Mandatory tests for assembling a Lithium-ion battery pack

This article by Paladugu Chandrasekhar (CEO, Futurelite Batteries) discusses the tests involved in assembling Lithium-ion cells into modules and battery packs.

Production Flow for testing of Li-ion cells up to making modules and battery pack

Formation and Grading

Machine needed – Individual Cell Capacity testing Machine

Formation – The cells received by the pack manufacturing unit could be at different levels of charge. In this step, the cells are first fully charged and then fully discharged to figure out the actual capacity of each cell.

Using the individual cell capacity testing machine, each cell is checked for its capacity and accordingly the cells are segregated into predefined groups (grading).

About this step – This is the first step of the process as a quality check in which we can segregate cells with similar capacity. Using same capacity cells increases the efficiency of the battery pack which reduces the premature discharge and charge cut off. If one of the strings of the battery pack contains lower capacity cells, that string tends to reach full capacity earlier while charging. The same string tends to discharge to its depth of discharge quickly which will trigger the BMS to turn off the MOSFET earlier than expected Capacity/Voltage/Range.

Sorting of the cells

Machine needed – Individual cell open voltage and internal resistance (IR) testing machine

After capacity testing, the graded cells are carried into the cell open voltage and internal resistance testing machine. Here, the cells are tested at 50% charge and segregated according to measured open voltage and internal resistance parameters.

To get clear and 4 digit after decimal reading, we use Japan Made HIOKI Meter for the voltage and IR check. This machine can check 100 cells per minute.

About this step – Open voltage and internal resistance of the cell plays a vital role in balancing the current flow through a particular cell. Each cell has its own resistance (in milli-Ohms) that tends to restrict the current flow. Mixing up the cells with different internal resistance in a single battery pack leads to an imbalance of the current distribution. Also, the temperature rises more in higher resistance cells compared to lower resistance cells.

After processing, the cells are divided into 10 different groups by a robotic hand (sorting). The grouping parameters are programmable and can be predefined by the R&D Team.

Polarity check by voltmeter

Place the cells in a module and check the polarity of the module before moving onto welding.

Welding

Machine needed – Semi-Automatic and manual welding machine

After segregation of the cells into different categories, we arrange the cells in a pre-designated series and parallel combination. To join the cells together into modules, we use spot welding. (Alternatively, wire bonding can also be used to connect the cells together).

The speed and quality of spot is very important. Using proper nickel strips, advanced machinery and an experienced technician are paramount to achieving a good pack quality.

Module Testing

In this stage, we test all modules individually to measure the capacity and quality of each module.

BMS Testing

Machine Needed – BMS Tester Machine

BMS contains common circuitry as well as discrete circuitry and plays a vital role in managing the cut off of the battery pack while charging, discharging, over temperature, low temperature, short circuit conditions. This circuitry also balances the strings of the battery pack. BMS tester machine is used to check every BMS function. Around 35 such parameters are checked in this process.

Wiring | BMS and Power Wiring

Checking each string of cells in the module (before and after connecting in series) is very important.

Next, we will connect the BMS control wires into the battery pack in order. Battery pack positive terminal is directly connected to the output. BMS negative and Battery pack negative will connect to the battery pack.

Assembling

In this stage we connect the BMS, assemble the packs, check the sensing voltages and all connections by Quality Check using a multimeter. If any of the battery pack’s strings and modules show irregularities in the expected voltage, resistance or capacity range, it can be found here and rectified.

Battery Pack Testing

Machine needed – Individual Battery Capacity Testing Machine

After wiring the semi-finished battery pack, we go for capacity testing using the individual Battery Capacity Testing Machine.

Using an advanced computer software, each process will be programmed before connecting the machine with semi-finished battery pack. Here, we observe and compare the final outcome of the battery pack with what we expected from the very beginning – checking for malfunction, temperature rise in the battery pack, capacity, steady current flow, current carrying capacity of the nickel strip and power cable etc.

After this process, the battery pack will go for final completion of casing and then to finished goods area.

Final Quality Control or Double Check

Check process data for every battery pack and verify it. Final inspection also includes visual and total quality check of the pack.

Final Packing before delivering to customer

Clean the final product and pack the product in cotton case as per the instruction. Now it’s ready to dispatch.

Random checking of finished pack

At this stage, the R&D person will take the randomly selected packed products and check every parameter.

About the author

Paladugu Chandrasekhar – CEO, Futurelite Batteries

Futurelite Batteries (based in Punganur, Chittoor District, Andhra Pradesh) provides services for battery pack testing and battery pack manufacturing and is open to collaborating with start-ups in the EV domain to assist them with battery testing needs. Futurelite is also planning to start Li-ion cell manufacturing in Punganur.