Analysis of several 18650 lithium rechargeable battery protection circuits

　　Among lithium-ion battery protection IC products, currently there are only up to 4 cells connected in series. Therefore, when many engineers design lithium-ion battery protection circuits larger than 4 cells, they feel that there is too little information or there is a lack of ready-made product circuits to develop. .

　　In the past two years, I have come across four or five kinds of 7-cell lithium-ion battery series protection circuits developed by other companies. I use my spare time to draw them one by one for everyone's reference, so that engineers can avoid detours.

　　The picture below is the schematic diagram of the first type

　　The circuit above is an early version of a 7-cell series lithium-ion battery protection circuit developed by a foreign company. The protection function of the circuit is relatively reliable and has a simple charge balancing circuit.

　　However, this circuit has major shortcomings: the power consumption of the battery cells in the 7S is unbalanced, and the static power consumption of the battery cells in the lower cells is too large.

　　There are also some small disadvantages: the circuit is slightly complex, difficult to expand, and the cost is not low.

　　But as an early 7S protection circuit product, its protection function is fully realized, with overcharge, over-discharge, over-current and short-circuit protection functions. It also has a simple charge balancing function. The picture below is the circuit board corresponding to this circuit. There is a chip LM339 burned out.

　　The picture below is a 18650 lithium rechargeable battery protection circuit designed by another foreign company.

　　It can be seen from the circuit diagram that its circuit is simpler than the circuit in the first type, the cost is also slightly reduced, and the protection function can also be fully realized.

　　The disadvantage is that the cost is still high. The protection switch is placed in the middle of the battery core, which makes production assembly and testing during mass production more troublesome.

　　What the above two circuits have in common is:

　　They all use two 4S lithium-ion battery protection ICs (MM1414 or S-8254) to form a 18650 lithium rechargeable battery protection circuit. This circuit structure is still used by manufacturers.

　　However, this circuit structure is difficult to expand and is limited by the protection IC itself. For example, if you want a 5S protection circuit, this circuit structure cannot be realized. The following is the third 18650 lithium rechargeable battery protection circuit, which is domestic A company provides a 7S protection circuit composed of a single protection IC that many people are paying attention to.

　　The characteristics of this protection circuit are good scalability, relatively low cost, and minimum power consumption in static state. Another feature is that the charging circuit and the discharging circuit are separated for the first time, which will reduce the cost and the internal resistance during discharge. It will also be smaller.

　　Another feature of this circuit is that when overcharging is protected, the current consumption of the overcharged cell will be very large, and it has some balancing functions.

　　The disadvantage is that the power consumption after over-discharge protection will be slightly higher than the static state. As you can see from the picture, the circuit of each battery cell in the upper part is the same, and it can continue to be expanded to 10 cells or even 13 sections. Of course, when there are more sections, the withstand voltage of the MOSFET must be considered.

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