18650 battery 3.7v 1800mah management circuit design ideas

　　Portable power supplies such as mobile power supplies, electric vehicle power supplies, car power supplies, etc. are composed of battery units. Due to the inherent errors of battery products, in order to balance this error and make the 18650 battery 3.7v 1800mah work better, BMS management design is required. Then How to design the management circuit of the 18650 battery 3.7v 1800mah?

　　Overall structure of 18650 battery 3.7v 1800mah management system

　　The designed application entity is a portable 18650 battery 3.7v 1800mah device used in industry, using Altera's FPGA and NIOS on it

　　II embedded processor, and uses a USB interface to connect to the computer, targeting large data volume applications. This device requires 30V DC voltage, so it is planned to use a 18650 battery 3.7v 1800mah of four 1000mAh lithium polymer batteries connected in series; in addition, for waterproof and dustproof considerations, only a square USB interface (USBB

　　TypeSockr, this USB port has both data transmission and charging functions.

　　The control core includes FPGA and its connected interfaces and display circuits. It requires a low voltage of 3.3V, which is obtained directly from the 4-cell lithium 18650 battery 3.7v 1800mah by a high-efficiency DC/DC chip. This voltage is very important, so it needs to be kept stable and continuous. Unless the 18650 battery 3.7v 1800mah is low in power or has overcurrent protection, this voltage is always supplied.

　　The actuator requires 30V DC voltage and a current of about 80mA. It uses a boost DC/DC circuit. This circuit is controlled by the control core. It does not work normally and is only turned on before action is required.

　　Charging uses an external 20V power supply and is connected through the USB interface. The consideration for using this power supply is for high-current high-speed charging of 1C or 0.5C. Since it shares the same port with ordinary USB, in order to avoid entering the charging process when connecting to ordinary USB, a voltage judgment circuit is needed for judgment.

　　Since it is difficult to find a suitable chip solution on the market, it was decided to use the remaining logic resources of the FPGA to implement the charger's control function and add a small amount of analog circuitry to assist. This requires that the power supply to the control circuit cannot be interrupted, the 18650 battery 3.7v 1800mah must always be online, and the negative terminal of the battery must always be connected to GND.

　　1. Voltage sampling

　　The most important part is the design of the voltage sampling circuit, which requires high accuracy and is less affected by temperature. The difficulty in this design is that the battery voltage is floating with respect to GND. Many solutions use a differential operational amplifier to convert the voltage to ground and then input it into a dedicated ADC for AD conversion. However, this solution caused many problems due to the introduction of a differential operational amplifier. First of all, the voltage is relatively high and the op amp is difficult to find; secondly, the op amp's power supply and the input voltage use the same power supply, which requires the op amp to need a rail-to-rail input function; thirdly, a negative power supply may be needed. Using DC/DC introduces noise; in addition, the op amp and the use of matched resistors reduce the accuracy.

　　RC charging circuit

　　In order to simplify the circuit as much as possible, an integral ADC is constructed here to convert the high precision of FPGA timing into the high precision of voltage measurement. The working process is: J1 is closed first, releasing the charge on C1; then J1 is opened, and R1 charges C1; the voltage comparator U1 compares the voltage on C1 with the reference voltage V2, and outputs a high voltage when the voltage of C1 exceeds V2. flat. By counting the time from when J1 is turned on to when U1 outputs a high level, the voltage of V1 can be determined. It can be intuitively seen that the higher the V1, the shorter this period of time.

　　Actual sampling circuit diagram

　　The actual circuit is shown in Figure 3. Note that this picture only shows the measurement circuit of the first battery. Among them, R1 and C1 are the resistors and capacitors used for integration, Q1 is a commonly used P-MOSFET, which is used to realize the function of J1 to discharge the capacitor, and U5

　　At the same time, it realizes the dual functions of voltage reference and voltage comparator. X1 is the discharge control, coming from the FPGA, and X2 is the switching output, going to the FPGA.

　　This circuit only consumes the 4uA current of MAX921 and the leakage current of C1, Q1, and Q2 in the static state, which is basically negligible and is very power-saving.

　　Another feature of this circuit is that it eliminates the commonly used photocoupler and uses capacitor C2 instead. When in static state, the voltage at both ends of C2 is balanced and no power is consumed. At this time, the voltage of X2 is 0. When U5 outputs a high level, the voltage of X2 is increased because the voltage across C2 cannot transient. The two Schottky diodes D1 and D2 play a limiting role. Carefully adjusting the values of C2 and R4 can successfully transmit switching information.

　　2. Balanced charging

　　Balance charging is a charging method required for all lithium 18650 battery 3.7v 1800mahs, but many low-power applications do not actually have balanced charging, such as most laptop 18650 battery 3.7v 1800mahs. This actually has a considerable impact on battery life.

　　Existing balancing technologies are mainly divided into energy transfer balancing between batteries and external energy input balancing. Energy balancing between batteries is to charge the energy of the high-power battery to the low-power battery. The biggest problem with this method is that it is very complicated to control.

　　Many special-purpose chips or microcontroller solutions now use external equalization, which is achieved through controllable energy consumption. In this method, an energy-consuming component is generally used to consume energy, thereby waiting for other battery cells to be fully charged or reducing the voltage of some cells. The disadvantage of this solution is that the energy consumption on the Zener diode is too large, and the heat generated is intolerable.

　　Actual charging method diagram

　　Of course, this is just a schematic diagram and does not include the current detection circuit (between input to transformer) and voltage detection circuit (transformer secondary winding). Among them, the switch array is implemented with power MOSFET.

　　In this way, the tubes all work in the switching state and consume very little energy. In addition, the battery does not have a diode in series, so the maximum output can be obtained. The disadvantage is that the circuit is relatively complicated. Since the voltage of each battery must be matched, the input charging circuit is required to be isolated. The T1 transformer is used as isolation here because the switching frequency can be made very high and the size of the T1 transformer is small. The entire charging circuit works in a switching state without adding any control modules. The FPGA directly controls the field effect transistor. The outputs of the current detection and voltage detection circuits are also converted into switching values and directly transmitted to the FPGA.

　　Charging is divided into four steps:

　　a) Check whether any battery cells are lower than 2.5V. If so, use a 5% duty cycle to charge the batteries lower than 2.5V in turn to boost the voltage to 2.5V;

　　b) Open J1 and J8, charge the whole unit with high current, and measure the voltage of the battery cells at the same time. If any battery cell reaches 4.2V, go to the next step;

　　c) Gradually reduce the duty cycle to maintain the maximum voltage of the single cell at 4.2V until the duty cycle is <5%;

　　d) Charge the batteries that have not reached 4.2V in turn. When the duty cycle drops to 5%, the charging ends.

　　What needs to be noted here is that charging in turns in steps a) and d) is implemented through a switch matrix, and charging in turns will not extend the charging time. This is because the duty cycle at this time is far less than 25%, and it can be used in a Charge the four batteries separately during the charging cycle.

　　3. Overcurrent and low voltage protection

　　In order to ensure the absolute safety of the 18650 battery 3.7v 1800mah, the over-current and low-voltage protection of the 18650 battery 3.7v 1800mah are set independently. When a problem occurs, the output of the 18650 battery 3.7v 1800mah can be directly cut off. This type of circuit is also very common.

Read recommendations:

LR20

Prospects for the Development of Lithium Batteries

Introduction to Lithium Battery

18650 battery 3.7v 2200mah

CR2032 button cell battery