Some people compare the charging and discharging of batteries to a child drinking breast milk, which is very vivid:
1. If the child is allowed to drink milk all the time without parental control, the milk may be drunk up, similar to battery over-discharge;
2. If the parent does not give the child milk all the time, the milk will accumulate more and more, similar to battery overcharge;
3. If the child drinks milk in a hurry, it is easy to choke on the milk, similar to the overcurrent protection of the battery;
For lithium batteries, if the milk cannot be fed and drunk scientifically, in addition to reducing the battery life, it may sometimes cause dangerous scenarios such as battery deflagration and explosion. So how to control this situation?
Usually, there is a dedicated protection PCB board inside the lithium battery, which is packaged together with the battery unit. Under its protection, the output voltage of the lithium battery can be controlled within a safe voltage range, that is, the battery's charge and discharge termination voltage and cut-off voltage.
If the operating voltage of the battery exceeds the safe range, irreversible damage may occur inside the battery, causing battery degradation, which is reflected in the increase of the battery's internal resistance and the decrease of capacity.
▌Lithium battery protection board
The lithium battery protection board usually integrates control IC, MOS tube, resistor capacitor, fuse FUSE, etc., as shown in the figure below.
In the external port, TH is temperature detection, and a 10K NTC is connected to the negative electrode of the battery; ID is battery in-place detection, generally a 47K/10K resistor is connected to the negative electrode of the resistor, and some are 0R resistors; TH and ID are optional, not all lithium batteries have them.
▌Overcharge protection
When the battery is charging, the current flows into the positive electrode of the battery pack and flows out from the negative electrode after passing through the FUSE. The two MOS tubes at the bottom are both in the on state. As shown by the red arrow in the figure below:
When charging, the control IC X1 will always monitor the voltage between the 5th pin VDD and the 6th pin VSS. When this voltage is greater than or equal to the overcharge cut-off voltage and meets the delay time of the overcharge voltage, X1 will turn off the MOS tube Q2 by controlling the 3rd pin. After Q2 is turned off, the charging circuit is cut off (the body diode D2 of Q2 is also reverse cut-off). At this time, the battery can only discharge.
When one of the following two conditions is met, the charging protection can be released: 1. The voltage at both ends of the battery cell drops to the overcharge recovery voltage of the protection IC. 2. Add a load to the output end of the battery pack to discharge until the voltage is less than the overcharge protection voltage.
▌Over-discharge protection
When the battery outputs current to the load at both ends, the current flows according to the red arrow in the figure below.
When discharging the battery, the control chip IC X1 will detect the voltage on C1 through the 5th pin. When this voltage is less than the discharge cut-off voltage and lasts for a period of time, the control IC will control Q1 to cut off through the DO pin, and the discharge circuit is cut off at this time.
When the following conditions are met, IC X1 contacts the over-discharge protection: remove the load, charge the battery pack, and when the voltage between VM-VDD reaches the over-discharge recovery voltage value, the control IC X1 will reopen the MOS tube Q1.
▌Overcurrent/short circuit protection
Overcurrent protection detects the voltage flowing through the control MOS tube through the second pin (VM) of IC X1. If the voltage is too large and lasts for a certain period of time, the control IC will turn off Q1 and disconnect the discharge circuit. Remove the output load, and the control IC will automatically reopen Q1.
▲ Protection current is 21A lithium battery protection board
The overcurrent protection voltage VM is often 0.1 ~ 0.2V, and this value is related to the IC model.
In addition to VM, the overcurrent protection value is also related to the on-resistance of the two MOS tubes Q1 and Q2. If the on-resistance of the MOS tube is larger, the protection current value will be smaller.
For example, if the internal resistance of a MOS tube is 20mΩ and the selected control IC has an overcurrent value of 0.15V, the overcurrent protection current should be: 0.15V/(0.02*2)=3.75A.
▌FUSE protection when the control IC fails
Some protection boards will have fuses inside, which will play a secondary protection role after the control IC fails to avoid worse results, but of course will also increase costs.
