2.1 Cell Voltage Limits

This page helps you set the cell voltage limits of the BMS from the datasheet of your cells: when the BMS warns, when it switches the charge or the discharge path off, when it switches the path on again, and when a voltage difference between the cells stops the battery.

Applies to

Boards: BMS1820, BMS3610, BCU2002, BMS1601, BMS1810.

2.1.1 How It Works

The BMS compares every cell with the limits on its own. One cell past a limit is enough to act; the other cells do not need to be there.

Fig.2.1.1: Cell Voltage Limits

3.0 3.2 3.4 3.6 3.8 4.0 4.2 Cell voltage (V) discharge path OFF warning normal operation, all paths ON warning charge path OFF U.V. Protection U.V. Warning O.V. Warning O.V. Protection U.V.P. Release O.V.P Release discharge path ON again charge path ON again Factory default values; choose your own from the cell datasheet. Protections act after their delay time, warnings after 1 s.

  • Warning: a cell past O.V. Warning or U.V. Warning for 1 s raises a WARNING event. No path is switched off. The warning clears when the cell is back a little inside the warning limit.
  • Over-voltage protection: a cell at or above O.V. Protection for O.V.P. Delay Time switches off the charge path. The discharge path stays on, so the battery can still supply the load.
  • Under-voltage protection: a cell at or below U.V. Protection for U.V.P Delay Time switches off the discharge path. The charge path stays on, so the battery can still be charged.
  • Release: the path switches on again when every cell that tripped is back past its release value, O.V.P Release or U.V.P. Release, for 1 s. Each trip counts towards Error Recovery Attempts (3.1 Protection and Recovery).
  • Cell delta: when the difference between the highest and the lowest cell of the whole system stays at or above Imbalanced Max. Cell Delta for about 30 s, the BMS raises Max. Cell Delta Voltage Protection. This is a CRITICAL event: both paths switch off until the BMS restarts.

2.1.2 Settings

Table.2.1.1: Cell Voltage Settings (LiMon™ Cell tab)

Setting What it does Unit Range Default
O.V. Warning Warning when a cell is at or above it. mV 0…5000 4050
O.V. Protection Charge path off when a cell stays at or above it. mV 0…5000 4175
O.V.P. Delay Time Time a cell must stay at or above O.V. Protection. ms 0…60000 2000
O.V.P Release Charge path on again when the cells are at or below it. mV 0…5000 4100
U.V. Warning Warning when a cell is at or below it. mV 0…5000 3300
U.V. Protection Discharge path off when a cell stays at or below it. mV 0…5000 3050
U.V.P Delay Time Time a cell must stay at or below U.V. Protection. ms 0…60000 3000
U.V.P. Release Discharge path on again when the cells are at or above it. mV 0…5000 3150
Imbalanced Max. Cell Delta Highest allowed difference between the highest and the lowest cell. mV 0…5000 100

The LiMon™ reference describes every field of the screen: 7.2 Cell Settings.

The defaults are a starting point only. Set every value for your cells before the first charge.

2.1.3 Choosing the Values

Take the limits from the datasheet of the cell: the maximum charge voltage and the discharge cut-off voltage. Keep the values in this order:

U.V. Protection < U.V.P. Release < U.V. Warning < normal range < O.V. Warning < O.V.P Release < O.V. Protection

  • O.V. Protection: at or below the maximum charge voltage of the cell. It is the last line of defence against overcharge, not the normal end of charge.
  • O.V. Warning: above the voltage per cell at which your charger ends the charge, so that a normal full charge does not raise a warning, and below O.V. Protection.
  • Charger voltage: set the charger so that no cell reaches O.V. Protection in a normal charge. With unbalanced cells the highest cell is above the average, so leave room for it.
  • O.V.P Release: clearly below O.V. Protection. A cell relaxes after the charge stops; a release value only a few mV below the limit lets the charger switch on and off again and again.
  • U.V. Protection: at or above the discharge cut-off voltage of the cell.
  • U.V. Warning: high enough above U.V. Protection to give the system time to react, for example to reduce the load or to start charging.
  • U.V.P. Release: clearly above U.V. Protection. A cell recovers when the load stops; if the release value is close to the limit, the load starts again, pulls the cell down and the trips add up to Error Recovery Attempts.
  • Delay times: long enough to ride through short voltage peaks and dips, for example the dip at a motor start, and short enough to protect the cell. The defaults, 2 s for over-voltage and 3 s for under-voltage, suit most systems.
  • Imbalanced Max. Cell Delta: above the spread that your battery shows in normal use, at full load and at the ends of the charge, where the spread is largest. Too low stops a healthy battery with a CRITICAL event; too high lets a weak or disconnected cell go unseen.

2.1.4 Checking the Result

  • Live View shows the highest and the lowest cell voltage and their delta; both cells must stay inside the warning limits in normal use.
  • At the end of a full charge: no O.V. Warning, and the highest cell clearly below O.V. Protection.
  • At full load near the end of the discharge: the lowest cell stays above U.V. Protection for as long as the application needs.
  • Events › Event Status shows every warning and protection with its time.

2.1.5 Common Mistakes

  • Limits not set for the cell chemistry. The defaults do not fit every cell. A limit above the maximum charge voltage overcharges the cells; a limit far below it never lets the battery reach full charge.
  • O.V. Warning below the charger end voltage. Every full charge raises a warning, and real warnings are no longer noticed.
  • Charger voltage too high. The highest cell reaches O.V. Protection at the end of every charge, the charge path switches off and on, and after Error Recovery Attempts it stays off. Lower the charger voltage or improve the balance.
  • Release value too close to the protection limit. The path switches on, the voltage crosses the limit again at once and the trip count rises quickly. Leave a clear gap.
  • U.V. Protection used as the normal end of discharge. Every discharge ends with a protection trip that is counted. Stop the load with the inverter or the vehicle controller before the cells reach U.V. Protection, for example at U.V. Warning or at a low state of charge.
  • Imbalanced Max. Cell Delta too low. Under a high current, cells with different internal resistance spread out for a short time. The CRITICAL event then switches off both paths of a healthy battery until a restart.
  • Order of the values mixed up. A release value outside the protection limit, or a warning past the protection, gives no warning before a trip or a path that does not switch on again. Check the order above after every change.