 100Ah of charge then I would require a second battery so as not to discharge the first battery to less than 25% SOC. However, would my solar panels be capable of regenerating enough charge to make adding the second battery viable? Or would there be other advantages to add the second battery in terms of operating on average at a higher SOC even if I were unable to regenerate all of the charge that I had consumed in a day?

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    \$\begingroup\$ It is close, but not quite enough. For a 12V battery, 100 Ah is around 100 * 12 = 1200 Watt-hours (Wh). Your panels, rated nominally at 100 W, will probably produce around 80 W each under real world conditions, assuming a high quality MPPT charge controller. So 80 W * 2 panels * 6 hours = 960 Wh per day. You could add a third panel, or find a way to reduce your energy consumption a bit. Adding a battery won't really help. \$\endgroup\$ – mkeith Dec 24 '17 at 0:30
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You need more than 150% of the power you use if you are using very low resistance parts. Otherwise you will need more. In the real world, typical solar panels don't actually produce what they are advertised to produce. In the real world batteries need more power than used to replenish their power with. Only the purest, least resistance batteries give extremely efficient recharge rates. Most produce heat and some swelling in the battery (AGM) which is loss of power. And you need a higher level of power offered to the battery than it currently has in order to charge it. I recommend double your perceived power achieved. Remember, only 1/3 the day is usable daylight typically, so if you want to use the battery after dark, the numbers get alot bigger. Never discharge a 12v battery under 12v whenever possible, and ABSOLUTELY NEVER allow a battery to remain in a discharged state for any period of time if possible. I have assumed lead acid batteries here, but they are all similar in the problems associated with them.

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  • \$\begingroup\$ I’ve noticed the battery’s voltage is about 0.2V to 0.3V less than the resting voltage when measured while the inverter is drawing power. The inverter has a setting to cut off supply of charge at a nominated voltage. Ideally what should this be set to? Should I set it to 12V or can I safely set it to 11.7V or 11.8V as the inverter is likely to be in a state of drawing power when the voltage reaches these low values? \$\endgroup\$ – Guru Josh Dec 24 '17 at 23:53
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100w panels have about 5.9 amps of short circuit current.

A PWM charge controller can deliver up to 83% of the current from the panel to the battery. An MPPT charge controller can deliver about 96% to the battery.

There's a lot of real world variables to consider, maybe 40-50 amps best case scenario.

Ideally you shouldn't discharge an AGM more than 50% (or 40%) if you don't want to shorten its' lifespan.

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