In recent years, an increasing number of products, such as PDAs, digital cameras, mobile phones, portable audio devices, and Bluetooth devices, have used lithium batteries as their main power source. Lithium batteries have the advantages of small size, high energy density, no memory effect, long cycle life, high voltage batteries, and low self discharge rate. They are different from nickel cadmium and nickel hydrogen batteries. Consider charging and discharging lithium batteries. Security to avoid feature degradation. The maintenance of overcharging, over discharging, overcurrent, and short circuits in lithium batteries is important, so maintenance circuits are usually designed in battery packs to maintain lithium batteries.
Due to the high energy density of lithium-ion batteries. In the case of overcharging, the energy of the battery will be excess as the temperature rises, causing the electrolyte to decompose and produce gas. Due to the increase in internal pressure, it may lead to spontaneous combustion or rupture; On the contrary, under over discharge conditions, the electrolyte will decompose and cause battery characteristics, and durability will decrease, thereby reducing the number of rechargeable times.
The maintenance circuit of lithium-ion batteries should ensure safety under such overcharging and discharging conditions and avoid characteristic degradation. The maintenance circuit of lithium-ion batteries consists of a maintenance IC and two power MOSFETs. During this period, it switches to an external power MOSFET to maintain the battery in case of overcharging or discharging. The maintenance IC has too many functions. Charging maintenance, over discharge maintenance, and over current/short circuit maintenance.
The principle of overcharging maintenance IC is that when the external charger charges the lithium battery, the charging conditions must be stopped to avoid internal pressure increase due to temperature rise. At this point, maintaining the IC requires detecting the battery voltage. When the battery voltage reaches 4.25V (assuming the battery overcharging point is 4.25V), it will activate overcharging maintenance, change the power MOS from open to closed, and then stop charging.
In addition, attention should be paid to faults in overcharging detection caused by noise to avoid being judged as overcharging maintenance. Therefore, a 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 decrease in charging times. Using lithium batteries to maintain ICs can avoid over discharge and complete battery maintenance functions. The principle of over discharge maintenance IC: In order to avoid over discharge of lithium batteries, it is assumed that the lithium batteries are 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 enable the power MOSFET to switch from on to off and cut off discharge, to avoid over discharge of the battery and maintain the battery in a low static current standby mode. 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 current over discharge voltage, the over discharge maintenance function can be avoided. In addition, considering the conditions of pulse discharge, the over discharge detection circuit has a delay time to avoid faults.
