Principle, function and characteristic analysis of lithium battery protection circuit


In recent years, more and more products, such as PDAs, digital cameras, mobile phones, portable audio devices and Bluetooth devices, use lithium batteries as their main power supply. Lithium battery has the advantages of small volume, high energy density, no memory effect, long cycle life, high voltage battery and low self discharge rate. They are different from nickel cadmium and nickel hydrogen batteries. It is necessary to consider charging and discharging lithium batteries. Reliability to avoid characteristic deterioration. The maintenance of overcharge, overdischarge, overcurrent and short circuit of lithium battery is very important, so the maintenance circuit is usually designed in the battery pack to maintain the lithium battery.

Due to the high energy density of lithium-ion batteries, it is difficult to ensure the reliability of batteries. In the case of overcharge, the energy of the battery will be surplus after the temperature rises, so the electrolyte will decompose to produce gas, which may lead to spontaneous combustion or rupture due to the increase of internal pressure; On the contrary, under the condition of over discharge, the electrolyte will decompose and cause battery characteristics, and the durability will be reduced, thus reducing the number of rechargeable times.

The maintenance circuit of lithium ion battery shall ensure reliability under such overcharge and discharge conditions and avoid characteristic degradation. The maintenance circuit of lithium ion battery consists of maintenance IC and two power MOSFETs. During this period, the maintenance IC monitors the battery voltage and switches to external power MOSFET to maintain the battery in case of overcharge or discharge. The maintenance IC has too many functions. Charge maintenance, over discharge maintenance and overcurrent / short circuit maintenance.

The principle of overcharge maintenance IC is that when the external charger charges the lithium battery, the charging conditions must be stopped to avoid the increase of internal pressure due to the increase of temperature. At this time, the maintenance IC needs to detect the battery voltage. When the battery voltage reaches 4.25v (assuming that the battery overcharge point is 4.25v), it will activate overcharge maintenance, change the power MOS from on to off, and then stop charging.

In addition, it is necessary to pay attention to the fault of overcharge detection caused by noise to avoid judging as overcharge maintenance. Therefore, the delay time needs to be set, and the delay time cannot be shorter than the duration of the noise.

In the case of over discharge, the electrolyte decomposes and the battery characteristics deteriorate, resulting in a reduction in the number of charges. Using lithium battery to maintain IC can avoid over discharge and complete battery maintenance function. Principle of over discharge maintenance IC: in order to avoid over discharge of lithium battery, it is assumed that the lithium battery is connected to the load. When the voltage of the lithium battery is lower than its over discharge voltage detection point (assumed to be 2.3V), the over discharge maintenance function will be activated to turn the power MOSFET from on to off and cut off the discharge, so as to avoid the over discharge of the battery and keep the battery in the standby mode of low quiescent current. At this time, the current is only 0.1ua.

When the lithium battery is connected to the charger and the voltage of the lithium battery is higher than the over discharge voltage at this time, the over discharge maintenance function can be avoided. In addition, considering the condition of pulse discharge, the over discharge detection circuit has a delay time to avoid failure.