Which Voltage is accurate for 50% battery condition? Loaded or not loaded?

[Sage Oldmann]

I'm new to solar and I have some questions. I have 3, 100 watt solar panels, 6, 35 Amp AGM batteries, 1 Sunforce 1,000 watt pure sine inverter, all fused, and running #2 wire between batteries and between batteries and inverter.

While testing my setup I ran a big screen TV drawing 1.5 amps for 6 hours. My starting resting voltage was 12.7 volts. As the TV operated the voltage dropped slowly for 4 hours, then began a rapid drop to 12.0 volts which as I understand it is 50% and my cutoff point. I drew 9 amps over 6 hours from a 210 amp battery bank and my batteries were down to 50% voltage? I know the inverter draws some power to operate ( I have no idea how much) but does this kind of voltage drop sound right? Do I have a problem?

What's really weird to me is as soon as I turned off the TV, within minutes the voltage meters (3 of them) showed the batteries jumping up to 12.4 volts. The next morning they showed 12.6 volts. How is this possible with no charging going on?

The bigger question for me is which voltage reading do I use to determine when I'm actually at 50% of my battery bank? When the system is showing 12.0 volts under load is that when I should power down? Or should I go down to 11.6 volts under load when the batteries would show 12.0 volts minutes after disconnecting the load?

Is it possible I really only used 1/10 of a volt to run the TV for those 6 hours and the battery bank is actually 12.6 volts afterwards?

[lkruper]

I'm new to solar and I have some questions. I have 3, 100 watt solar panels, 6, 35 Amp AGM batteries, 1 Sunforce 1,000 watt pure sine inverter, all fused, and running #2 wire between batteries and between batteries and inverter.

While testing my setup I ran a big screen TV drawing 1.5 amps for 6 hours. My starting resting voltage was 12.7 volts. As the TV operated the voltage dropped slowly for 4 hours, then began a rapid drop to 12.0 volts which as I understand it is 50% and my cutoff point. I drew 9 amps over 6 hours from a 210 amp battery bank and my batteries were down to 50% voltage? I know the inverter draws some power to operate ( I have no idea how much) but does this kind of voltage drop sound right? Do I have a problem?

What's really weird to me is as soon as I turned off the TV, within minutes the voltage meters (3 of them) showed the batteries jumping up to 12.4 volts. The next morning they showed 12.6 volts. How is this possible with no charging going on?

The bigger question for me is which voltage reading do I use to determine when I'm actually at 50% of my battery bank? When the system is showing 12.0 volts under load is that when I should power down? Or should I go down to 11.6 volts under load when the batteries would show 12.0 volts minutes after disconnecting the load?

Is it possible I really only used 1/10 of a volt to run the TV for those 6 hours and the battery bank is actually 12.6 volts afterwards?

There is a chart on this web page (http://batteryuniversity.com/learn/a...tate_of_charge) that gives the open circuit voltage at 50% discharge of 12.24. To measure this, the batteries must not be under load and need to have rested for a few hours. Even then, voltage is not an accurate way to determine state of charge. However since they are AGMs you cannot measure the specific gravity.

By my rough calculations, your load would use up approximately 50% of your battery capacity. However, six 12v AGMs in parallel to make a 12v battery bank will have problems, if they do not already.

[Sage Oldmann]

However, six 12v AGMs in parallel to make a 12v battery bank will have problems, if they do not already.

What kind of problems are you talking about? I have all the battery cables the same length. I draw the positive of off battery #1 and the negative off the far end of battery #6. How should multi AGM battery banks be hooked up?

[lkruper]

What kind of problems are you talking about? I have all the battery cables the same length. I draw the positive of off battery #1 and the negative off the far end of battery #6. How should multi AGM battery banks be hooked up?

This site gives some examples of how to minimize problems -> http://www.smartgauge.co.uk/batt_con.html

[lkruper]

What kind of problems are you talking about? I have all the battery cables the same length. I draw the positive of off battery #1 and the negative off the far end of battery #6. How should multi AGM battery banks be hooked up?

When batteries are in parallel, if one battery is weak, it degrades the rest so that they are the same as the weak one.

[inetdog]

If you take the time and effort to create a table which gives you voltage under a number of load conditions corresponding to each resting voltage value you will stand a chance of getting useful SOC information from voltage readings taken under load.
Otherwise you will not be able to rely on loaded voltage as a good indicator of SOC.

As a practical matter, if your loads are more or less constant, then setting the Low Battery Cut Off (LBCO) voltage to the loaded value corresponding to 50% should do what you want to do better than setting LBCO to the rested voltage for 50%.

As a side note, for those who have FLA batteries, the SG reading is a far more reliable SOC indicator than any measured voltage.
Those with sealed cells have to do the best they can using voltage only (or for those with the equipment to do it, voltage and internal resistance.)

[SunEagle]

The other issue you have is a 210Ah battery system with maybe 16 amps of charging from those 100 watt panels. That is around a C/13 charge rate which is on the low side and will more than likely result in a less than 100% SOC.

You may want to add a 4th 100 watt panel if your CC can handle the higher charging amps.

[Sunking]

Voltage does not really mean anything. SOC charts are based on Open Circuit Voltage aka OCV in a battery that has been rested for several hours. Even then it is only an indicator. Only way to tell the true SOC in real time is with a Hydrometer.

The big drop off you seen is not 50% it is DEAD.

[Sage Oldmann]

The other issue you have is a 210Ah battery system with maybe 16 amps of charging from those 100 watt panels. That is around a C/13 charge rate which is on the low side and will more than likely result in a less than 100% SOC.

You may want to add a 4th 100 watt panel if your CC can handle the higher charging amps.

A 4th panel is on the way.

[Sage Oldmann]

Voltage does not really mean anything. SOC charts are based on Open Circuit Voltage aka OCV in a battery that has been rested for several hours. Even then it is only an indicator. Only way to tell the true SOC in real time is with a Hydrometer.

The big drop off you seen is not 50% it is DEAD.

I'm a bit confused. It's a brand new system with new batteries. How can it be dead? It charges to 13.8 volts then drops to 12.7 volts at rest just like it should. It runs the TV for 6 hours which if I calculate correctly is just about right for the wattage of the TV and the capacity of the battery bank. If it was dead why is it working? The TV is still working fine at 12.0 volts which is 50%. Then I let it recharge which it did very quickly. Could you explain what you mean that 12.0 volts is dead? Every chart I've seen says it isn't 100% dead until10.5 volts. I'm a long way from that. Is it possible it's just one battery that may be bad?

[inetdog]

I'm a bit confused. It's a brand new system with new batteries. How can it be dead? It charges to 13.8 volts then drops to 12.7 volts at rest just like it should. It runs the TV for 6 hours which if I calculate correctly is just about right for the wattage of the TV and the capacity of the battery bank. If it was dead why is it working? The TV is still working fine at 12.0 volts which is 50%. Then I let it recharge which it did very quickly. Could you explain what you mean that 12.0 volts is dead? Every chart I've seen says it isn't 100% dead until10.5 volts. I'm a long way from that. Is it possible it's just one battery that may be bad?

It is possible that just one battery in a series string is bad. Or one cell in one battery.
It is not possible that only one battery in a parallel configuration is bad to get the symptoms you describe.

[Sage Oldmann]

It is possible that just one battery in a series string is bad. Or one cell in one battery.
It is not possible that only one battery in a parallel configuration is bad to get the symptoms you describe.

So how do I find out what the problem is if I can't test the sealed AGM batteries? How can I determine what is dead or malfunctioning?

[lkruper]

So how do I find out what the problem is if I can't test the sealed AGM batteries? How can I determine what is dead or malfunctioning?

The only thing I can think of is to charge and use each battery independently to see how they perform individually, but that would be time consuming.

[inetdog]

So how do I find out what the problem is if I can't test the sealed AGM batteries? How can I determine what is dead or malfunctioning?

A fair question. Taking the batteries out of the bank one at a time for offline charging and an informal load capacity test would be one way to go.
Getting a clamp-on DC ammeter such as the one from Sears at about $50 will allow you to current measurements of individual batteries while still connected into the bank while charging and discharging. That could help if the problem is one battery.

If you do a single battery capacity test and all of them turn out low, you have gotten a batch of batteries that have been on the distributors shelf for a couple of years or you have killed them somehow.

Do they have readable date codes?

[lkruper]

A fair question. Taking the batteries out of the bank one at a time for offline charging and an informal load capacity test would be one way to go.
Getting a clamp-on DC ammeter such as the one from Sears at about $50 will allow you to current measurements of individual batteries while still connected into the bank while charging and discharging. That could help if the problem is one battery.

If you do a single battery capacity test and all of them turn out low, you have gotten a batch of batteries that have been on the distributors shelf for a couple of years or you have killed them somehow.

Do they have readable date codes?

Or sounds like a job for Optimate 6!