Cell Balancing in Electric Vehicle Battery Pack

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Cell Balancing in Electric Vehicle Battery Pack

Passive and Active cell balancing techniques

M Sanath Kumar

Dept. Of Electrical and Electronics PES University, RR campus Bengaluru, India

Fakeerappa Rathod

Dept. of Electrical and Electronics PES University, RR Campus Bengaluru, India

Abstract Electric vehicles are the future of Transportation. There has been an increase in EV purchases of 109% worldwide in 2021[5] and in the last decade, EVs have become popular because of their zero tailpipe emissions and least reliance on oil and its byproducts [6]. The Latest fully Electric vehicles use Lithium-ion battery packs as their main energy source which comes with both merits and demerits. If the battery parameters are not maintained optimally in all conditions, catastrophic casualties can occur, e.g., toxic smoke or fire. The main aim of this paper is to demonstrate ways to balance the voltages in every cell of the Battery pack using more than one technique. This ensures the optimum performance of the Battery pack by not allowing any cell to over-charge or over-discharge hence, increasing its life and usable capacity. Detailed models and simulations of different types of cell balancing techniques are shown in this paper

Keywords Cell balancing; Active balancing; Passive balancing; Vehicle to grid


Electric vehicles will be of great relevance in the coming years and so will the related technologies. Battery technology is one of them. Maintaining a Battery pack at its full capability is a crucial part of running an EV safely because if not, there can be severe concerns related to safety and logistics. Battery packs are made of many li-ion cells connected in series and parallel configurations to achieve a total battery voltage and power at the output [2]. These cells can often have voltage imbalances in them causing over-charging or undercharging. Differences can occur in the initial charge capacities and cell voltages due to temperature changes [3]. This will result in not being able to use the cells capacity to its maximum hence, causing poor battery performance, Compromised battery life, and a potential fire hazard. Cell voltage balancing is one such technique that can be used to eliminate these imbalances where it makes sure that all the cells are equally charged and equally discharged by using either of these mentioned types of cell balancing: 1) Passive balancing 2) Active balancing. Both methods will be simulated and analyzed thoroughly.

G Pavan Kumar

Dept. of Electrical and Electronics PES University, RR campus Bengaluru, India

Sangeeta Modi

Dept. of Electrical and Electronics PES University, RR campus Bengaluru, India

fig 1.0 – Increase in EV market share [4]

fig 1.1 – Cell Balancing techniques block diagram

  1. Cell Model

    Battery Model: Several cells of a specific nominal voltage and capacity are configured in series and parallel to create a whole battery pack of a particular rating of power, voltage and current. One such cell model is shown in fig 1.2 with a cell voltage of 7.2V and capacity of 0.00475Ah. Without any balancing circuits, this cell discharges from 100% to 0% in approx. 2500 seconds (fig 1.3).

    fig 1.2 – Battery model without Balancing

    fig 1.3 – Discharge Characteristics

  2. Vehicle to grid/ Grid to vehicle

    This is a Bidirectional system where power can be transferred from both sides. The battery can transfer energy back to the grid through the inverter and the grid can supply energy to the battery through a rectifier. By implementing V2G technology, the supply grid operation can be improved and there will not be a high demand for EV loads from the supply and distribution grids, hence bypassing the expenditure for upgrading the existing grid infrastructure [6]. The inverter circuit that can convert DC to AC power is shown in fig 1.4 and the output in fig 1.5. The rectifier circuit that can convert AC to DC power is shown in fig 1.6 and the output in fig 1.7. The SPWM control that was used to generate gate pulses for the switches is shown in fig 1.8 and the output in fig 1.9. The Mathematical equations that were used to simulate these systems are mentioned below.

    Inverter filter design

    Rectifier filter design

    fig 1.4 – Inverter model

    fig 1.5 – Inverter output

    fig 1.6 – Rectifier model

    fig 1.7 – Rectifier output

    fig 1.8 – SPWM model

    fig 1.9 – SPWM output

    SPWM control

  3. Passive cell Balancing

    There will be a combination of both weak and strong cells in the pack and there is a high chance that the weaker cells are over discharged and the stronger cells are overcharged. In this method, a resistor with a switch is connected across the cell where if in case the stronger cell happens to achieve more potential than the weaker cell, the excess energy from the stronger cell is wasted in the resistor. The transistor switch enables quick control over the working of the resistor at the right time.

    SOC(i)>SOC(j) then connects resistor R(i) via MOSFET

    fig 2.0 – Passive cell balancing circuit

  4. Active cell Balancing

In this method, the concept of a strong and a weak cell remains the same as the passive cell balancing method but the technique is improved. Here, the circuit tries to have more control over the cell voltages and extracts the full capacity of the cells to use. The excess energy in a stronger cell can be transferred into a weaker cell with less energy by means of different components like inductors or capacitors. The two main varieties are shown below.

fig 2.1 – Active cell balancing circuit

(Multiple winding type)

2.2 – Active cell balancing output (Multiple winding type)

fig 2.3 – Active cell balancing circuit

fig 2.4 – Capacitor current

fig 2.5 – SOC cell 1

fig 2.6 – SOC cell 2

The SOC of Cell 1 was set to 100% and the SOC of Cell 2 was set to 60%. Here, Cell 1 starts charging the capacitor and as the capacitor is fully charged, it starts sending that energy into Cell 2 hence balancing the energy in both the cells. A comparison was made between Active and Passive balancing techniques and the following results were found (fig 2.7).

fig 2.7: SOC Table


Electric vehicle Battery packs were considered to have voltage and energy imbalances in them that were hurting the overall performance of the pack. This issue was solved using Cell balancing techniques namely Passive balancing and Active Balancing. This made sure that the cells in the battery pack did not over-charge and maintained a balance of voltages throughout every cell hence, increasing the overall capacity of the Battery. The Balancing circuits were designed and simulated in MATLAB and the results were obtained in a graphical manner. A comparative study was done and it was found that the Active balancing technique was a better me of accounting for the imbalances in the cells compared to the passive balancing technique as it is more efficient with the energy management between the cells.


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