ANALOG DEVICES
temperature, a fast charger can determine when to reduce the charging current and/ or lower the termination voltage to assure safety and improve the lifespan of the battery, according to cell manufacturer specifications and recommendations. The voltage and current can be adjusted over temperature to comply with the six- zone JEITA temperature settings (see Figure 6) and with three-zone step-charging based on the battery voltage.
Battery lifespan can be further improved using a step-charging profile that changes charge current according to battery voltage. Figure 7 shows a step-charging profile that uses three charge voltages and three corresponding charge currents. Transitioning between the stages can be managed through a state machine (also see Figure 7).
Note that current, voltage, and temperature are all interrelated (see Table 1 and Table 2).
Figure 5: A single cell fast charging for a 3.6V lithium cell Parallel charging
Parallel charging of multiple cells requires additional management. For example, the charger must prevent cross charging when the two batteries’ voltages differ by more than 400mV. Cross charging can be tolerated for a limited time only when the lowest cell charge is too low to support system loading (see Table 3 and Figure 8). The implementation of a fast-charging system with parallel batteries can be done using evaluation kits and a Raspberry Pi board.
Figure 6: Six-zone JEITA temperature ranges
The following example shows a way to charge a single 3.6V lithium cell. Figure 5 shows the time domain shape of the voltages and the currents in the charging system. Specifically, the graph shows the battery voltage, battery current, and output voltage of the buck converter.
As can be seen, the step-down converter
output (VPCK) is set to 50mV above the battery voltage. This output voltage is constantly increased to avoid dropout as well as to minimise overall power dissipation.
Battery safety management With the high currents involved in fast charging, OEMs must be able to guarantee safe charging. Thus, a smart fast charger must monitor several important parameters as part of its overall battery management. For example, by monitoring battery temperature and ambient/room
18 JUNE 2023 | ELECTRONICS TODAY
Figure 7: A step-charging profile with a state machine for managing the transition between stages
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