One question I often get asked is "What is the voltage of Lithium ion cells and batteries?" or "Are Lithium Ion cells 3.6V or 3.7V?"
The headline voltage that is quoted on cell manufacturers data sheets is usually the nominal or average voltage measured at the mid point between full charged and fully discharged based on a 0.2C discharge (where C is the rated capacity of the cell in mAh). This nominal voltage is usually shown as either 3.6V, 3.7V or 3.8V.
The actual voltage profile during discharge depends on the cell chemistry being used, the discharge rate, the temperature and the age of the cells or battery being discharged as I will explain below:
Chemistry. Different cell chemistries have different voltage profiles. The most common system consists of a Cobalt Oxide cathode and a graphite carbon anode. This chemistry provides high capacity for low drain applications and has a running voltage of around 3.6V at 0.2C. For higher drain applications Manganese Oxide can be used as a cathode but this has a lower energy density. It does however have a higher nominal voltage of between 3.7V and 3.8V at 0.2C. Nickel Cobalt Manganese (NCM) provides a compromise between high current capability and high energy density and has a nominal voltage of 3.7V at 0.2C.
Discharge Rate. The discharge rate makes a significant difference to the discharge voltage, especially if the cell is not designed for higher rates. As mentioned above, the nominal voltage is usually based on a 0.2C discharge and the actual voltage will be far lower if the cell is being discharged at 5C or 10C (note that cobalt cells are typically only capable of a 2C max discharge).
Temperature. Data sheet figures are normally based on a discharge at room temperature (+20°C to +25°C). The voltage profile will be slightly improved if the cell is discharged at a higher temperatures (up to +60°C) due to lower internal cell impedance. At lower temperatures the impedance is increased and the voltage is suppressed. At application temperatures below 0°C the selection of a lithium ion cell needs careful consideration, especially if the discharge rate is high as performance between models varies considerably.
Age. The impedance of a lithium ion cell increases as it is cycled and this further suppresses the discharge voltage profile.
The headline voltage of 3.6V or 3.7V on a cell data sheet is only ever going to be a nominal figure that ends up being shown as 10.8V or 11.1V on a three cell series battery. The actual voltage depends on all the factors I have mentioned above.
So what should you write on your battery? We recommend using the nominal voltage from the cell manufacture data sheet and multiplying it by the number of cells in series. It just makes life far more simple!