It can be said that the lithium battery IC is the patron saint of the battery; in recent years, more and more products have rapidly adopted lithium batteries as their main power supply, nothing more than: small size, high energy density, no memory effect, cycle life High, high-voltage batteries, low self-discharge rate, etc., are also different from nickel-cadmium and nickel-metal hydride batteries, so the safety of charging and discharging must be considered to ensure the prevention of characteristic degradation, but also because of this, for lithium The protection of overcharge, overdischarge, overcurrent and short circuit current of the battery is more important, so the protection circuit is usually designed in the battery pack to protect the lithium battery, which shows the importance of the lithium battery protection IC.
Lithium-ion batteries have high energy density, making it difficult to ensure battery safety. Specifically, in an overcharged state, when the temperature of the battery rises, there will be excess energy, and the electrolyte solution will decompose to generate gas, which may cause ignition or rupture due to the increase in internal pressure. Conversely, in an over-discharged state, the electrolyte solution decomposes, resulting in deterioration of battery characteristics and durability (that is, a reduction in the number of charging cycles). The protection circuit of lithium-ion batteries is to ensure safety during such overcharge and discharge states, and to prevent deterioration of characteristics.Also read:How to extinguish lithium battery fire
Requirements for IC performance of lithium battery protection board
① High precision of overcharge protection When the lithium-ion battery is in an overcharged state, in order to prevent the internal pressure from rising due to temperature rise, the charging state must be cut off. The protection IC will detect the battery voltage, and when overcharged is detected, then The power MOSFET for overcharge detection is turned off to stop charging. At this time, attention should be paid to the high accuracy of the overcharge detection voltage. When charging the battery, it is a matter of great concern to the user to charge the battery to a full state. At the same time, taking into account safety issues, it is necessary to cut off the charging state when the allowable voltage is reached. To meet these two conditions at the same time, a high-precision detector must be provided. The current accuracy of the detector is 25mv, and the accuracy will need to be further improved.
②Reduce the power consumption of the protection IC With the increase of the use time, the voltage of the charged lithium-ion battery will gradually decrease, and finally it will be lower than the standard value. At this time, it needs to be charged again. If it is not charged and continues to be used, it may The battery cannot continue to be used due to over-discharge. In order to prevent over-discharge, the protection IC must detect the battery voltage. Once it reaches the over-discharge detection voltage, the power MOSFET on the discharge side must be turned off to stop the discharge. But at this time the battery itself There is still natural discharge and the current consumption of the protection IC, so it is necessary to minimize the current consumption of the protection IC.
③ Over-current/short-circuit protection requires low detection voltage and high precision requirements. When short-circuit occurs due to unknown reasons, the discharge must be stopped immediately. Over-current detection uses the Rds(on) of the power MOSFET as the inductive impedance to monitor its voltage drop , if the voltage at this time is higher than the overcurrent detection voltage, the discharge will stop. In order to make the Rds(on) of the power MOSFET effective in the charging current and discharging current, it is necessary to make the impedance value as low as possible. Currently, the impedance is about 20mQ~30mQ, so the overcurrent detection voltage can be lower.
④ High voltage resistance When the battery pack is connected to the charger, a high voltage will be generated instantaneously, so the protection IC should meet the high voltage resistance requirements.
⑤Low battery power consumption When in the protection state, its static current consumption must be less than 0.luA. ⑥Zero-volt rechargeable Some batteries may have a voltage as low as 0V during storage due to long-term or abnormal reasons. Therefore, the protection IC needs to be able to charge even at 0V.