In short
- Cell OVP 3.65 V and Cell UVP 2.80 V are the usual hard limits for LiFePO4; the inverter should stop well before either.
- The balancer only runs above its start voltage: if the inverter stops charging below it, the cells drift apart over time.
- Never allow charging below 0 °C: keep Charge UTP at a few degrees above zero.
Every JK board ships with defaults that work, more or less. But a pack built from 280 Ah cells behind a hybrid inverter is not a scooter battery, and a few values are worth setting on purpose. The settings helper gives the numbers for your cell count; this guide explains why.
The hard limits: OVP and UVP
Cell OVP is the voltage at which the BMS stops charging. For LiFePO4 it is usually 3.65 V, the maximum in most datasheets; some builders choose 3.60 V for a little margin. Cell UVP stops the discharge, usually around 2.80 V. These are safety nets, not working points: in normal use the inverter should stop charging and discharging long before the BMS has to step in. The recovery values, OVPR and UVPR, must sit inside the range, around 3.40 V and 3.00 V.
The balancer: start voltage and trigger
The JK active balancer works only when two conditions are met: one cell is above the Start Balance Vol. and the gap between highest and lowest cell is above the Balance Trig. Volt. On LiFePO4, 3.40 V and 5 to 10 mV are common choices. Below about 3.35 V the voltage curve is so flat that a few millivolts say almost nothing about charge, and balancing there can move energy the wrong way.
RCV and RFV: what the BMS asks the inverter
On the inverter series, the BMS tells the inverter what voltage to charge to. Cell RCV (charge) at 3.45 V and Cell RFV (float) around 3.375 V give 55.2 V and 54.0 V on a 16-cell pack. The trap: if RCV is at or below the balance start voltage, no cell ever reaches the point where the balancer starts. The pack looks fine for months, then the cells have drifted apart. Keep RCV at least 30 to 50 mV above the start voltage.
The state of charge counter
The JK counts amp-hours in and out, then resets its counter when the cells reach SOC-100% Volt. or SOC-0% Volt. Set SOC-100% at or slightly below RCV, otherwise the counter is never corrected and drifts by several percent.
Temperatures and currents
- Charge UTP: 2 to 5 °C. LiFePO4 must not be charged below 0 °C.
- Charge OTP around 45 °C, Discharge OTP around 60 °C.
- MOS OTP: leave the board default unless you have a reason.
- Continued charge current: at most 0.5C of your cells, within the board rating; discharge: up to 1C, within the board and cable ratings.
Check them without opening the app
JK BMS Monitor reads these settings over RS485, never writes them, and compares them with the usual LiFePO4 ranges on the dashboard, including the RCV and start voltage trap. You see at a glance if a value deserves a second look.
Common questions
What is a good balance start voltage for a JK BMS with LiFePO4?
Most builders use 3.40 V, sometimes 3.35 V. Lower than that the balancer works on the flat part of the curve where voltage says little about charge; higher, it may run too rarely if your inverter stops charging early.
Should I set the inverter or the BMS?
With closed-loop communication over CAN or RS485, the BMS sends RCV and RFV and the inverter follows. Set the charge voltages in the inverter by hand only when it runs without communication, and keep them consistent with the BMS.