Industry Solutions | Automotive Battery Health Detection Solutions

1. Industry Background

With the popularization of new energy vehicles, battery safety and performance have received widespread attention in the industry. Battery health (State of Health, SOH) is an important indicator used to measure the performance and life status of lithium batteries. It usually represents the ratio of the current battery capacity to the new battery capacity, reflecting the capacity attenuation of the battery during use.

100% means that the battery is completely healthy, with optimal capacity and performance. If a battery’s SOH is 80%, it means that the battery’s available capacity is equivalent to 80% of its initial design capacity. The battery’s performance and endurance have significantly decreased and should be replaced if necessary.

2. How to avoid battery health degradation

● Number of charge and discharge cycles: As the number of charge and discharge cycles increases, the active materials of the lithium battery will gradually age, resulting in capacity decay and increased internal resistance, thereby reducing SOH. ●
Depth of charge and discharge: Deep charge and discharge will accelerate battery aging, while shallow charge and discharge will help extend battery life and improve SOH.
● Battery operating temperature: Too high or too low temperature will have an adverse effect on the performance of lithium batteries. High temperature will accelerate the chemical reaction inside the battery, resulting in capacity decay and shortened life; low temperature will reduce the battery’s charge and discharge performance.

● Charge and discharge rate: Fast charge and discharge will generate greater heat and stress inside the battery, accelerate battery aging and reduce SOH; while slow charge and discharge is relatively gentle and causes less damage to the battery.

3. Battery capacity calculation

The capacity is determined by the chemical properties of the battery cell, so how do we determine the battery capacity decay? We can measure the battery capacity decay degree in other ways.

Calculation of the whole package capacity:

The battery nameplate of new energy vehicles (Figure 3.1) will have nominal voltage and battery capacity. The nominal voltage refers to the voltage value of the battery pack in normal operation, and the unit is Ford (V); the battery capacity refers to the amount of electricity discharged by the battery under certain conditions (discharge rate, temperature, termination voltage, etc.), and the unit is ampere-hour (AH). 1AH means that the battery can last for 1 hour when discharged at a current of 1A.

Figure 3.1 New energy vehicle battery nameplate

Knowing the nominal voltage and battery capacity of the battery pack, the battery capacity (unit: WH) can be calculated. The capacity represents the storage capacity of the battery pack. The formula is as follows:

Battery charge = nominal voltage × battery capacity

The voltage of the battery pack in Figure 3.1 is 367.7V, and the battery capacity is 174AH. The power of the battery pack is: 367.7*174=63979.9wh.

When the battery is in the car, a fast charging pile can be used to charge the car from 0% SOC to 100% SOC. The ratio of the actual charging capacity to the rated capacity is the ratio of the actual capacity of the battery to the rated capacity. This method can be used to estimate the capacity attenuation of the entire pack.

There are two disadvantages to calculating battery capacity in this way. First, the estimated value is inaccurate and cannot be used as a standard basis for battery maintenance. Second, if the battery is removed from the car, it is not convenient to calculate in this way.

For a whole pack of batteries that has been removed from the car, the battery pack charge and discharge maintenance instrument (Figure 3.2) can be used to measure the capacity of the whole pack of batteries.

Figure 3.2 Battery pack charge and discharge maintenance instrument

The whole pack charge and discharge test can intuitively show the real capacity of the whole pack of batteries:

Our equipment can set process parameters. Click “Add” to add a work step. The four steps can be set as a set of parameters. Turning on maintenance can automatically complete the four steps of capacity calculation, which is very convenient.

The cumulative charge and discharge capacity can be viewed on the operation data interface, and the data can be exported by inserting a USB flash drive.

Battery pack capacity calculation:

The battery pack is composed of multiple single cells, and its nominal voltage and battery capacity are closely related to the arrangement of the single cells. Series connection increases voltage, while parallel connection increases capacity.

Series boost

Series boost: Multiple single cells are connected in series, only the voltage is increased, and the capacity remains unchanged. For example: three 3.7V 30AH cells are connected in series, the battery pack voltage is: 3.7*3＝11.1V, the capacity remains 30AH, and the battery pack has a voltage of 11.1V and a capacity of 30AH.

Parallel expansion

Parallel capacity increase: multiple single cells are connected in parallel, only the capacity is increased, and the voltage remains unchanged. For example: three 3.7V 30AH cells are connected in parallel, the battery pack capacity is: 30*3＝90AH, the voltage remains unchanged at 3.7V, and the battery pack has a voltage of 3.7V and a capacity of 90AH.

The nominal voltage and battery capacity of the battery pack can be calculated according to the above method, and the module charge and discharge maintenance instrument (Figure 3.3) can be used to measure the battery pack capacity.

Figure 3.3 Module charge and discharge maintenance instrument

The first step is to charge the battery pack to the cut-off voltage (number of cells × maximum cut-off voltage of the single cell);

Step 2: Rest for 10 minutes;

The third step is to discharge the battery pack to the cut-off voltage (number of cells × lowest cut-off voltage of the single cell);

Step 4: Charge the battery pack to the cut-off voltage;

Compare the accumulated capacity after full charge and discharge with the actual capacity of the battery pack to determine the degree of capacity attenuation of the battery pack.

Single battery capacity calculation:

If the type of single cell and battery capacity are known, the single cell charge and discharge maintenance instrument (Figure 3.4) can be used to calculate the single cell capacity.

Figure 3.4 Single-cell charge and discharge maintenance instrument

The first step is to charge the single battery to the cut-off voltage;

Step 2: Rest for 10 minutes;

The third step is to discharge the single battery to the cut-off voltage;

Step 4: Charge the single battery to the cut-off voltage;

Compare the accumulated capacity after full charge and discharge with the actual capacity of the single battery to determine the degree of capacity attenuation of the single battery.

You can also use a DC impedance tester (Figure 3.5) to estimate the capacity decay of a single cell. The internal resistance of a lithium battery will increase with the number of uses and working hours. The internal resistance of a battery is usually less than 20 milliohms in a new state. After more than 300 cycles, the internal resistance will rise to more than 30 milliohms, at which point the battery capacity is poor.

Figure 3.5 DC impedance tester