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  • I have a 12v battery that is rated at 220ah.
  • I have solar panels rated at 500 watts total.
  • I'm using a MPPT charge controller rated for 12/24 volts

I only have a rudimentary understanding of electricity, so please forgive the elementary nature of these question that I hope you can help me work out:

Assuming we start at zero, and ignoring real-world inefficiencies...

  1. If this is a 12v panel and it produces ~41 amps, would it take roughly 5.3 hours to fill up the battery?

  2. If this is a 24v panel it produces ~21 amps, would it take roughly 11 hours to fill up the battery?

That seems counter-intuitive, but if "amps" = "the amount of electricity" I'm getting lost as to why that isn't true.

  1. Is there any difference in usable power generation between 12 and 24 volt panels as long as they are at the same wattage?

  2. Finally, if I have some amount of "amp hours" I'm trying to replace in a battery (let's say I ran that 220ah battery down to 110ah remaining and I want to recharge it), what's the formula for figuring out how many hours of sunlight I would need, if I have X watts worth of solar panels?

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The key to understanding your photovoltaic system is thinking in terms of energy and power. The battery capacity is measured in Ampere-hours, but this measure only tells you how many electrons can flow trough the battery before it discharges. How much energy each electron carries is determined by the battery voltage, so less Ah does not always mean less energy. To calculate the energy storage capacity of a battery (Wh), you multiply the capacity (Ah) by the nominal voltage (V) of the battery. In your case the battery has an energy storage capacity of 220 Ah * 12 V = 2640 Wh = 2.64 kWh.

The MPPT charge controller contains a DC/DC converter which allows it to step the fluctuating solar panel voltage up or down to match the battery. For example, if your 24V panel outputs 10A (240W), the charge controller steps it down to 12V and 20A which is then fed to the battery (still 240W of power, minus any conversion losses). The charge time difference between using a 500W 24V panel or 500W 12V panel should be neglible.

To calculate the charge time, divide the energy by the power. For example fully charging a half discharged battery would require 220 Ah / 2 * 12 V = 1.32 kWh of energy. If your panels output 500W, this would take 1320 Wh / 500 W = 2.64 h = 2 h 38 min. If the charging efficiency is say 90%, it would take 1.11 times longer still.

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  • \$\begingroup\$ Eureka! Your explanation finally made it click! In particular, this bit really helped: "The battery capacity is measured in Ampere-hours, but this measure only tells you how many electrons can flow trough the battery before it discharges. How much energy each electron carries is determined by the battery voltage, so less Ah does not always mean less energy." Thank you! \$\endgroup\$ – user1142433 Jan 28 '16 at 5:28

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