12/06/2025
It’s interesting how both clients and some installers are pushing for communication between batteries and inverters, thinking it’s always a benefit. But in reality, it can create system instability.
This is likely driven by the need to understand how full a battery is, the human mind understands percentages a lot better than trying to learn that at xV my battery is empty and at yV it is full.
Let’s say you use SOC (State of Charge) to set your battery's lower discharge limit. The minimum safe voltage for a typical lithium iron phosphate (LiFePO₄) cell is around 3.0V, so for a 16-cell battery (16s), that equates to 48V.
However, SOC and voltage do not have a linear relationship, especially on lithium batteries. What would the voltage be if you discharged to 20% State of Charge (SOC)? You can’t say with certainty. Lithium batteries "flatten" their voltage curve across most of their SOC range. Voltage doesn’t drop steadily from 100% to 0%. Instead, most voltage drop happens rapidly near the bottom 10–20% and the top 5–10%. This makes voltage an unreliable indicator for precise SOC measurement.
More importantly, SOC is a calculated value, not directly measured. It only becomes reasonably accurate if the battery is left idle for several hours. On a solar system, this rarely happens. That’s why we prefer voltage-based cutoffs for system settings. It’s more consistent and safer.
Real-World Example:
We recently tested a popular battery brand paired with a Sunsynk inverter. When enabling CAN communication between them, the battery was overcharged by more than 1V. And we hadn’t changed anything except enabling communication!
Using Swift, our monitoring platform, we could clearly compare the BMS voltage with the inverter-reported voltage. The discrepancy was obvious and significant. That kind of overvoltage can void the battery’s warranty. How is that fair to the client?
More Supporting Data:
Three batteries, identical models, about 9 months old:
Battery 1: 50.17V, 12.02% SOC
Battery 2: 50.30V, 9.79% SOC
Battery 3: 50.60V, 27.53% SOC
(Bus bars were in place, and cable lengths and thickness were verified.)
A six-battery system, different brand. All report 53.4 or 53.5V, but SOC varies from 44% to 95%.
Another system with three batteries shows SOC differences between 70% and 74%, even though the voltages are identical.
(Not dramatic, but still relevant if the system relies on SOC.)
Takeaway:
Just because I have this data does not mean your system definitely has the issue. But here’s a useful clue: if your battery LEDs (those little lights on the front) are often out of sync, for example, one showing 3 lights and another showing 4, it might be a sign of imbalance or communication mismatch.
