It will depend many things, what inverter you have will limit the low voltage disconnect, what type of battery will limit the lowest % of discharge you will want to go before damaging the battery, what amount of load will create different voltage sags, what size of wires will vary the amount of voltage sag, what temp your batteries are at will vary the available capacity, what time of day will effect the amount of assistance the array is providing, what the future weather will be like will effect the possibility of recharging hence how low you can take down your battery while being able to replacing the energy without damaging the battery. The age of the batteries, as batteries age the have less capacity, when young they have less capacity...
Sorry, I'm not up yet, I'm sure I can give you a few hundred more...
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites, Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
In general, with lead acid batteries, we don't like to go to 50% state of charge or lower too many times--Pretty stressful on the battery bank. Plus it takes a lot of energy (days of sun) to get them back charged again.
The absolute maximum discharge would be ~20% state of charge. If you go below that, a weak cell in your bank could go dead and actually begin to "reverse charge" (battery cell goes from ~+2.0 volts to ~-2.0 volts charging. In pretty much all rechargeable battery chemistries, "reverse biasing/charging" a cell will kill them.
Voltage wise, lead acid battery bank state of charge is difficult to estimate by purely measuring voltage. The bank voltage depends on SOC, temperature, charging/discharging current levels, age, etc... So it is difficult to set a specific cut-off voltage.
For a 24 volt battery bank, setting a warning voltage at ~23.0 volts is a good start. Under moderate loads, ~23.0 volts would be ~50% state of charge. It could dip lower under heavier loads for a few seconds or minutes (some systems let you set different voltage set points at different times--~1 hour average, ~5 minute average, ~1 minute average voltage, etc..).
To give you an idea:
New poster "leaf" has a really nice set of charts that compare battery voltage against different rates of discharging and charging (as well as resting voltage readings).
I don't quite a agree with the resting voltage line (I think the voltage is a bit low)--But it shows how to estimate a battery's state of charge while operating.
Note, where the charts "flatten out"--the room for error estimating state of charge is pretty high.
-Bill
There are also "maximum" current draw from your battery bank. For general flooded cell lead acid batteries, using the 20 Hour Rate/capacity (i.e., 225 AH capacity):
225 AH * 1/20 hour rate = ~11 amp draw (20 hours to take battery bank dead)--Good average current draw (efficient)
225 AH * 1/8 hour rate = 28 Amp draw--Maximum recommended continuous current draw
225 AH * 1/5 hour rate = 45 Amp draw--Maximum draw for an hour or so
225 AH * 1/2.5 hour rate = 90 Amp draw--Maximum surge (seconds to minute or so--i.e., starting pump, etc.).
-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Bill, I know this is the second time I've seen these charts, and I still think either the legend in wrong or they haven't been done correctly. Most flooded lead acid batteries have a resting voltage at 50% SOC of near 12 volts. Yet the first chart as I read it says a 12 volt battery under a 10% of capacity load will have a voltage of 12 volts. I would guess normally it would be closer to the c/5 or 20% load...
Perhaps I'm reading it wrong or missed something?
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites, Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
You are probably correct that the discharge curves seem like they should be a few tenths of a volt lower (maybe even -0.5 volts in some cases)... These are supposed to be measured values by one person on one battery bank at one time.
In any case, use them as a guide to understand what is happening in "your" system.
Battery voltage is a poor estimate of state of charge (in general).
-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Comments
It will depend many things, what inverter you have will limit the low voltage disconnect, what type of battery will limit the lowest % of discharge you will want to go before damaging the battery, what amount of load will create different voltage sags, what size of wires will vary the amount of voltage sag, what temp your batteries are at will vary the available capacity, what time of day will effect the amount of assistance the array is providing, what the future weather will be like will effect the possibility of recharging hence how low you can take down your battery while being able to replacing the energy without damaging the battery. The age of the batteries, as batteries age the have less capacity, when young they have less capacity...
Sorry, I'm not up yet, I'm sure I can give you a few hundred more...
In general, with lead acid batteries, we don't like to go to 50% state of charge or lower too many times--Pretty stressful on the battery bank. Plus it takes a lot of energy (days of sun) to get them back charged again.
The absolute maximum discharge would be ~20% state of charge. If you go below that, a weak cell in your bank could go dead and actually begin to "reverse charge" (battery cell goes from ~+2.0 volts to ~-2.0 volts charging. In pretty much all rechargeable battery chemistries, "reverse biasing/charging" a cell will kill them.
Voltage wise, lead acid battery bank state of charge is difficult to estimate by purely measuring voltage. The bank voltage depends on SOC, temperature, charging/discharging current levels, age, etc... So it is difficult to set a specific cut-off voltage.
For a 24 volt battery bank, setting a warning voltage at ~23.0 volts is a good start. Under moderate loads, ~23.0 volts would be ~50% state of charge. It could dip lower under heavier loads for a few seconds or minutes (some systems let you set different voltage set points at different times--~1 hour average, ~5 minute average, ~1 minute average voltage, etc..).
To give you an idea:
There are also "maximum" current draw from your battery bank. For general flooded cell lead acid batteries, using the 20 Hour Rate/capacity (i.e., 225 AH capacity):
225 AH * 1/20 hour rate = ~11 amp draw (20 hours to take battery bank dead)--Good average current draw (efficient)
225 AH * 1/8 hour rate = 28 Amp draw--Maximum recommended continuous current draw
225 AH * 1/5 hour rate = 45 Amp draw--Maximum draw for an hour or so
225 AH * 1/2.5 hour rate = 90 Amp draw--Maximum surge (seconds to minute or so--i.e., starting pump, etc.).
-Bill
Bill, I know this is the second time I've seen these charts, and I still think either the legend in wrong or they haven't been done correctly. Most flooded lead acid batteries have a resting voltage at 50% SOC of near 12 volts. Yet the first chart as I read it says a 12 volt battery under a 10% of capacity load will have a voltage of 12 volts. I would guess normally it would be closer to the c/5 or 20% load...
Perhaps I'm reading it wrong or missed something?
Photowhit,
You are probably correct that the discharge curves seem like they should be a few tenths of a volt lower (maybe even -0.5 volts in some cases)... These are supposed to be measured values by one person on one battery bank at one time.
In any case, use them as a guide to understand what is happening in "your" system.
Battery voltage is a poor estimate of state of charge (in general).
-Bill