Portable Power Module for Güralp Certimus
Appendices
6
Appendices
6.1
Understanding Li-ion batteries
6.1.1 Charge characteristics
The Power Module uses a battery of rechargeable lithium-ion (Li-ion) cells. These
have a good capacity/volume ratio and are commonly used in, for example,
electrically-powered vehicles.
As with most cell technologies, a Li-ion cell's terminal voltage depends on its state of
charge. The graph below shows the relationship between the cell terminal voltage
and the extent to which the cell has been discharged. Note that the graph does not
extend down to zero volts: the curve is actually very flat across the majority of
charge states - much more so than for a lead-acid battery, for example. The terminal
voltage varies by less than 0.3 V between 20% and 80% charge.
Each cell provides a terminal voltage in the range 2.5 V to 4.2 V. Only the useful part
of the operating curve is shown above. Once discharged down to 3 V, very little of
the available charge is left. If the cell is discharged below this point, internal damage
starts to occur because of the chemistry involved. This process accelerates if the
cell is discharged to below 2.5 V. If left in this condition for a long period of time, the
internal cell damage can make it dangerous to recharge at any more than a very
small recovery current because of the internal heating effects.
To protect the cells from this damage, the PPM’s control system will turn off the
power output if the cell voltage drops below 3V. The system will then enter a low-
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Issue A - May 2021
