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Case/lithium battery is for oil gradient tester

Case brief (Customer request)


The lithium battery is for oil gradient tester. It will works at 80℃ high temperature for a long term. The cycle life should not less than 200~300 times. And the battery should be placed into a stainless steel tube which is 29mm inner diameter, 630mm length. It can be used more than 6 hours, with a high cost-effective. 

Some parameters of the tester as below: 


① Continuous operating current: 500mA. 

② Peak current: 1.5A. 

③ Operating voltage: 22V~35V.

                                                                                                                                                                 

Solve the case (case analysis)


1.Difficulties: 


①80℃ High temperature.

②8pcs cell in a line. It can’t be misplaced between cell and cell. After connected, the battery pack can’t be bended. Otherwise, it can’t be put into steel tube. 

③It is a little bit difficult to develop the PCM (Protection Circuit Board). If the PCM is round shape, the components can’t be arranged well. It can be achieved, if the PCM is made into multilayer round board. But the structures are very complex, with bigger hidden danger. And the processing is very difficult. If using long strip shape board, the width of board can’t exceed 24mm, so the only way is lenghten the board. If like that, there is another problem that is the PCM how to be fixed on the battery pack. 

④The steel tube is conductive. 

⑤With high cost-dffective.


2.Emphasis:


① The high temperature performance.

② The technological structure.

        

                                                                                                                                                                 

Set up the project (project design)


1.Ideas for desing:


① Meeting high temperature performance. There are two choices for this point. First, use the materials with more than 300 times cycle life to produce cells. Second, use the materials with cycle life around 200 times to produce cells. The first option is the twice cost of the second option. Obviously, the first option can’t meet the requirements of high cost-effective. So, choose second option. 

② Technological structure. The cell we chose is 26.3mm max diameter. The different value between cell and the inner diameter of steel tube is 2.7mm. So, just make these 8pcs cells into a line, the 2.7mm will not be a problem. In order to meeting the dimension, the negative wire through the whole battery pack should use 27# (it can meet the requirements of charging and discharging current basically). Considering the insulation and intensity, the cells must be coverd by a whole insulation paper (0.3mm thickness). Then, the PCM how to be fixed on the battery pack? We will use a silent stock tube with 25mm diameter to cover the whole PCM to achieve the seamless connection between silent stock tube and battery pack, and using a whole insulation paper with 0.3mm thickness to make them be a whole part. 


2.  Concrete proposal: 


① The number of this case: 50065-1

② Cells: ICR26650 3.7V 4500mAh (The percent conversion is more then 70% in the situation of 80℃ high temperature. Cycle life more than 300 times.)

③ PCM: Single cell overcharge protection voltage: 4.25±0.05V. Single cell overcharge release voltage: 4.05±0.05V. Single cell overdischarge protection voltage: 2.80±0.08V. Under voltage protection delay: 0.13S. After connected, overcurrent protection value (10ms): 5~7A. With the function of short-circuit protection, after remove load, can be resumed and used. 

④ Electric wires: Silicone-line 24# 300℃.

⑤ Make fixture. It is used to ensure 8pcs cells in a line.

                                                                                                                                                                 

Implementation (process introduction)



1.Match cells in a group

2.Stick insulating gasket

3.Make 8pcs cells in series using nickel strap

4.Stick high temperature glue and preliminary fixed

5.Connect the PCM and battery pack using electric wires

6.Wrap the insulation paper, and fix the battery pack well

7.Cover PCV shrink (finish)

                                                                                                                                                                 

Validation (difficulty and keynote)

1.Test if the battery packs can be put into and taken out from the stainless steel tube with 29mm diameter and 630mm length. 

2.Test capacity in the high-temperature cabinet with 80℃.

3.Set up the experimental working steps, and then start to test. 

4.Finish the experiment and then export datum. 


                                                                                                                                                                 

Test Report of Capacity



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