Many DIY builders choose a JK BMS for one reason: its active balancer. Instead of bleeding excess energy from the highest cell through a resistor, it moves charge from the fullest cells to the emptiest ones. It works faster than passive balancing, and it raises questions too: when does it run, how much current is normal, and why does the delta still move?
Cell delta: the number to watch
The cell delta is the difference between the highest and lowest cell voltages in the pack. On lithium iron phosphate cells, the delta is small in mid charge because the voltage curve is flat, and grows near the top and bottom of the charge where the curve gets steep. A delta must always be read together with the state of charge.
- Mid charge, at rest: a few millivolts is normal and says little about balance.
- Near full charge: the delta rises as the first cells reach the knee of the curve; this is where the balancer matters.
- After a long full charge: a well balanced pack comes back to a small delta.
When the balancer runs
A JK BMS balances when two conditions are met: the cell voltage is above a start threshold you set, and the delta is above a trigger value you set. Below that voltage, the balancer stays idle even if the delta looks large. This is deliberate: on the flat part of the curve, voltage differences do not reliably reflect energy differences, so balancing there could make things worse.
| Reading | Meaning |
|---|---|
| Balancing: off, delta small | Nothing to do: the pack is balanced or below the start voltage |
| Balancing: on near the top of charge | Normal: the balancer is evening out the cells |
| Balancing on for hours every day, delta not falling | One cell may differ in capacity, or a connection may add resistance |
| Delta high at the bottom of discharge only | A lower-capacity cell, visible only when the pack is nearly empty |
Balance current: what it tells you
The balance current shows how hard the balancer is working at that moment. It peaks when the delta is large and the pack is in the balancing window, then falls as the cells converge. A balancer that runs at full current for long periods, day after day, points to a pack that drifts faster than it can be corrected. Check busbars and cell terminals first: a poor connection adds a voltage drop under load that looks like a misbehaving cell.
Settings: read them, do not guess them
Start voltage, trigger delta and charge voltage work together. If your inverter stops charging before the cells ever reach the start voltage, the balancer never runs. If the start voltage is set very low, the balancer may chase meaningless differences on the plateau. Changing these values belongs to you and the JK app; JK BMS Monitor only reads them, along with every cell, the temperatures and the alarms.
Following the balancer over weeks
The JK app shows a live picture, within Bluetooth range. JK BMS Monitor connects to the RS485 port, shows the delta and balancing state on its screen and on your Wi-Fi, and publishes one sensor per cell to Home Assistant. With the free Cloud you keep up to three years of history. A simple check: note the delta at the end of each full charge for a month. A flat line means the balancer keeps up; a rising one tells you which cell to look at.