In short
- The BMS counts the current that goes in and out of the cells; the inverter either receives that figure or makes its own estimate.
- Without a working communication link, the inverter guesses from voltage, and the guess drifts with lithium.
- A full charge from time to time resets the count on the BMS side; after that, the figures usually line up again.
A Pytes E-Box or V5 battery has its own BMS, which measures every amp that goes in or out and keeps a running total. That total, compared with the capacity, gives the state of charge. The inverter, on its side, needs a figure to decide when to stop discharging.
Two ways the inverter gets its number
When the battery and the inverter talk over a CAN or RS485 link with a protocol they both understand, the inverter simply shows the BMS value. That is the normal setup with most hybrid inverters that list Pytes as compatible. If the link is missing, wired to the wrong port or set to the wrong protocol, the inverter falls back on its own estimate, usually based on voltage. With LiFePO4 cells, whose voltage barely moves across most of the range, that estimate can be well off.
When even the BMS drifts
Counting current is precise but not perfect. Small measurement errors add up over days of partial cycles. The BMS corrects itself when the pack reaches full charge: at that point it knows it is at 100 % and starts counting again from a known value. A battery that never quite fills, in winter or behind an inverter set to stop charging early, can drift by several points.
Reading both side by side
Pytes Monitor reads the BMS through the console port and shows its state of charge, its voltage and current, and the highest and lowest cells. Put that next to what your inverter shows. If the two differ by a few points and come back together after a sunny day, all is well. If the inverter figure wanders far from the BMS one, check the communication cable and the battery protocol selected in the inverter.
Which one to trust?
For decisions about the battery itself, the BMS figure. It is the one built from the cells. For understanding why the inverter switched to the grid at a given moment, look at the inverter figure: that is the one it acted on.
Common questions
Does Pytes Monitor change the inverter’s value?
No. It only reads the BMS. Your inverter keeps receiving what it received before.
How often should the battery reach 100 %?
Pytes publishes its own recommendations; as a rule of thumb, a full charge every week or two keeps the count accurate.
The difference is more than 10 points. What should I check?
The communication cable between battery and inverter, the port it is plugged into, and the battery protocol selected in the inverter settings.
