Which Low Battery Cut Out Voltage to Use?

Elke · September 2011

Greetings,

I wanted to take my grid-tie system (Xantrex XW4024) off the grid and switch to battery power for a test run. The plan was to run only of the batteries until 25% of discharge and then let the sun recharge them. To not let the batteries discharge any more I wanted to set the 'Low Battery Cut Out' voltage to an appropriate value.

I am using SunXtender AGM batteries and have followed the manufacturer's recommendations to set all the charging parameters. It is a 24V system (25.6V OPEN circuit voltage). A 50% depth of discharge would be equivalent to a 25V OPEN circuit voltage. The maximum value for the Xantrex Low Battery Cut Out voltage is 24V. I know, this is not an open circuit voltage value. But, how do I know, what a safe value is? Trial and error? Or is there a rule of thumb?

I would appreciate some advice. I am reluctant to discharge the batteries to just whatever depth of discharge.

Thank you.
Elke

BB. · September 2011

Re: Which Low Battery Cut Out Voltage to Use?

A common battery cutoff level is around 11.5/46 volts (12/48 volt bank). But, there is a good chance that will take your system well below 75% state of charge.

If you do not have a battery monitor, you could just calculate/measure your AC loads and time your discharge...

For example, say your loads are 1,000 watts average. And you have a 600 AH battery bank:

600 AH * 48 volt battery bank * 0.25 discharge 0.85 inverter efficiency * 1/1,000 watt load = 6.12 hours

So, as a first estimate, you would run (from my mythical system) your loads for ~6 hours to discharge your battery bank by 25%.

Plug your numbers in for your test loading.

-Bill

Cariboocoot · September 2011

Re: Which Low Battery Cut Out Voltage to Use?

The absolute lowest you can run a 24 Volt system to is 21 Volts. But you don't want to do that because that is basically dead batteries.

Battery Voltage is not a good indicator of state of charge, as you know. Especially not when under load which any battery would be when connected to an inverter. If you're going to try and correlate the two I'd suggest getting a battery monitor: when it reads "25%" you can see what the under load Voltage is and set the cut-out accordingly. Otherwise I wouldn't go any lower than 24.5, depending on how heavy the load is you expect to keep running (heavier load = lower Voltage set-point). The at rest Voltage for 24 Volt system @ 50% SOC is about 24 Volts.

Keep in mind your inverter will not read battery Voltage with 100% accuracy. Neither will a multimeter for that matter. To err on the side of caution you might want to limit it to 25 Volts under load.

BB. · September 2011

Re: Which Low Battery Cut Out Voltage to Use?

Obviously, you would substitute the values for your 24 volt battery bank (yea--I forgot that you said 24 volt system as I was typing my response :roll:).

Still would need to know the Amp*Hour rating of your battery bank and the average load(s) you would like to run to make the calculations for your system.

-Bill

Joe_B · September 2011

Re: Which Low Battery Cut Out Voltage to Use?

This is one of the most dificult and frustrating parameters to set on any system. Like others have said, it really depends on your loads. I do not recomend using the X bargraph to monitor SOC. I have a Victron BMV-600 and the X meter never reads even close to the SOC that the Victron indicates. The victron is not perfect either but if you correlate the SOC on the Victron with the SG, they will be fairly close.

So here is what I do for my LBCO, I do not totally rely on the LBCO value in the inverter section, I set the Victron to output an relay closure at 50% SOC. Then connect the relay from the Victron to the RPD input on the XW. This will insure that if the XW loses it marbles for some reason, the Victron will shut down the inverter before it wrecks the battery bank. Of course the inverters LBCO settings will still function but the Victron acts like a watchdog "just in case".

Elke · September 2011

Re: Which Low Battery Cut Out Voltage to Use?

Thank you so much for all your advice.

I have noticed something odd: The Xantrex software does not allow me to set the low battery cut out voltage to any value greater than 24V! Is this a bug? Do I have old firmware?

Eventually, I would like to get a battery monitor, especially before upgrading to a larger battery bank. For now, I have simply 4 SunExtender batteries in series for a combined 24V, 305Ah bank. This is small, but the main reason for the batteries is to be able to have daytime electricity in cases when the grid is down and the sun is out with the panels producing.
So at least I will not do much damage, if I mess up during my experiments.

With these numbers I calculated that I could run my 1.1kW vacuum for roughly 1.5 hours and expect around 25% depth of discharge.
After this exercise the control panel should give me some voltage for the batteries - I would think. And maybe I can use this number for the cut out voltage - as long as it is not larger than the 24V factory limit?

Elke

Joe_B · September 2011

Re: Which Low Battery Cut Out Voltage to Use?

Elke,

The LBCO in the inverter section is a threshold that will start a timer. The LBCO delay will control how long the voltage must remain below the LBCO threshold before the inverter will shut down. As soon as you remove the load (turn off the vacuum cleaner) the bank voltage will immediately rise. If you want to be the most accurate, you will have to use some sort of battery meter (with a shunt) that will keep track of how many amp hours you remove and replace from the battery bank.

Solar Guppy · September 2011

Re: Which Low Battery Cut Out Voltage to Use?

Also, LVD on the XW when triggered faults the inverter, it is not a control feature to engage or disengage the AC1 input. Its intented as a last resort to prevent battery damage

Elke · September 2011

Re: Which Low Battery Cut Out Voltage to Use?

Interesting! That's good to know. From the manual it is not exactly clear.

Thank you.

Vic · September 2011

Re: Which Low Battery Cut Out Voltage to Use?

Elke,

If you have the time to do the discharge manually, it is no too difficult, altho, still impefect.

If you assume that the batteries still have 100% of the rated capacity, you could either make a dc resistive Load Bank, or use some heating devices run on 120/240 VAC.

Then you can extract 25% of the rated AH capacity of the bank in a conveniently short period of time-- a few hours let's say. I have not looked, but there may not be 8 Hour capacity ratings of your batteries, but you could approximate the values from data given.

Have done this with one of the banks here, and it seems to have worked well. The CC used here can be set to read out the AH returned to the bank, so that one could determine (retrospectively) approximately how deep was the discharge, from the return AH data. The batteries under test here had a good family of capacity data vs discharge time.

It would probably be a good idea to monitor the terminal voltage of the whole bank, just in case something goes south, or even LVD some safe value.

Just a thought, would never be perfect, but at least AGMs are quite efficient, so the AH return data would need a very small derate for recharge inefficency.

Vic

Which Low Battery Cut Out Voltage to Use?

Comments

Categories