Basic min & max voltage logic

@jamcandles
Are you operating the battery with an approximate 80% DoD during your normal daily cycling?

Summer daily average discharge is 40% total ( 95% to 55% SOC), winter 31% total (95% to ~65% SOC), with solar or grid charging overnight.  I have never taken any of them (the current 4-pack, nor my original during my first year) below a 41% total reported pack SOC (and that low only once). 

I have automation set to flip everything to grid if the pack drops below 45%.

Individual cell voltage min and max, highest and lowest in the 4-battery pack, is usually 0.003 between the highest and lowest cell at no load, to 0.008 during charging.  If I top the batteries, and keep them there (e.g. allow them to remain topped), what I think is the firmware balancing cycle kicks in and cycles repeatedly tops and discharges every few minutes; during those cycles, I have seen 0.02-0.07V differences during those times, but as soon as the pack drops below 100% SOC the differences return to normal. 

The battery has has the most wander, the one at 34% in my post, is one of the new batteries, reports 69 cycles total, and is around 8mo old.  The batteries at 55 and 62% are the same age, reporting 71 cycles each.  While the battery reporting 46% in my post is ~20mo old and reports 181 total cycles.

All are on the latest firmware, RS232 and RS485 were both updated about a month after the last firmware came out.  

@eg4eric Sorry I posted my response without hitting reply... and I don't see an edit button to fix it.  Details posted at 3:05pm here: https://forum.eg4electronics.com/community/postid/21498/

Correct: Cell delta accross the pack reaches a maximum of 0.08V during balancing cycles; I had the decimal shifted in my prior post.

@jamcandles

If you have our EG4 Indoor WallMount batteries, we recommend operating the batteries with an 80% DoD. This helps minimize SOC drift, maintain optimal battery performance, and maximize the overall lifespan of the batteries.

I’m also sharing a document that provides additional information on battery health and the recommended 80% DoD.

Battery-Health-and-DoD-Document.pdf

Thank you @eg4eric.  I read and understand he document, however, it feels like the recommendation to regularly use a full 80% DoD (as opposed to 70%, 50%, etc.) is a bit dangerous from both a power-reliability and battery longevity perspective.  Let me explain, as I love my batteries and just want to use them to their fullest.

Is there only strict voltage based logic built-in at 80% (or lower) DoD, and then not again until they are topped?   I get the voltage changes a lot faster at the high and low levels of discharge; I was mainly wondering (or requesting if its not there) additional logic at the higher, but not quite over-voltage end. 

Using my batteries as another example

• My 4-pack is at 53.31V at the moment, discharging but at almost no load (500W, 0.01C on my 4-pack)
• One of my batteries thinks it is at 99%, and the lowest thinks is reporting 95%. 
• Based on my past observations, they will all sit at 99% for a good long while when I top them to sync reported SOC in a few days; meaning they are reporting higher than they really are at the moment
• My purely user-experience based estimate is I am probably somewhere between 85-95% true SOC right now

A consistent, regular full 80% discharge, like the document says, would mean a 1.5-2 day cycle rate for me, and anyone who sized their battery system to cover a couple days of load.  We would lose:

• We'd be at risk of dropping well below 80% due to
  • Reported SOC wander.  I have seen the batteries think they are much higher (and lower) than they actually are.
  • Any utility outage would put us below 80% very quickly.  If the grid went down at 1AM, the batteries would be dead by morning.  
• When we got down to 70% or 80% DoD, we would have effectively no backup for utility outages
  • I'm in PG&E territory, they're notoriously unreliable and expensive in summer (heat) and winter (wind, rain, cars)
• Any daily automations and schedules would be lost, or at best much more complicated.  A daily cycle, like I'm doing now, is something we can plan our lives around.

I was thinking if there was firmer voltage logic at the higher, but not quite 100% end, it would keep the batteries reported SOC in a fairly-close range, even over extended time periods.  It would allow for or add the ability to safely:

• Keep a daily cycle at whatever the users real load is
• Most of us would have backup power, without risk, for utility outages
• In my case, I wouldn't lose the 2-4kWh of solar or grid power that's lost every time I top the batteries to sync reported SOC.  They really do get crazily out of sync within 2-3 weeks unless they are topped

Using my battery levels as one last example, that battery thinking it is at 99% is not realistically possible.  To pull my batteries down to 53.3 volts at 99% would take an enormous load, well beyond 0.5C, and my dual 12000XP's would trip long before that.  Just a little extra voltage based logic would tell the battery that it is, at most, at 95% if there was 0.5C output rate.  I suspect just that little bit of knocking the reported SOC back in line would keep the batteries from getting egregiously out of sync over time; they may not be perfect, but they would keep within an acceptable and safe 5-10% of each other.