Batteries in Series and/or parallel-connected battery pack

Batteries in Series and/or parallel-connected battery pack Simulation of a single battery may be complicated; simulation of a battery pack of multiple batteries is more difficult and tedious.

Most studies have focused on a single battery, very little were done on multiple batteries, especially lithium-ion batteries.

Battery packs of multi-batteries supply high voltage when batteries are connected in series and high capacity when connected in parallel.Generally in battery packs of parallel connection, batteries of similar parameters (type, resistance, and capacity) are chosen.Gan and Takeuchi have shown that a battery of high discharge capability with a battery of large capacity in parallel has better discharge performance and better discharge efficiency than any of the single battery Numerical simulation for the discharge behaviors of batteries in seriesand/or parallel-connected battery packMao-Sung Wu , Chang-Yen Lin , Yung-Yun Wang , Chi-Chao Wan , C.R. Yang A calculation method has been proposed based on a semi-experimental equationDischarge data of different types of single lithium-ion battery has been used as inputDischarge behavior of the entire battery pack composed of several different-type batteries in series and/or in parallel may be calculatedEvery single battery in a pack may be monitored.

Discharge curve of lithium ion batteries of 750 mAh, 1300 mAh, 1900 mAh and 2000 mAh at different discharge currents have been collected Voltage variation prediction in constant current discharge can be madeA new discharge curve at another current can be predicted which is helpful for predicting the behavior similar batteries in series and parallel.

Similar batteries in series will have the same current of the entire system(Battery Pack)System voltage is the sum of each individual voltagesIn parallel connection the systems current is equally distributed to each battery for similar type of batteries.Practically parallel connection is more complex for batteries of different discharge capacities.

A iteration method is used. For two batteries for example A and B in parallel IB(t)=Isystem(t)-IA(t)

After IA(t) is preset IB(t) can be calculated to obtain their corresponding voltage. This iteration continues with a view to getting same voltage across both the batteries.Binary search in data structures is common for picking correct current value.Convergence condition is set at |VA-VB|= IA(t)+IB(t) as IC(t) cannot be to extreme(Isystem(t) or zero) Parallel connection between A and B has been considered and discharge curves between Isystem(t) and zero have been extracted.A and B is considered as a new battery D. Now Isystem(t)=IC(t) + ID(t). Current for C and D are calculatedThen current for A and B are calculated from ID(t).A method for the estimation of the battery pack state of charge basedon in-pack cells uniformity analysisLiang Zhong, Chenbin Zhang, Yao He, Zonghai ChenDifference among the cells and impact of balance control have been consideredSeries and parallel connection are studiedUnsccented particle filter algorithm has been used Based on the model of the first overcharged cell and that of the first over-discharged cell, the estimation of the SOC of a battery pack is realizedExperimentally verified that an accurate estimation value can be obtainedan accurate estimation value can be obtainedA method for the estimation of the battery pack state of charge based on in-pack cells uniformity analysisLiang Zhong, Chenbin Zhang, Yao He, Zonghai ChenThe in-pack cell with the lowest available capacity will determine the available capacity of the entire packThe charging of the pack will stop when the in-pack cell with the highest available capacity is fullThe variations in the individual cells are primarily the cell total capacity, internal resistance and the initial value of the SOCA method for the estimation of the battery pack state of charge based on in-pack cells uniformity analysisLiang Zhong, Chenbin Zhang, Yao He, Zonghai ChenA battery management system (BMS) with a balance control unit addresses the uneven cell problemIn this study, the relationship between the pack parameters (SOC and total capacity) and the parameters of the in-pack cells with consideration of the different balance control strategies has been establishedA method for the estimation of the battery pack state of charge based on in-pack cells uniformity analysisLiang Zhong, Chenbin Zhang, Yao He, Zonghai ChenThe SOC of B1 and B2 are determined by the UPF algorithm with their corresponding battery models, and then the pack SOC can be calculated based on their relationship.B1=First Over discharged cellB2=First over charged cellDefinition of the pack SOC is first describedThe relationship between the parameters of the pack and those of the in-pack cells has been analyzedA method for the estimation of the battery pack state of charge based on in-pack cells uniformity analysisLiang Zhong, Chenbin Zhang, Yao He, Zonghai ChenCommon Battery model for B1 and B2The UPF algorithm is applied to estimate the SOC of B1 and B2Using the parameters of B1 and B2, the pack SOC is calculatedSOC of a single cell

A method for the estimation of the battery pack state of charge based on in-pack cells uniformity analysisLiang Zhong, Chenbin Zhang, Yao He, Zonghai ChenThe relationship between battery units and the pack can be converted into the relationship between the in-pack cells and the pack

A method for the estimation of the battery pack state of charge based on in-pack cells uniformity analysisLiang Zhong, Chenbin Zhang, Yao He, Zonghai ChenSeries ConnectionThe SOC and total capacity of each cell are denoted by SOCi and CiWithout Balance Controla)The battery pack capacity is the sum of the minimum cell capacity that can be charged and the minimum remaining cell capacity that can be dischargedA method for the estimation of the battery pack state of charge based on in-pack cells uniformity analysisLiang Zhong, Chenbin Zhang, Yao He, Zonghai Chenb) The total capacity of the pack

c) SOC of the Pack

A method for the estimation of the battery pack state of charge based on in-pack cells uniformity analysisLiang Zhong, Chenbin Zhang, Yao He, Zonghai Chenc) The minimum remaining cell capacity in the fully charged pack as cmin, the corresponding cell SOC as SOCcell and the total capacity of the cell as Ccell The SOC of the pack is

A method for the estimation of the battery pack state of charge based on in-pack cells uniformity analysisLiang Zhong, Chenbin Zhang, Yao He, Zonghai Chen Passive Balance ControlPack total capacity

The pack SOC with passive control is

A method for the estimation of the battery pack state of charge based on in-pack cells uniformity analysisLiang Zhong, Chenbin Zhang, Yao He, Zonghai ChenActive Balance ControlCharge is transferred between cells to achieve the balanceb) Total capacity of the pack is the mean capacity of the cells

c) SOC of the pack is

A method for the estimation of the battery pack state of charge based on in-pack cells uniformity analysisLiang Zhong, Chenbin Zhang, Yao He, Zonghai ChenParallel ConnectionEach branch can be regarded to be a one unit, with the SOC and total capacity of each branch denoted by SOCpack _i and Cpack_iTotal Capacity of the pack is SOC of the pack is

A method for the estimation of the battery pack state of charge based on in-pack cells uniformity analysisLiang Zhong, Chenbin Zhang, Yao He, Zonghai Chen Cell with the minimum chargeable capacity is first to be over-charged(B2) Minimum remaining capacity is first to be over-discharged(B1)pack SOC can be simplified by the parameters of B1 and B2

Passive Balance: cell with the minimum total capacity is first to be over-discharged, so the pack SOC can be expressed as the SOC of B1

Active Balance: The pack SOC can be the SOC of any one of the cells SOC in the pack. To reduce the estimation error, the pack SOC is represented by the mean SOC of B1 and B2

A method for the estimation of the battery pack state of charge based on in-pack cells uniformity analysisLiang Zhong, Chenbin Zhang, Yao He, Zonghai Chen SOC and total capacity of each branch can be calculated as mentioned below.

SOC and total capacity of each branch will lead to the calculation of SOC and capacity of total parallel pack.

A method for the estimation of the battery pack state of charge based on in-pack cells uniformity analysisLiang Zhong, Chenbin Zhang, Yao He, Zonghai ChenLithium Ion Battery ModelSingle Battery Model is

To estimate the SOC of B1 and B2, their respective models are constructedLinear state space equations are derived through discretization

A method for the estimation of the battery pack state of charge based on in-pack cells uniformity analysisLiang Zhong, Chenbin Zhang, Yao He, Zonghai ChenUPF algorithm has been used to estimate the single cells SOC and the pack SOC based on the non-linear state space equations of B1 and B2.Simulation and Experimental resultsTo analyze the impact of the cell-to-cell variation on the estimation of the pack SOCTo verify the applicability of the algorithm based on UPF The pack is composed of IPF1865140 type cells in series with a nominal capacity of 10 Ah. The cells are with a nominal voltage of 3.2 V

Battery Model Identification and validation has been done i.e. parameters of the battery model were determined experimentally and comparedAnalysis of the UPF algorithm and the estimation of the pack SOC the battery pack was first discharged at C1 rate for approximately 20 min and then operated for approximately 10 cyclesThe error of the estimation of a single battery SOC based on the UPF algorithm is 1.0779%, while the error based on the UKF algorithm is 2.803%Cell SOC Estimation Curve for two methods

C. With the choice of the appropriate battery model, the SOC of the first over-discharged battery B1 and the first over-charged battery B2 could be obtained by using the UPF algorithm D. For the case when the initial value of the SOC of each in-pack cell is different from each other, the SOC estimation of the pack was analyzed. Assume the SOC of B1 is 50% and B2 is 55%.SOC estimation error of B1 and B2 were 1.0789% and 1.3903% respectivelyCell SOC Estimation with different initial value

SOC Estimation of a battery Pack with inconsistency of the initial SOC value among in-pack cells

SOC Estimation of a battery Pack with inconsistency of the initial SOC value among in-pack cellsErrors for different types balance are calculated

SOC Estimation of cells with different internal resistance and same Initial SOC for cells

The SOC estimation of the cells with different total capacity and their SOC comparisonAssumption(B1-50% and B2-55%)

The SOC estimation of the battery pack with inconsistency of the in-packcell total capacity

ConclusionThe pack SOC estimation was transformed into the estimation of the parameters of the first over-charged cell and the first over-discharged cell. Applying the UPF algorithm to the single cellmodel, the SOC of the single cell was obtained, and thus the pack SOC was calculatedParallel connection was not studied