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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 2600mAh. 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 Click here for
an instruction video on issues to be aware of when using Lipo Batteries
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
herefor 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 2600mAh
To increase the amperage (storage capacity) of the
pack we add additional cells in parallel.
The pack voltage is now 11.1v 5200mAh
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!
