Over current and short-circuit current 

　　For unknown reason (or discharge is the cathode metal by accident) cause over current or short circuit, in order to ensure safety, must immediately stop discharge. 

　　For over current protection IC principle, when the discharge current is too large, or short circuit occurs, protection IC will activate a short circuit current protection, over current at this time of testing is to power MOSFET Rds (on) as inductive impedance to monitor its voltage drop, if is higher than set by the over-current detection voltage is stop discharging, in the same way, over-current detection must also be equipped with delay time in case when the breaking current is flowing into produce false action. 

　　Usually after the over current to produce, if can remove overcurrent factors (e.g., immediately and load out), will return to its normal state, charge and discharge can be normal again. 

Polymer battery comes in 

　　Liquid lithium ion battery (Li - ion) as an efficient energy carrier is widely used in communications, electronics industry, especially on the personal communications tools such as mobile phone, PDA, after Japan's SONY commercialized in 1992 was the first to get a lot of popular, especially with the help of the communication tools, the mobile phone lithium batteries because of the light, the advantages of large capacity, environmental protection gradually accepted by the masses of users. 

　　In recent one or two years, however, with the rapid development of communication technology, mobile phone screen, MMS, bluetooth and which camera technology, the battery capacity, volume, weight, and the electrochemical properties of indicators put forward higher request, the traditional liquid lithium electricity has been more and more can't adapt to the new requirements. New type of Polymer lithium ion battery (Li - Polymer), to cater to the demand. Polymer li-ion battery is in the original steel battery shell, aluminum shell developed on the basis of the third generation of lithium ion battery, with its lighter and thinner, the characteristics of higher energy density, get the favour of communication terminal manufacturers at home and abroad and design company. Polymer lithium ion battery and lithium ion battery liquid adopted by the most fundamental difference between the two different electrolytes. Polymer lithium battery electrolyte look from the exterior to solid, known as the solid polymer electrolytes. This is a kind of in a solid state electrolyte, but could dissolve support like liquid electrolyte, polymer materials and the effect ion migration can occur. Solid and liquid lithium, luce, liquid electrolyte, because of the fluidity of liquid by the change of external environment factors such as high temperature, etc., have a greater impact than polymer lithium ion battery. 

Overcharge protection

　　Overcharge protection IC principle is: when the external charger for lithium battery, in order to prevent due to temperature rise caused by internal pressure rise, need to terminate the charging status. At this point, the protection of IC to detect the battery voltage, when arrived at 4.25 V (assuming the battery overcharge point of 4.25 V) that activate the overcharge protection, cutting the power MOSFET from open to, and stop charging. 

　　In addition, you must pay attention to because of the noise generated by the excessive charging detection misoperation, lest judge for protection. Therefore, we need to set the delay time and delay time can not be shorter than the duration of the noise. 

Cell phone lithium battery is made up of what parts and what is the function of each part

　　Cell phone lithium battery is mainly composed of plastic shell and cover of lithium batteries, protection of circuit board (PCB) and recoverable fuse polyswitch some manufacturer also configured the NTC resistance identification or vibration motor or charging circuit components. 

Each part is as follows: 

　　Lithium batteries: provide a source of charging and discharging. 

　　Worked as a short circuit protection circuit board: prevent the battery overcharge. 

　　Recoverable fuse (PTC) : high temperature protective thermistor and protection of circuit board after the actual effect of double protection. 

　　Recoverable fuse (NTC) : negative thermistor, induction battery internal temperature of low temperature protection effect. 

　　Recognition resistance: original battery is not original battery can't use. 

Excessive discharge protection 

　　In the case of excessive discharge, caused by decomposition of electrolyte battery degradation characteristics, and cause the reduce the number of charging. Using lithium battery protection IC can avoid excessive discharge phenomenon, realize the battery protection function. 

　　Excessive discharge protection IC principle: in order to prevent the excessive discharge of the lithium battery, assuming that the lithium battery connected to the load, when the lithium battery voltage is less than its excessive discharge voltage testing point (2.3 V) is assumed to activate the excessive discharge protection, make the power MOSFET by open to cut off by the discharge, in order to avoid excessive battery discharge phenomenon, and keep the battery in low static current standby mode, the current is only 0.1 mu A. 

　　When lithium battery charger is connected, and the lithium battery voltage is higher than excessive discharge voltage, excessive discharge protection function can lift. In addition, considering the pulse discharge, discharge detection circuit have delay time in order to avoid misoperation. 

Lithium battery protection circuit

　　　Due to the lithium ion battery energy density is high, so difficult to ensure the safety of the battery. Under excessive charging status, the battery temperature rise after the excess energy will then the electrolyte decomposition and produce gas, due to internal pressure increase the risk of spontaneous combustion or broken; On the other hand, in the condition of excessive discharge, electrolyte battery characteristics and durability degradation caused by decomposition, and thus reduce the number of rechargeable. 

　　Lithium ion battery protection circuit is to ensure that such excessive charging and discharging state security, and prevent the degradation characteristics. Lithium ion battery protection circuit is constitute by the protection IC and two power MOSFET, the protection of IC to monitor the battery voltage, when there is a switch to the charge and discharge as outside hang power MOSFET to protect the battery, protect the function of the IC with excessive overcharge protection, discharge protection and over current/short circuit protection.