Recently, projectors of professional battery manufacturer have begun to develop in the direction of miniaturization. Portability brings great convenience and is used for product introductions, temporary small meetings, etc. On this basis, a new demand has emerged, that is, the standby function, that is, a battery module is configured inside the pico projector, which can be used for about two hours for the pico projector without external power supply. The emergence of this function makes the application of the micro projector more convenient. It only needs to fully charge the battery pack in advance, and it can be used in various occasions, especially in some occasions where it is inconvenient to connect to the power supply. The long power cord is messy, and the modern business atmosphere is stronger.
The projector system load, the current is about 2A, the voltage range is 9V-13V, and the standby time is about 2 hours. According to this requirement, the capacity of the battery pack is calculated: 2A*2H=4000mAh. Considering the cost and production efficiency factors, a general-purpose 18650 lipo battery is used (polymer needs to be customized, the cycle is long, and the cost is high). The standard voltage of 18650 cells is 3.7V, and the common capacity is 2600mAh. According to the above requirements for voltage and capacity, it can be calculated; 3 batteries are required in series, the standard voltage reaches 11.1V, the highest is 12.6V, and the lowest is 9V, which meets the voltage requirements; 1 group needs to be connected in parallel, and the capacity reaches 5200mAh, which meets the requirements of the capacity. Require. That is, a total of 6 cells with 3 series and 2 parallel are used. The summary is: the battery pack is a 3-series 2-parallel scheme, with a nominal voltage of 11.1V and a nominal capacity of 5200mAh.
Since the material of lithium batteries itself determines that it cannot be overcharged, overdischarged, overcurrent, short-circuited and ultra-high temperature charge and discharge, the lithium battery pack needs a control board to be used safely. Based on the small size and relatively high temperature of the mini projector, the size of the control board should be as small as possible, and the over-temperature protection function should be added. When the pico projector is in standby mode, the system demand current is normally 2A, and the maximum is 5A. Summarizing the above requirements, the functional design requirements for the control board are as follows:
(1) Overcharge protection function, for each cell, the overcharge voltage is 4.22V;
(2) Over-discharge protection function, for each cell, the over-discharge voltage is 2.75V;
(3) Short circuit protection function, for each cell;
(4) Overcurrent protection function, choose 1.3 times the maximum current, that is, 6.8A;
(5) Over-temperature protection function, for the surface of custom lithium battery packs;
(6) The size of the control board should be as small as possible.
The protection board usually includes control IC, MOS switch, resistor, capacitor, NTC, etc. Among them, the control IC controls the switch to be turned on when everything is normal, so that the cell and the external circuit are connected, and when the cell voltage or loop current exceeds the specified value, it immediately controls the switch to turn off to protect the safety of the cell. According to the required five-fold protection function, we first select the appropriate control IC. Also considering the cost constraints, choose a more mature power battery chip to control the SMD type, this IC has all the above protection functions.
According to the reserved space of the system, considering the fixing reliability of the lithium battery cell and the PCB, a bracket needs to be designed to fix it. Consider placing the cell against both sides of the bracket, fixing it with double-sided tape, designing the position of the PCB to be fixed by a bayonet, and then covering the outside with a sleeve, so as to ensure the stability of the structure.
The size of the bracket should be the same as that of the control board. After correcting the size drawing of the control board, and there is no problem in the simulation assembly, the control board and the bracket can be sent out for the proofing board. Other accessories are slightly designed and sent out for proofing. Assemble the finished product after the samples arrive. There is only one output interface for the system, and the whole is still relatively beautiful.