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DIY - Custom Lithium Ion Battery Packs 

 Want to build your own pack? Then read on and discover how!

The typical 18650 cell (Also Applies to Lithium polymer (Li-Po) 

The cell has an operating voltage range from 2.8V minimum to 4.2 V maximum. Storage capacity 2200mAh. Some time back the manufacturers settled on a nominal voltage for the cell at 3.6V and over the past few years have decided settle for 3.7V, not that it makes a difference other than to confuse some. 

What happens if the cell is completely discharged?

The cell will be permanently damaged and unlikely to be capable of a recharge. We have observed that the cell begins to heat up as it approaches 2V and then does a rapid about turn and the voltage falls to zero never to rise again. Attempts to push it through this barrier may result in ignition.

What happens when the cell is overcharged beyond 4.2V?

Manufacturers specify a max voltage of 4.2V. If you charge beyond this voltage the cell is likely to ignite. An example of an ignition can be seen in the YouTube video below. They are dangerous ' take extra care.
  An example of what could happen if you throw it in your refuse bin. Consider those who ride the rubbish trucks

Management Circuit Boards

To keep each lithium ion cell within the above limits lithium ion  protection  circuits are used. Generally there are one, two, thee & four cell protection circuits. Click here for info on our lithium ion  protection circuits

An Example of a 3 cell Lithium Ion Pack

Each cell has a nominal voltage of 3.7V. If we put 3 cells in series the total pack nominal voltage will be 11.1V.

Maximum full charge voltage per cell will be 4.2V therefore the maximum pack voltage will be 12.6V. Each cell is rated at 2200mAh. Since they are in series the amperage does not increase so the pack amperage is 2200mAh

To increase the amperage (storage capacity) of the pack we add additional cells in parallel.

The pack voltage is now 11.1v 4400mAh

To ensure that cells in the pack are properly managed we need a electronic management circuit for 3 cells

The cells above will be connected to the board as per the diagram.

Note this 3 cell management board sees each cell bank as one cell. Adding addition cells in parallel changes the storage capacity of the pack but the circuit only sees each cell bank as one cell.

If you have a 3 cell protection circuit the management chip on the board has been programmed to manage 3 cells. The board cannot be used for a different number of cells in series unless it is reprogrammed to do so.

 

Examples of Packs

Insulation

If you place two cells along side one another and connect them in series be aware that there is a voltage difference on the casings of the two cells. The plastic jackets on the cells are made from heat shrink sleeving and if the cell gets too hot the jacket may burst and cause a short circuit and consequently a fire. It is therefore a good idea to place insulation material between the cells.

Soldering Tagging Material onto the Cells.

There is a fallacy that these cells can only be spot welded. This is only true for those who are not in the know! 

You will need at least a 40 watt soldering iron, solder and especially flux paste. A light coat of flux on the cell surface will enable you to wet the cell with solder very quickly and will prevent heating the cell causing damage.

Warning ' Cells deliver High Amperage

Take care to not short any cells with wire or tagging material. The cells deliver enormous amperage and have no mercy. The wire or tagging material will melt immediately! If you are holding the wire or tagging material at the time ' Shame! 

Lithium polymer cells have a C rating. A 10C cell will deliver 10 times it's rated amperage immediately. A 4 amp cell can deliver 40 amps immediately. Beware!

Materials available for building packs

Click here for Cell and management board pricing

 Power Connectors Male and  Female @R10

Sleaving:  We can supply sleaving @ R10 per meter

Tagging material - We will supply small quantities free of charge

 Chargers

Click here  for a list of available Lithium Ion chargers

 
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