Lithium-ion batteries are most afraid of over-charging and over-discharging in use.
Voltage characteristics of batteries in different materials
Lithium iron phosphate(LiFePO) series: Factory standard charging cut-off voltage ≤3.85V, discharge cut-off voltage≥2.5V
Nickel, Cobalt, Maganese(NCM) series:Cut-off voltage ≤4.2V, discharge cut-off voltage ≥2.7V
Lithium manganate(LMO) series:Cut-off voltage ≤4.2V, discharge cut-off voltage ≥2.7V
Term:
Over-charge: The charging voltage exceeds the upper limit voltage.
Over-discharge: The discharge cut-off voltage is lower than the lower limit voltage.
What are the consequences of lithium-ion battery over-charge and over-discharge?
Over-charge: A large amount of gas will be generated in the battery, which causes the internal pressure to rise rapidly, resulting in the battery explosion.
Over-discharge: reduced capacity, shortened battery life, direct damage lead to the battery scrap.
When the lithium battery is used in PACK, it is more likely to over-charge and over-discharge, which is caused by the consistency difference of the cell. If the charging and discharging process is not properly controlled, it will be further increased, resulting in the phenomenon of over-charging and over-discharging of part of the cell.
Lithium batteries are great, but they need protection. In order to ensure the safety of use, there are many requirements:
Basic protection requirements: over-charge protection, over-discharge protection.
Strengthen protection requirements: over-current protection, high-temperature protection, low-temperature protection, short circuit protection, reverse protection.
Expansion requirements: good consistency, small dropout voltage, small temperature difference.
Because of the material characteristics of the lithium battery itself, it can not be over-charge, over-discharge, over-current, short-circuit and ultra high or low temperature charge and discharge, so the application of lithium battery always needs a protection circuit.
After solving the security problem, according to different use environments and use occasions, the customer put forward many additional functional requirements:
In order to meet different needs, there are many types of lithium battery protection products.
Hardware-type protection board:
Use special lithium battery protection chip, when the battery voltage reaches the upper limit or lower limit, the control switch device MOS tube cut off the charging circuit or discharging circuit, to achieve the purpose of protecting the battery pack.
Characteristics:
1. Only over-charge and over-discharge protection can be realized. Partial can realize over current protection, short circuit protection, over temperature protection, and reverse connection protection. None of the other additional functions can be satisfied.
2. Protection threshold cannot be changed (general protection point is 3.9V and 2.0V)
3. The balance threshold cannot be changed (general balance current below 150mA)
Classification:
Hardware-type protection board can be divided into:
Separate port protection board – Totally three ports: Charging port, discharge port, common terminal
Common port(Same port) protection board – Totally two ports: Positive and negative port
Below is the physical appearance of the Hardware-type protection board:
Charging and discharging circuit is different, i.e., charging and discharging port are separated.
Charging and discharging circuit is the same circuit, i.e., charging and discharging share a same port.
Why do we need to distinguish between common and separate port?
1. For customers, generally do not care about the separate port or common port. All we have to do is provide them with positive and negative battery electrodes. But some customers’ charge port and discharge port share a common port, some customers require the charging port and discharge port can be separated wiring.
2. The over-current capacity of the protective board is determined by the over-current capacity and quantity of the MOS tube. The MOS tube accounts for most of the cost of the protective board. Generally speaking, the charging current is smaller and the discharge current is larger. For this application requirement, we designed a separate port protection board to reduce the cost.
Some knowledge about the separate port protection board and common port protection board:
1. The control mode of the separate port protection board and the common port protection board is the same;
2. The over-current capacity of the protection board is determined by the over-current capacity and quantity of the MOS tube;
3. The charging MOS tube is connected in series with the discharging MOS tube in the case of common port and half-separate port; The full separate port is parallel;
4. The number of charging MOS tubes is less than or equal to the number of discharging MOS tubes;
5. The number of charging MOS tubes of the common port protection board is equal to the number of discharging MOS tubes, and the total number of MOS tubes must be even number.
6. The number of charging MOS tubes of separate port protection boards is generally less than discharging MOS tubes.
7. The common port protection board can be used directly as the separate port protection board, choosing common port protection board when the charging current larger than the discharge current;
8. In principle, the separate port can not be used as a common port, either charging MOS tube overload and over-current, or discharging MOS tube waste;
9. The MOS tube of the protection board is relatively expensive, in the final analysis, the purpose of the separate protection board is to make reasonable use of the MOS tube flow capacity, not waste and save money.
The basic principle:
1. Some customers who understand the principle and cost of the protection board will specify the use of separate or common port, in which case the customer has the final say.
2. If the charging current is low, the discharge current is high. For example, charge 5A, discharge 20A. Separate port is recommended. (1 MOS tube for charging and 4 MOS tubes for discharge)
3. If the charge current is similar to the discharge current, or the charge current is larger than the discharge current. Common port board is recommended. (There are as many charging MOS as discharging MOS)
Single section general protection board schematic diagram (typical)
U1:control IC; All functions of the protection board are realized by IC monitoring the voltage difference between VDD-VSS and VM-VSS and controlling C-MOS to perform switching actions.
Cout:overcharge control terminal; The switch of the MOS tube is controlled by T2 grid voltage of MOS tube.
Dout: over discharge, over current, short circuit control terminal; The switch of the MOS tube is controlled by the T1 grid voltage of the MOS tube.
VM (V-) : over current and short circuit protection voltage detection terminal; The over current and short circuit protection can be realized by detecting the voltage of VM terminal.（U（VM）=I*R（MOSFET））
When the protection board is normal, Vdd is high level, Vss and VM are low level, and Dout and Cout are high level. When any parameter of Vdd, Vss and VM is transformed, the level of Dout or Cout will change. At this time, MOSFET executes the corresponding action (open or close the circuit), so as to realize the protection and recovery function of the circuit.
Protective board schematic diagram (simplified diagram)
1: General state
When the battery voltage is above the over-discharge detection voltage (above 2.75V) and below the over-charge detection voltage (below 4.3V), the voltage of the VM terminal is above the charger detection voltage, and when the over-current/detection voltage is below the OV, IC controls the MOS tube by monitoring the voltage difference between VDD-VSS and VM-VSS. The DO and CO terminals are high level, and the MOS tube is in the on-off state. At this time, it can charge and discharge freely.
As shown in the picture below, IC is powered by a battery cell and can work reliably at a voltage of 2v-5v.
2: Over-charge protection and recovery
When the battery is charged to the voltage exceeds the set value VC(4.25-4.35V, the specific overcharge protection voltage depends on IC), VD1 reverses so that Cout becomes low level, T1 cutoff, and charging stops. When the battery voltage falls back to VCR(3.8-4.1V, the specific recovery voltage of overcharge protection depends on IC), Cout becomes high level, T1 switches on charging to continue, and VCR must be less than VC with a fixed value to prevent frequent jump.
In addition, adding load discharge at the end of the output to the battery voltage less than the over-charge protection voltage can be also recovered.
3: Over-discharge protection and recovery
When the battery voltage is reduced to the set value VD (2.3-2.5V, the specific overcharge protection voltage depends on IC) due to discharge, VD2 reverses, and after a short time delay fixed in IC, Dout becomes low level, T2 is cut off, and discharge stops.
Remove the load or when the battery is placed on charge, the interior or door is turned over so that T2 is switched on again in preparation for the next discharge.
4: Discharge over-current, short circuit protection and recovery
When the circuit discharge current exceeds the set value or the output is short-circuited, the over-current, short-circuit detection circuit action, so that the MOS tube (T2) off, current cut-off.
Remove the load or when the
