20Ah battery (sealed lead acid) and 1500w generic inverter

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Gerome
Gerome Registered Users Posts: 2
When the battery is under load with the inverter, is it okay if the voltage of the battery dropped? If okay how much it is allowed to drop and what is the suggested voltage cutoff of the battery?


(Also I don't use the full 1500w of the inverter, approx. 20w of it, I'm using for my led lights)

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
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    For lead acid batteries, their output voltage is difficult to estimate the state of charge by the battery voltage. There are lots of variables (state of charge, amount of discharge/charge current, temperature, age, etc.).

    Lead acid batteries, if you can discharge to 50% state of charge, and recharge the next sunny day--That will give you a longer battery life. Generally, like to suggest discharging to 75% SoC for daily/normal cycling. And down to 50% SoC during bad weather/occasional heavy loads (running tools a couple days a week, etc.).

    For a first approximation, with "average loads" (max ~1/8 or 12.5% rate of discharge) use 11.5 volts as your max discharge voltage for normal usage. For surge current (starting a motor, you might see 10.5 Volts for a few seconds).

    Which brings us to your battery/battery bank... A 20 AH @ 12 volt battery is pretty small... A typical "automotive" battery is around 50-80 AH @ 12 volts. And in the USA, we suggest a pair of 6 volt @ 200 AH "golf cart" batteries in series for a 12 volt @ 200 AH battery bank for a first try. They are (at least here) relatively inexpensive and pretty rugged/forgiving.

    As an example, using a maximum of 8 hour discharge rate:
    • 20 AH / 8 hour discharge rate = 2.5 Amps max suggested continuous discharge rate
    • 2.5 Amps * 12 volts = 30 Watts
    • 20 AH * 0.50 max suggested/planned discharge = 10 AH "usable" capacity
    • 10 AH * 12 volts = 120 Watt*Hours of useful power
    A 1,500 Watt AC inverter is a pretty heavy load for a 20 AH battery... Normally, suggest a 100 AH battery for every 250 Watt rated inverter--Or for your 1,500 Watt inverter:
    • 1,500 Watts * 1/250 Watts per 100 AH (at 12 volts) = 600 AH @ 12 volt battery bank to use a "fully loaded" 1,500 Watt AC inverter
    Note your AC inverter might draw 10-20 Watts "tare losses" (using that much power "just turned on". 20 Watt tare + 20 Watt LED loads--Your inverter is only 50% efficient.

    Highly suggest a much smaller AC inverter (maybe 6 Watts or less Tare losses) for your LED lightning.

    Or, a better option. See if you can purchase 12 (or 12/24 volt) LED lights. No inverter losses. And there are lots of options. I believe you are in the Philippines... So, what is available in your region may be different. Some Amazon examples:

    https://www.amazon.com/12-volt-led-lights/s?k=12-volt+led+lights (12 volt fixtures and even "light bulbs")
    https://www.amazon.com/s?k=12+24+volt+led+lights (12/24 volt compatible LED lights)

    For example, if you are using 3x 7 Watt 12 VDC LED Lamps:
    • 3 * 7 Watts = 21 Watts LED lighting load
    • 21 Watts / 12 volts = 1.75 Amp draw
    • 10 AH (50% capacity) / 1.75 Amp draw = 5.7 hour run time @ 20 Hour Lead Acid battery (suggested)
    Your thoughts?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Gerome
    Gerome Registered Users Posts: 2
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    BB. said:
    For lead acid batteries, their output voltage is difficult to estimate the state of charge by the battery voltage. There are lots of variables (state of charge, amount of discharge/charge current, temperature, age, etc.).

    Lead acid batteries, if you can discharge to 50% state of charge, and recharge the next sunny day--That will give you a longer battery life. Generally, like to suggest discharging to 75% SoC for daily/normal cycling. And down to 50% SoC during bad weather/occasional heavy loads (running tools a couple days a week, etc.).

    For a first approximation, with "average loads" (max ~1/8 or 12.5% rate of discharge) use 11.5 volts as your max discharge voltage for normal usage. For surge current (starting a motor, you might see 10.5 Volts for a few seconds).

    Which brings us to your battery/battery bank... A 20 AH @ 12 volt battery is pretty small... A typical "automotive" battery is around 50-80 AH @ 12 volts. And in the USA, we suggest a pair of 6 volt @ 200 AH "golf cart" batteries in series for a 12 volt @ 200 AH battery bank for a first try. They are (at least here) relatively inexpensive and pretty rugged/forgiving.

    As an example, using a maximum of 8 hour discharge rate:
    • 20 AH / 8 hour discharge rate = 2.5 Amps max suggested continuous discharge rate
    • 2.5 Amps * 12 volts = 30 Watts
    • 20 AH * 0.50 max suggested/planned discharge = 10 AH "usable" capacity
    • 10 AH * 12 volts = 120 Watt*Hours of useful power
    A 1,500 Watt AC inverter is a pretty heavy load for a 20 AH battery... Normally, suggest a 100 AH battery for every 250 Watt rated inverter--Or for your 1,500 Watt inverter:
    • 1,500 Watts * 1/250 Watts per 100 AH (at 12 volts) = 600 AH @ 12 volt battery bank to use a "fully loaded" 1,500 Watt AC inverter
    Note your AC inverter might draw 10-20 Watts "tare losses" (using that much power "just turned on". 20 Watt tare + 20 Watt LED loads--Your inverter is only 50% efficient.

    Highly suggest a much smaller AC inverter (maybe 6 Watts or less Tare losses) for your LED lightning.

    Or, a better option. See if you can purchase 12 (or 12/24 volt) LED lights. No inverter losses. And there are lots of options. I believe you are in the Philippines... So, what is available in your region may be different. Some Amazon examples:

    https://www.amazon.com/12-volt-led-lights/s?k=12-volt+led+lights (12 volt fixtures and even "light bulbs")
    https://www.amazon.com/s?k=12+24+volt+led+lights (12/24 volt compatible LED lights)

    For example, if you are using 3x 7 Watt 12 VDC LED Lamps:
    • 3 * 7 Watts = 21 Watts LED lighting load
    • 21 Watts / 12 volts = 1.75 Amp draw
    • 10 AH (50% capacity) / 1.75 Amp draw = 5.7 hour run time @ 20 Hour Lead Acid battery (suggested)
    Your thoughts?

    -Bill
    Thanks Bill, really appreciated your response. I'll try ordering 12v lights on shopee to be more efficient. Thanks again!