Li-ion Voltage Analysis

It is important to note that all Li-ion cells, including the Li-ion cells contained in our Mobile Power Centers, are sensitive to voltage. A Prolonged low voltage condition within a Li-ion cell may cause the dissolution of metals (principally copper). Copper dissolves into the electrolyte solution at open circuit voltages below ~0.7 volts. This dissolved copper is re-plated within the cell upon subsequent charging and can cause undesirable effects and almost certainly will compromise cell performance (e.g. low capacity, poor cycle life, high self-discharge).

 

Similarly, high voltage can also cause the degradation of Li-ion cells, especially at elevated temperature. When a Li-ion battery is plugged into a charger, charging continues along a prescribed path until a state of charge ("SOC") of 100% is sensed by the circuitry. The charging is then terminated and the battery is allowed to very slowly discharge. It is detrimental to the cells to be kept at 98-100% SOC for prolonged periods of time (i.e. more than 10 days). This is why many cells are allowed to discharge to around 95% SOC or less before charging is re-initiated, even while connected to a charger.

 

The nature of Li-ion cells is such that the relationship between state of charge ("SOC") and voltage is fairly flat throughout much of the cell’s discharge range. A typical discharge voltage curve is shown below:

 

Li-ion Voltage Discharge Curve

 

 

The rapid fall of voltage at the end of the discharge cycle provides a relatively accurate means of determining when energy will run out. However, this also means that the SOC drops much more rapidly and can lead to an over-discharged condition if the cell or battery is left to sit for prolonged periods at a low SOC. This is due to the fact that Li-ion cells have an inherent self-discharge rate independent of any circuit load. This self-discharge rate is quite low at room temperatures around 20-25°C or less. Values of around 2% per month are typical. However, this rate can more than double when cells are exposed to high temperatures. Furthermore, ambient temperature can have a profound effect on the discharge voltage curve and must be considered during transportation and storage when the cells may be exposed to extreme high or low temperatures.

 

Well-designed lithium ion batteries, such as those from Silicon Lightworks, include safety circuits that protect cells from both high and low voltage conditions. However, inherent self-discharge within the cells can lead to a low voltage condition if the cells are left uncharged for long periods of time. As such, we recommend fully cycling your Mobile Power Center at least once every four months (i.e. fully charging and discharging) and then recharging to a SOC of approximately 50-75% (i.e. until 2 or 3 indicator lights are illuminated on the fuel gauge of the Mobile Power Center). This will ensure maximize battery performance and increase battery life.

 

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