3.4 Battery parameters

3.4.1 Cell defaults

To change the default cell settings, select the menu "Cells → Cell defaults":

 In this section:

• Capacity – nominal capacity of cells, Ah;
• Resistance – nominal (maximum) internal resistance of the cell, Ohm;
• Relax time (after charging) – a relaxation time after charging, second;
• Relax time (atfer discharging) – the relaxation time after discharging, second;
• Cycles – a number of charge-discharge cycles;
• Reset battery cycles – a command to reset charge-discharge cycles;
• Reset cell parameters (SOC, capacity, resistance) – a command to reset cells state of charge, capacity, and resistance.

The values “Capacity”, “Resistance”, “Cycles” are used to calculate the SOC of cells and battery.

The values of “Relax time” are used to determine the state of the battery. If the battery is in a state of relaxation, the system recalculates the voltage on the cells to the state of charge of the battery.

The “Reset cell parameters” command is used for starting-up and adjustment of battery.

3.4.2 SOC estimation

The BMS Main 2.x board calculates the state of charge of the battery (SOC) using two algorithms:

• by open circuit voltage;
• by current and voltage.

It is recommended to use the algorithm of calculation of SOC by voltage and current.

To change the parameters of the algorithm for calculating the battery SOC, select the menu "Cells → SOC estimation":

In this section:

• Estimation algorithm – SOC calculation algorithm:
  • Voltage – by open circuit voltage;
  • Current and voltage (simplified);
  • Current and voltage (enhanced);
• Final SOC – a method of calculating the battery SOC:
  • Minimum cell SOC – the battery SOC is assumed to be equal to the minimum SOC of cells;
  • Average cell SOC – the battery SOC is assumed to be equal to the average SOC of cells;
• Scale the final SOC – flag to scale the battery SOC by the following values;
• Internal SOC corresponding to 0% – battery SOC that sets to be 0%;
• Internal SOC corresponding to 100% – battery SOC that sets to be 100%.
• Uocv = Uocv(SOC, t °C) – the dependence of the cell open circuit voltage Uocv on SOC and the cell temperature (selected for specific batteries, can be established experimentally – see section Determining the discharge characteristic);
• Linear zone – linear zone of dependence Uocv = Uocv(SOC, t °C):
  • Uocv _{[point 1]} – starting point of the linear zone;
  • Uocv _{[point 2]} – end point of the linear zone;
• Temperature correction – the dependence of battery capacity on temperature;
• Cycle correction – the dependence of battery capacity on the number of charge-discharge cycles.

The SOC calculation algorithm for voltage calculates SOC cells based on the tabular dependence Uocv = Uocv(SOC, t °C) .

The SOC calculation algorithm “Current and voltage (simplified)” works as follows:

• if I = 0, the battery is in a state of relaxation and the cell voltage Uocv is outside the [U_{ocv[point 1]}; U_{ocv[point 2]}], the SOC calculation based on the tabular dependence Uocv = Uocv(SOC, t °C);
• in any other cases, the SOC value is proportional to the charge (coulomb) passed through the battery (current time integral).

The SOC calculation algorithm “Current and voltage (enhanced)” differs from the simplified algorithm by online correction of effective capacitance. When using this algorithm, it is necessary to fine tune the tabular dependence Uocv = Uocv (SOC, t °C). 

3.4.3 Cell resistance estimation

Calculation of the resistance of cells is carried out in two ways. The first method is used when the battery passes from a relaxation state to a charge or discharge state, wherein the cell resistance value

R = (U-Uocv) / Istable,

where U is the cell voltage measured in the charge or discharge state, V; Uocv is the cell voltage measured in the state of relaxation (before switching to the state of charge or discharge); Istable – stabilized current through the cell in the state of charge or discharge.

The second method is used for a stepwise change in the current through the cell, while the value of the cell resistance: 

R = (U₂-U₁) / (I_stable2-I_stable1) provided that | I_stable2-I_stable1 | > 0.2 × Q_max

(Q_max is the maximum cell capacity),

where U₂ is the voltage on the cell at the moment when the stabilized current I_stable2 is flowing through it; U₁ – the voltage on the cell at the moment when the stabilized current I_stable1 flowing through it.

The stabilized current I_stable = I, if during the stabilization time the instantaneous current I is in the range from 0.95 × I to 1.05 × I.

To change parameters of the algorithm for calculating the cell resistance, select the menu "Cells → Cell resistance estimation":

SOC correction

The BMS Main 2.x board can recalculate the battery SOC after long-term storage or after long-term working in the case when the battery was not charged fully or discharged totally. Recalculation is done based on the tabular dependency Uocv = Uocv (SOC, t) (see section Calculating the state of charge (SOC)).

To configure parameters for periodically correcting the battery state of charge, select the menu "Cells → SOC correction":

 In this section:

• Enable – a flag to enable SOC correction;
• Shutdown period – a time the battery is off, day. If the BMS detects on its startup that it was off during the “Shutdown period” time, the BMS recalculates the battery state of charge base on the tabular dependency Uocv = Uocv (SOC, t);
• Correction period – a period of correcting the battery SOC, day. If the BMS detects that the last correction was more than the “Correction period” ago, the BMS recalculates the battery state of charge base on the tabular dependency Uocv = Uocv (SOC, t) and tunes it gradually during the “SOC change time”;
• SOC change time – a duration of the linear changing the battery SOC to the value calculated by the correction algorithm, minute.