Convert Battery Percentage Values to Voltages?

[dreancestere]

My 4 module LiFePO4 HomeGrid Stack'd battery (19.2 kWh/400 Ah, 15 cell, not 16 cell) is connected to my Sol-Ark 15 inverter via closed-loop communications. Currently, I'm using percentage values to control TOU and Discharge levels (Shutdown at 1%, Low Batt at 5%, Restart at 10%). I've read some posts here recommending not using percentages to control the battery and to switch over to voltages. How do I convert the values? HomeGrid's setup guide for Sol-Ark inverters specifies percentages for those values and I can't find anything in the battery specifications talking about those values in volts.

 

[wpns]

My 4 module LiFePO4 HomeGrid Stack'd battery (19.2 kWh/400 Ah, 15 cell, not 16 cell) is connected to my Sol-Ark 15 inverter via closed-loop communications. Currently, I'm using percentage values to control TOU and Discharge levels (Shutdown at 1%, Low Batt at 5%, Restart at 10%). I've read some posts here recommending not using percentages to control the battery and to switch over to voltages. How do I convert the values? HomeGrid's setup guide for Sol-Ark inverters specifies percentages for those values and I can't find anything in the battery specifications talking about those values in volts.

Voltage is the second-worst way you can determine SOC of LFP batteries, just behind weighing them.

You need a shunt to do coulomb counting, with a reset to 100% when full. Your BMS has one, but it’s probably not very good over long time periods or small currents below it’s threshold. There are better shunts (Victron, etc) but nothing’s perfect.

 

[Backyard]

LFP's voltage is so flat it's difficult to get an accurate SOC until it hits the knee. Anything in between is a guessing game. Better off coulomb counting as mentioned above

 

[Chuck7454]

LFP's voltage is so flat it's difficult to get an accurate SOC until it hits the knee. Anything in between is a guessing game. Better off coulomb counting as mentioned above

Backyard is both correct, and not correct (sort of like Schrodinger's Post-er). The basic premise is correct "it's difficult to get an accurate SOC until it hits the knee", however the values of SOC you are interested in are at the knee (10%, 5% and 1%) so you may be OK, but it is not quite that simple as the charging/discharging current will also affect the voltage. And 1 other thing to keep, in mind most tables of this kind (Volts to SOC) are based on 1s, 4s, 8s & 16s so you should not just use a 48V (16s) table as your battery is 15s. And don't forget the effect of cell temperature on the cell voltage.

And yes, I have been of little help with this post, I just like making Quantum Mechanics references. Especially when 2 posts up there is a reference to Relativity.

 

[bolhuijo]

You can get OK estimates of SOC with voltage, but you need to let the battery rest for about 4 hours without any charge/discharge.

If they're in use all the time, then yeah you gotta weigh them as wpns says above.

 

[shazam]

Maybe I mis-read the OP's question?....he wanted a reference to convert SOC to voltage?

I saved this from an earlier discussion on the topic on this forum....I make no claims to its' accuracy. It was posted by one of the members in response to essentially the same question. OP can search and find that discussion at his pleasure

Jim

PS- On my SA15k and 50 kWh of SOKs, also using closed-loop, it seems farily accurate...fwiw....ymmv.

 

[dreancestere]

Thanks for that chart. I'll look for the thread. But, from the previous posts, I think I'm just going to leave it using percentages. It doesn't look like I'd gain anything by converting. I'm also assuming that since I'm in closed loop communication, the inverter and battery will maintain some kind of reasonable accuracy as long as I get to 100% SOC often enough. Looking at recent history, I usually reach that around noonish every day. The longest stretch I've seen (tentative glance through data) where I don't get to 100% SOC has been 4 days. But, I've modified a 2pm to 4pm TOU slot to charge from grid if necessary to get me to 100% every day. We'll see how that goes.