Finally, one must look at the reference data provided in an actual charger chip’s data sheet. This shows the pre-charge current and taper limit / termination as discussed before – but also a “thermal regulation” step just in case, when the battery voltage is much lower than the input source, there could be a scenario where running at the maximum charge current might overheat the power switch in the IC. What is done here is allow the current to go as high as possible without exceeding a maximum allowed die temperature. As the cell voltage increases slightly, and the difference between the charging source and the cell lets the current go to its maximum level and proceed with the “constant current” fast charge phase. Finally, one additional detail that may not be clear from this phase: most of these phases of the charge process have a time limit associated with them. For example, if the battery stays in the pre-charge phase for more than 30 minutes, charge will be terminated and declared a fault condition. This is because the cell voltage is not going up, even when current is being continuously applied, so there is probably an internal defect like a shorted cell. One other point that should be mentioned is that the entire charge process is gated on and off based on the cell temperature. If the cell temperature is too low or too hot, charging is disabled. Most chargers have some form of temperature sensor input to monitor the pack temperature, or at least use an internal temp sensor to estimate the pack temperature. There are also more sophisticated multi-level charge schemes implemented in some devices.