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Hello,
I´ve noticed that at 25% SOC the battery voltage was still 48.5V while discharging them at 2kW.
It´s 12 US2000 connected in three blocks through a hub.
So i wondered why so little SOC with still such high voltage..?
The specs say "minimum voltage 44V" which is the literal "dead end", but 44 to 48.5V is a big differene in my understanding.
Is the SOC count wrong for some reason..?
The discharge / charge curve is not flat. The battery will maintain a faily constant Voltage in the middle. If you allow further discharge below roughly 20% then you'll notice the Voltage drop down much more quickly and maybe even reach 44V at 15% depending upon the load.
Same with charging, the Voltage will remain fairly constant up until 90% and after that then it will rise much more quickly towards 100%. Again, maybe even go from 49V at 90% to 52 at 95% and then do some balancing of the cells before shooting up to 100%.
Especially with lithium batteries, the Voltage is a really bad way to determine SOC. Pay more attention to the actual SOC reported by the battery and don't worry about the Voltage so much, it's almost meaningless for battery level purposes.
The Voltage is really important to the very top and the very bottom of charge. At these point going beyond these parameters would damage the battery, but once again, the Pylontech battery will watch these and take care of these limits.
@WKirby Thanks a lot! That makes perfect sense.
What would be the minimum SOC aiming for a long battery life?
20%..?
Watch for steep Voltage dropoff towards the end and then keep SOC above there just before the dropoff to keep the Voltage at the flat part of the curve. This would help with longevity of the battery long term. That is what I do anyway, but the battery will also protect its self, so not too much to worry about really.
Point taken, thank you for your help!!!
In your GX device, what is set as the "Battery Monitor" (in "Settings"-"System setup")?
What would be the minimum SOC aiming for a long battery life?
20%..?
While LiFePO4 cell/battery voltage is not a good indicator for SOC (unless near empty or near full), it is the most important when setting the charge/discharge limits.
I would not discharge LiFePO4 under 3.0V (cell voltage).
I would not charge LiFePO4 above 3.6V (cell voltage).
For longer cell life (with the downside of lower usable capacity), rise the minimums (for instance, stop the discharge at 3.1V) and lower the maximums (for instance stop the charge at 3.55V). When lowering the maximum, you have to check your battery & BMS specifications to see at which voltage the BMS starts balancing the cells. You want to go slightly above that, otherwise the BMS never balances the cells.
Pylontech batteries have 15 cells.
Keep in mind that for instance setting the cut of at 3.0Vx15=45V, does not necessarily mean that all cells will be at 3.0V. Some can be at higher voltage (no problem with those), but some can be at lower voltage (and this can be a big problem, depending on how low). So set you battery voltage limits accounting for this cell imbalance.
Asuming you are using ESS, observe the voltage when close to empty (at your typical loads) and set the ESS Minimum SOC at such a value that the battery discharge is stopped before it reaches the Dynamic cut off voltages.
