Three questions internal BMS and battery safety questions

[garetwo]

Well, my roller coaster ride continues. First, just to head off the “never do that” stuff, it has already been done, I know better now, but it was not me to begin with. Also, since trying to help a friend AND joining this forum the 16 JITA 12.8vdc 300ah batteries arranged 4S4P will now have a Victron 500amp shunt on each group of four and an equalizer across each group of four – none are installed yet, but THANKS!!! To all the input that has moved us to this point.

Now, the story and the question. He just told me he found batteries down to 8vdc. Disregarding why this takes place (the equalizers will help now) what does the battery’s internal BMS actually do?

What does this mean?: DISCHARGE PROTECTION VOLTAGE 10.0-10.8. Is it just to set inverters to stop using batteries if they see whatever we set them to or is the internal BMS supposed to somehow stop allowing the battery to discharge?

There was a wild swing in voltages across the individual batteries. I am going to find out what equalizer he purchased. I can see you cannot have a single battery drop out in a group in series, but perhaps he can get an alarm when a single battery hits 10.8 VDC so he can at least open the disconnect for that set of batteries. Th=is brings up the second question, “Can a bank of batteries in series be somehow automatically dropped out if one battery gets too low”?

One more, is there a low-end voltage point where a LifePO4 battery is damaged? My friend told me he has seen them drop to zero and not have a problem.

 

[sunshine_eggo]

A BMS in discharge protect mode prevents any discharge from that battery. Since series components in the same circuit must experience the same current, NO current is allowed to pass through the entire string.

Think of it as though you've disconnected the cable from the (-) terminal of that battery. That's what's happened INSIDE the BMS circuitry.

One more, is there a low-end voltage point where a LifePO4 battery is damaged? My friend told me he has seen them drop to zero and not have a problem.

Anything observed below 10-ish volts is almost certainly an arbitrary voltage reported by the BMS in discharge protection mode, not the actual sum of cell voltages.

The deeper you discharge cells, the shorter their life. LFP batteries cycled 0-100% every single day will have a shorter cycle life than a battery used less aggressively.

 

[garetwo]

A BMS in discharge protect mode prevents any discharge from that battery. Since series components in the same circuit must experience the same current, NO current is allowed to pass through the entire string.

Think of it as though you've disconnected the cable from the (-) terminal of that battery. That's what's happened INSIDE the BMS circuitry.




Anything observed below 10-ish volts is almost certainly an arbitrary voltage reported by the BMS in discharge protection mode, not the actual sum of cell voltages.

The deeper you discharge cells, the shorter their life. LFP batteries cycled 0-100% every single day will have a shorter cycle life than a battery used less aggressively.

is zero percent the same as zero volts?

 

[garetwo]

A BMS in discharge protect mode prevents any discharge from that battery. Since series components in the same circuit must experience the same current, NO current is allowed to pass through the entire string.

Think of it as though you've disconnected the cable from the (-) terminal of that battery. That's what's happened INSIDE the BMS circuitry.




Anything observed below 10-ish volts is almost certainly an arbitrary voltage reported by the BMS in discharge protection mode, not the actual sum of cell voltages.

The deeper you discharge cells, the shorter their life. LFP batteries cycled 0-100% every single day will have a shorter cycle life than a battery used less aggressively.

How does it disallow the actual reading? What is between the DVM probe and the actual cell bank?

 

[garetwo]

Think of it as though you've disconnected the cable from the (-) terminal of that battery. That's what's happened INSIDE the BMS circuitry.

Oh...sheesh, you answered the earlier question, plus, I should have said thank you to start with - thanks!! How does it read 8 volts then, why would it not read zero? Or, if everything is still connected, is that a potential between one battery to another versus the actual singular battery voltage? (Does that even make sense?)

 

[garetwo]

A BMS in discharge protect mode prevents any discharge from that battery. Since series components in the same circuit must experience the same current, NO current is allowed to pass through the entire string.

WOW! This is a big deal! One could lose a whole bank, forcing the remaining to supply the load. This would certainly explain why such a drain on the batteries when the load was not that great over time, he may have had one or more banks no longer being used because of one cell...Awesome information, thank you.

 

[sunshine_eggo]

is zero percent the same as zero volts?

No. At 10V (2.5V/cell) the battery is at 0%.

How does it disallow the actual reading? What is between the DVM probe and the actual cell bank?

BMS circuitry. It is arbitrarily reporting a low value.

Oh...sheesh, you answered the earlier question, plus, I should have said thank you to start with - thanks!! How does it read 8 volts then, why would it not read zero? Or, if everything is still connected, is that a potential between one battery to another versus the actual singular battery voltage? (Does that even make sense?)

Because the BMS manufacturer chose to report that voltage via the BMS circuitry. Some might read near 0V, 2-3V, etc.

The BMS is still drawing a very small amount of power from the battery cells (very low milliamps or even microamps), so it can continue to communicate data via bluetooth or other monitoring (if so enabled) and/or maintain the protection state while providing an arbitrary voltage.

Some BMS/monitors may draw power from only the first 1-2 cells, so the BMS may simply report the measurement of the cells powering the BMS/monitor.

WOW! This is a big deal! One could lose a whole bank, forcing the remaining to supply the load. This would certainly explain why such a drain on the batteries when the load was not that great over time, he may have had one or more banks no longer being used because of one cell...Awesome information, thank you.

Yep.

That's why it's very important to have all batteries in a series string top balanced at 100% charge, so you can draw maximum capacity from the bank.