LBCO Voltages...

johnT · November 2015

Does anyone know at what voltage a 48v system will be around 50% (approximate range)? This seems to be an area with a wide variety of opinions. I realize that it will vary from battery bank to battery bank, but it seems that there would have to be a normal range that most relatively healthy battery banks would fall into.

Here is what it says in the manual of the Magnum ME-RC Remote under setting the LBCO:

"Where do I set the LBCO setting?

If you want to cycle the batteries

slightly – but don’t want to discharge them more than 20%*, then the

LBCO setting should be set from 11.5 to 12.2 VDC (12-volt models), 23.0

to 24.4 VDC (24-volt models), or 46.0 to 48.8 (48-volt models). In some

applications, such as those installed in an off-grid home or when doing a lot

of dry-camping in your RV, you may want to cycle down to 50%* by setting

the LBCO from 10.0 to 11.4 VDC (12-volt models), 20.0 to 22.8 VDC (24-volt

models), or 40.0 to 45.6 VDC (48-volt models). In extreme circumstances,

you have the ability to discharge the batteries to 80%* by setting the LBCO

to 9.0 or 9.5 VDC (12-volt models), 18.0 or 19.0 VDC (24-volt models), or

36.0 or 38.0 VDC (48-volt models) before recharging."

I would just go by this, but I have seen a lot of opinions from what seem like knowledgeable people that would greatly disagree with those voltages. Some say that 50% is closer to 48.0 VDC, which is a vast difference from "40.0 to 45.6 VDC", and some would say that at 40.0v a battery bank would be basically fully discharged (which is obviously not good). With rainy season setting in here, I know I am going to be cycling my batteries deeper than I was when it was sunny most days, but I would still like to keep from going below 50%...I just need to know where that range is. I also realize that there is a big difference between resting voltage and under load. But in reference to LBCO, I don't know why anyone would be talking about resting voltage, since that would not apply.

Over time, I have come to trust the information and opinions on this forum more so than any other, but I haven't been able to find much on here about it. Any information would be greatly appreciated. Thanks

CALLD · November 2015

It's really hard to use voltage as a state of charge indicator when the batteries are under load because it depends on knowing how your batteries perform under various loads. This is not universal.

The advice you will get from the most credible sources will be to keep battery voltages above 2vpc (12/24/48v) for the longest battery life and to avoid going under 1.9vpc (11.4/22.8/45.6v)under the heaviest loads on your system.

The reasons being that under these voltages you are starting to enter the "danger zone" for battery sulfation, plate damage and cell reversal. You are also getting to a level of discharge that makes fully recharging in one day (sunrise to sunset) impossible. A lead acid battery needs a minimum of 16hours of changing time to get back to 100% after a deep discharge irrespective of how much power the charger can deliver. The battery itself limits the maximum rate at which it can absorb charge particularly in the absorption phase. At voltages below 1.9vpc under heavy load the weakest cells in a battery start to experience a pronounced voltage drop and can quite easily "go over the cliff" and even go into reverse! Once a battery cell goes into reverse it is permanently damaged. Ensuring battery voltage doesn't go below 1.9vpc makes cell reversal very unlikely for systems below 48v nominal.

Another reason to avoid going below 1.9vpc (11.4/22.8/45.6) is to prolong your inverter life. As the battery voltage drops your inverter has to draw ever more current to keep the output power constant. This puts increasing strain on the inverter's components and will shorten it's life.

vtmaps · November 2015

a RESTING voltage of about 48 volts represents 50% SOC for many lead acid batteries. In a operating off grid system, resting voltage is an abstract concept. It doesn't occur.

If you set your LBCO (= LVD low voltage disconnect) to 48 volts you will assure yourself that you will never get below 50% SOC.

Suppose your batteries were at about 75% SOC (somewhere up around 50 volts). You would like to turn on a large load (e.g. a 1200 watt vacuum). You will only run it for 5 minutes.   5 minutes X 1200 watts = 0.1 kwh. This is a small amount of energy for your battery which stores many kwh. It should not drop your SOC very far. BUT during that 5 minutes the battery voltage may sag to 23 volts and activate your LBCO, even though the SOC is far above 50%. Once the LBCO is activated the battery voltage will begin to rebound (could be a few hours) to a resting voltage that truly represents its 75% SOC.

The only way you could actually get to 50% SOC is with a tiny load (leave a night light plugged in all the time). The reason is that a tiny load will not cause any voltage sag... the voltage is essentially the resting voltage.   By the time the voltage got down to 48 volts, the battery would really be at 50% SOC.

There are ways around this... the voltage sag is a function of the Peukert factor... buy a low Peukert factor battery and there is less voltage sag.

Another way around the problem is to use SOC rather than voltage to activate the LBCO. Of course that means you need some sort of shunt based battery monitor to calculate the SOC.

Another way, is to use voltage to activate the LBCO, but have it factor in the current draw and Peukert factor to lower the LBCO voltage as a function of load. In other words, 46 volts is OK when you are drawing 1200 watts, but 47.9 volts is not OK when you are drawing only 10 watts.

--vtMaps

Dave Angelini · November 2015

The XW + system uses 46V with a 10 second test time and a new hysteresis set-point that I have been just leaving at the default.
The older XW systems were always 44V and 22V for lights out after 600 seconds. They come back-up after sunrise if you have sun....

LBCO Voltages...

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