Case/lithium battery for power tools

Case brief (Customer request)

Lithium battery used for tree branch scissors, nominal voltage is 14.8V, nominal capacity is 4.4Ah, continuous working current is 15A, peak current is 30A, with PCM to protect the battery from over current, over charge, over discharge short circuit and over temperature, working voltage range is from 12V to 16.8V, pass PSE certification, need to choose high quality battery cell, battery pack size can’t exceed 133*75*28mm, and the battery pack needs to be assembled into a 150*104*34mm dimensions box with input/output connectors. Charger will be designed by the customer and the charge current is 1A.  

Solve the case (case analysis)

1.Difficulties: 

①The structure of battery pack must be very precise due to size limitation. 

②PCM needs to be developed, but the customer’s tree branch scissors is also under developing, so the parameter is not accurate, and they can't provide material object for reference, which increase the difficulty of development. 

③The Internal welding space of battery case is narrow and small. The battery wire needs to be welded with the I/O interfaces, but the distance is only 10mm between the baffle and the I/O pads, It is difficult to the operation of welding tools. If we are not well-handled, on the one hand, it may burn out the plastic baffles causing the housing to be scrapped, and on the other hand, there may be a virtual welding phenomenon.

④Short distance in the Positive and negative welding points will easily lead to short circuit by defective insulation. And the welding space is narrow, the insulation work is difficult. So how to solve this problem?

2.Key points:

①Optimize the structure of PCB.

②When set up the parameters of the PCB ,we must take the state of the tree branch scissors when they are actually used into account.

③How to weld the electronic wire with the I/O interface. 

④The size must be accurate while design the battery case.

⑤We strive to do our best at the appearance of connector.

Set up the project (project design)

1.Ideas for desing: 

①First of all, using the work fixture to make the battery’s Length width and height size to achieve the minimum.

②The bonding pad position of the PCM is seamlessly connected with the battery pack, and the combined size should be reduced as much as possible;  Under the circumstances of all technical parameters reach the normal standard, the PCB’s components should be as flat as possible, thus to reduce the thickness of the PCB. As the customer can’t provide power tools for reference, we can only do simulation experiment on the basis of the customers offered motor. According to experience, we must design two sets of parameter schemes for customers to do the application experiment.

③To prevent short circuit when battery box was assembly welding, the battery can’t be assembled with power. So we need to use the electronic wire to weld the connector first, then use the electronic wire which is welded with connector to connect with the electronic wire of the same battery pack. In this way, the limitation of the narrow space of the battery case can be avoided; ④Using 75 ℃ temperature control switch.

⑤Design the battery case and create 3D drawings, battery box will be produced by the mould factory.

⑥Label design, label will be produced by the printing factory.

2.Concrete plans:

①Case number: PLH80002-6.

②Cell: 18650,3.7v 2200mAh 5C discharge, after setting aside for 180 days, the capacity is higher than 90% after fully charged. Cycle life is not less than 300 times.

③Battery structure: 4S2P.

④PCM: Overcharge protection voltage of single cell: 4.25±0.05V; over discharge protection voltage of single cell: 3.0±0.08V; under-voltage protection delay: 0.5s; battery pack over current protection value: 30A; With short circuit protection and high temperature protection function, battery can recover after removing the load.

⑤Output electronic wire: Using 16#、200℃ Silicone wire; Input electronic wire: Using 20#、80℃ 1007 PVC wire.

⑥The case is black, using ABS material, with the flame retardant function.

⑦The connector is waterproof. Protection grade reaches IP55.

⑧The label is made of water-proof material with membrane-covering.

Implementation (process introduction)

1.Battery cell matching.

2.Paste the Insulating washer.

3.Use high temperature glue to insulate.

4.Make the battery cell in series and parallel with nickel strips.

5.Wrap the insulating paper to Insulate and fix the battery pack.

6.Put the PVC cover on.

7.Use electronic wire welding with I/O connector.

8.Put the battery pack into plastic box, Install the battery cover after passing the test, then packing.

9.Pass PSE certification, certificate is as below(Finished).

Validation (difficulty and keynote)

1.Verify whether there is a false solder between the electronic wire and I/O interfaces.

2.After the battery pack is assembled into the case, test it to check if it is working.

3.Check if the motor could work properly after connect it with the battery which is assembled with PCM.