How to design a lithium battery protection board leakage current tester based on ICL7107?

In portable applications, there is an urgent need for batteries with high capacity, small size, and light weight. At present, lithium batteries have replaced nickel-cadmium and nickel-hydrogen batteries in primary and secondary batteries. However, lithium batteries have the disadvantage of "poor tolerance to overcharging and overdischarging". Therefore, lithium batteries must be equipped with a protective circuit board to protect the lithium battery from overcharging and overdischarging. The protection board is powered by a lithium battery. The normal leakage current is 2μA~3μA. If the leakage current of the protection board exceeds 5μA, the battery storage life will be shortened, and the battery will be damaged in severe cases. Therefore, there is an urgent need for a tester to detect leakage current of a lithium battery protection board.

This study uses the integrated circuit ICL7107 to design a lithium battery protection board leakage current rapid detector.

1. Hazard analysis of excessive leakage current in protection circuit

1.1 Protection circuit analysis

The schematic diagram of the lithium battery protection circuit is shown in Figure 1. In the figure, U1 uses Ricoh R5421N151F lithium battery protection chip, and U2 uses Sanyo FTD2017 metal oxide semiconductor tube (MOST). When the lithium battery has abnormal conditions such as overcharge, overdischarge, overcurrent, short circuit, etc. during operation, U2 as a switching device will quickly cut off the circuit to ensure the safety of the lithium battery.

Figure 1 Schematic diagram of lithium battery protection circuit

1.2 The link to generate leakage current

It can be seen from Fig. 1 that the leakage current generated by the lithium battery protection circuit includes the following three steps: 1 leakage current generated by the capacitor C1; internal working current of the 2R5421N151F chip; and leakage current generated by the 3FTD2017 metal oxide semiconductor tube.

When the capacitor C1 generates excessive leakage current, or the R5421N151F chip abnormally generates excessive internal operating current, or the FTD2017 transistor GS is electrostatically breakdownd, or the PCB board copper foil is short-circuited due to the proximity, the protection circuit will be generated large leakage current.

1.3. Hazard analysis of excessive leakage current

As can be seen from Figure 1, the protective plate is attached to both ends of the battery. When the leakage current of the protection board itself is too large, on the one hand, the protection board itself cannot effectively protect the system; on the other hand, the storage life of the lithium battery is shortened, and the battery can be damaged in severe cases. According to statistics, the leakage current of the protection board is too large, accounting for about 3‰. Because the leakage current of the protection board is too large, it has a great concealment, it usually takes 3 to 6 months to be used, for example, the price of a lithium battery is 20 yuan/block and the output is 10,000 pieces/day. The battery manufacturer's loss of 600 yuan per day will result in a return of 200,000 yuan a year, and will have a negative reputation impact.

2. Protection board leakage current test principle analysis

2.1 The working principle of the tester

The tester consists of a reference circuit, a test voltage circuit, a comparison circuit, a digital display 312/microampere meter, an alarm circuit, etc., and its schematic diagram is shown in FIG.

Figure 2 Schematic diagram of the protection board leakage current tester

Reference circuit: R1, DW1 form a 2.5V low-temperature floating high-stability voltage signal source, which is divided by R2 and W1 to obtain a reference voltage of about 5mV. The reference voltage is applied to the inverting input of the comparison circuit LM358.

Test voltage circuit: This tester is designed according to the working voltage of lithium battery of 4.2V. After DW2 voltage regulation, 4.2V voltage is obtained by adjusting W2 as the protection board test voltage.

The test board test voltage is applied to the ends of the protection board through the OUT+, OUT-socket and test leads, and the leakage current of the protection board generates a voltage drop through the resistor R7:

UR7=ILeak &TImes;R7=XμA&TImes;(1&TImes;103)Ω=XmV (assuming leakage current is xμA)

The voltage drop generated by the leakage current on the resistor R7 is simultaneously filtered by the resistor R3 and the capacitor C4 and applied to the non-inverting input terminal of the LM358. When compared with the reference terminal of 5mV, when the leakage current is large, the voltage drop formed on the R7 exceeds 5mV. The voltage of the non-inverting input terminal of the LM358 exceeds the inverting input terminal, and the potential of the output terminal becomes high, driving the 9013, and the buzzer is driven to generate an audible alarm signal.

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