Measurement Error

The SuperBMS will attempt to detect measurement errors by comparing cell voltages between measurements.

A measurement error will be announced by a long tone by the buzzer.

If a measurement error occurs, the BMS will disable the Charge/Balance/Discharge mode in order to avoid possible damage. The BMS will also update the status summary at the corresponding index when requested.

Finding the Problem

A measurement error could be a result of 4 possibilities:

  • loose solder connections
  • faulty op-amps
  • faulty ADCs
  • faulty battery

If an op-amp is faulty or not soldered properly, the op-amp could saturate, outputting around 8 volts. This exceeds the maximum voltage range of the inputs of the ADCs. When the ADCs read a voltage that is too high, the ADC will be disconnected from it’s inputs, resulting in floating voltages to be recorded. The floating voltages are often close to 5 volts, or 0 volts, triggering a measurement error.

If the battery is damaged or faulty, it’s voltages could vary greatly between measurements.

Rule out improper solder connections by resoldering the op-amps and ADCs, and using a multimeter or oscilloscope to confirm accurate voltages are being sent to the ADCs.

If the op-amps and ADCs are working, test the battery by putting it under load, and monitor the voltage sag of the battery.

Recovering From Low Charge

If a battery cell has discharged below the shutdown voltage the BMS may be able to recover the battery, depending on the following:

  • How low is the cell?
  • How charged are other cells?
  • Is the discharged cell one of the first 4 cells?

How low is the cell?

If the cell has not discharged much more below the shutdown voltage the chances of recovery are high.

If the cell has continued to discharge below the shutdown voltage the chances of recovery are much less.

How charged are other cells?

This is a very important question. If other cells are much higher than the discharged cell, the BMS may not be able to charge the discharged cell as fast, depending on when the other cells reach the maximum cell voltage.

Is the discharged cell one of the first 4 cells?

If this is true, then this cell is partially responsible for powering the BMS. If any balancing is required during charging, the fan will draw a good amount of current from the first 4 cells, making it take longer to charge (not much longer but still significant with a very discharged cell).

Any of these 3 events could cause the discharged cell to never charge back up. If this is the case, the BMS will try to restore the battery to acceptable levels indefinitely.