# At what voltage is 50% AH rating?

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Registered Users Posts: 59 ✭✭
I am setting up a new Prosine 2.0 inverter charger on my 12 volt 260 ah battery bank. One of the parameters ask at what voltage do I want the prosine to shut down for low voltage. At what voltage is your wet cell battery considered at 50% ah while under a 15 amp load?

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• Super Moderators, Administrators Posts: 33,541 admin
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There is a nice chart here:

http://forum.solar-electric.com/forum/solar-news-reviews-product-announcements/solar-information-links-sources-event-announcements/4426-working-thread-for-solar-beginner-post-faq?p=194303#post194303

Probably, I would start at 12.0 volts and if you have too many shutdowns, drop it to 11.5 volts.

Battery voltage for lead acid is not a very accurate method of determining state of charge under charge/load/no current/temperature/age...

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 3,741 ✭✭✭✭
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bc buck wrote: »
I am setting up a new Prosine 2.0 inverter charger on my 12 volt 260 ah battery bank. One of the parameters ask at what voltage do I want the prosine to shut down for low voltage. At what voltage is your wet cell battery considered at 50% ah while under a 15 amp load?

Most lead acid batteries are at about 50% SOC when their RESTING voltage is at nominal voltage, i.e. 12.0 volts for a 12 volt battery.

Run your batteries down, disconnect them, wait a couple of hours, and measure the voltage. Do this until you get your RESTING voltage down to 12.0 volts. Then connect them, turn on a 15 amp load, wait a minute, and measure the voltage. That is your answer.

There is a problem with this approach to setting the LVD (low volt disconnect).... if you leave a 1 amp load turned on, by the time your battery gets to the LVD voltage it will be well below 50% SOC.

How long do your 15 amp loads last? Many inverters have adjustable timing for LVD.... i.e. the inverter doesn't shut down until the LVD has been reached for a period of time. My 24 volt system has a LVD of 24.0 volts for 1 minute. This means I can use my circular saw (which usually pulls the voltage down to about 23.3 volts) for less than a minute. For longer cuts I turn on the generator.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Solar Expert Posts: 252 ✭✭
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Same here, if it's set to close or to short you will get LVD when the fridge cycles or other short heavy draws. I have mine set at 11.5 Volts for a minute too. My pig of an inverter has a constant 2+ amp draw.
• Solar Expert Posts: 3,123 ✭✭✭✭
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Alaska Man wrote: »
Same here, if it's set to close or to short you will get LVD when the fridge cycles or other short heavy draws. I have mine set at 11.5 Volts for a minute too. My pig of an inverter has a constant 2+ amp draw.
If a fridge or freezer has a defrost timer, it will not be happy with an inverter that has a "sleep" mode where it does not supply power in the absence of a minimum load.

SMA SB 3000, old BP panels.
• Solar Expert Posts: 295 ✭✭✭
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I got a Victron Battery monitor to calculate the SOC.
5kVA Victron Multiplus II, 5.2kW array, 14kWh DIYLifepo4 bank, all grid-tied.