Case/portable monitoring equipment lithium battery

Case brief (Customer request)

Battery is used in Portable Security Camera. Customer requires: 7.4V lithium polymer battery 3000mAh~4000mAh. the parameters of Portable Security Camera: Constant output voltage: 5V. Continuous working current: 200~500mA. The max working current: 600mA.size requirement for the battery: 22.5*56*80.5mm, can be adjusted. But the thickness should be made as thin as possible. need battery charger, wall-plug, charging time as short as possible.

Solve the case (case analysis)

1.Difficulties: 

①5V constant output voltage. The actual voltage of the battery is 7.4V, the output need to be 5V. So, need to develop the PCM to achieve this pont. Besides, when design the PCM, there is an important point should be noted. That is the input voltage of the battery. The actual power of this battery won’t decrease because of becoming 5V output. So, when the battery is charging, the charging voltage is still 8.4V.

② Selecting the cells. Battery capacity is at least 3000mAh, and at the same time, the thickness as thin as possible.

③ Battery structure. For lithium polymer battery in series or paraller, the PCM should be put into the groove between cell and cell. While the size of the groove between cell and cell is relative with the thickness of cell. The thickness is thicker, the space of groove is larger. Otherwise, is smaller. Customer hope the thickness as thin as possible, that means the space of groove is not larger. But need to achieve 5V constant output voltage, must add some components into PCM. So, the size of PCM will be increase. If the size of PCM is larger than the space of groove, PCM can’t put on the groove.

2.Key points:

①5V constant output voltage. The actual voltage of the battery is 7.4V, the output need to be 5V.

Set up the project (project design)

1.Ideas for desing: 

①Confirm battery model. As customer’s requirements, we select this model 605176 3.7V 3000mAh. The size of this cell: thickness 6mm, width 51mm, length 76mm. The size of finished product will be around 13-13.5mm. 

② Think abou the battery structure, where the PCM should be put in? Two cells pile up together face to face. The groove of between these two cells is 5.7mm (height). The first choice: the PCM put on the groove. The second choice: the PCM put on the top of cells, infront of tap. 

③ Design and development of the PCM. Achieve 5V constant output. We have mentioned in previous, battery’s actual power won’t decrease for 5V output. So, the charging port is still 8.4V. So, the PCM should be made into discharge 5V (P+ P-), and charge 8.4V(C+ C-). Achieve these functions, need to add some components on the PCM. So, this PCM’s size will be larger than ordinary 2S-PCM. So, the size of PCM should combine with (where the PCM should be put in). We mentioned two choices. The first choice, the PCM put on the groove, but the space of groove is 5.7mm (height)*56mm (length). This size can’t contain all the components we needed. So, we need to choose the second choice.

2.Concrete plans:

①The number of this case: HX90009

② Cells: 605176 3.7V 3000mAh

③ PCM:

P+ P- discharge is 5V 1A. 

C+ C- charge is 8.4V 2.5A.

Because this is li-polymer battery, and the taps are connected with the bonding pad of PCM directly. So, the position of B+ B- BM should be accurate. Otherwise, can’t connect. 

④ Electric wires: UL 26AGW, length: 38mm. Connector: Molex51021-2 farward direction.

Implementation (process introduction)

1.Match cells in a group 

 2.Stick insulating gasket 

 3.Stick high temperature glue for insulation

4.Put the batteries in series and parallel respectively using nickel strap

5.Connect the PCM and battery pack using electric wires

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

7.C over PCV shrink (finish)

Validation (difficulty and keynote)

1.Whether the battery pack with nominal votlage 7.4V can pass the test of constant output 5V or not?