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  • foofoodog commented on deba168's instructable DIY OFF GRID SOLAR SYSTEM7 months ago
    DIY OFF GRID SOLAR SYSTEM

    AGM deep cycle batteries should only be taken to 50% depth of discharge to maximize battery life span. Your example math assumes it will be 100% DOD and that will destroy the battery in a very short time.Battery amp hours are normally quoted at a 20 hour rate for greatest efficiency. So a 35 Ah battery at that rate will give you 1.75 amps an hour or 21 watts an hour at 12 volts. Expect only 50% of that to be usable without trashing the battery.Running a 200 watt inverter full bore from a 35 Ah battery will last about an hour and also destroy the battery quickly. At the 1 hour rate you can expect about 300 watts and your inverter will be tripping the low voltage shutoff alarm very quickly. For comparison a 180 watt cigarette lighter plug car inverter circuit would need a 15 amp fuse and ...see more »AGM deep cycle batteries should only be taken to 50% depth of discharge to maximize battery life span. Your example math assumes it will be 100% DOD and that will destroy the battery in a very short time.Battery amp hours are normally quoted at a 20 hour rate for greatest efficiency. So a 35 Ah battery at that rate will give you 1.75 amps an hour or 21 watts an hour at 12 volts. Expect only 50% of that to be usable without trashing the battery.Running a 200 watt inverter full bore from a 35 Ah battery will last about an hour and also destroy the battery quickly. At the 1 hour rate you can expect about 300 watts and your inverter will be tripping the low voltage shutoff alarm very quickly. For comparison a 180 watt cigarette lighter plug car inverter circuit would need a 15 amp fuse and you would only use it with the car running and with enough spare capacity from the alternator.For your example you will probably want 2 35 Ah batteries to go with your 125 watt panel to maximize battery lifespan. If you need to pull the full 200 watts for any length of time you will need a whole lot more battery and hence more solar.

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