Others will be damaged or possibly overheat or catch fire. Such high charging rates are possible only with some battery types. High C-rates typically require the charger to carefully monitor battery parameters such as terminal voltage and temperature to prevent overcharging and so damage to the cells. But high C-ratings are attractive to end users because such batteries can be charged more quickly, and produce higher current output in use. Battery cells which have been built to allow higher C-rates than usual must make provision for increased heating. As some batteries reach their full charge, cooling may also be observed. This unit charges batteries until they reach a specific voltage, then trickle charges them until disconnected.Īll charging and discharging of batteries generates internal heat, and the amount of heat generated is roughly proportional to the current involved (a battery's current state of charge, condition / history, etc. For example, when the 100 kWh battery in a Tesla Model S P100D is undergoing supercharging at 120 kW the C-rate is 1.2C and when that battery delivers its maximum power of 451 kW, its C-rate is 4.51C. If both the (dis)charge current and the battery capacity in the C-rate ratio is multiplied by the battery voltage, the C-rate becomes a ratio of the (dis)charge power to the battery's energy capacity. Since the unit of the C-rate is typically implied, some care is required when using it to avoid confusing it with the battery's capacity to store a charge, which in the SI has unit coulomb with unit symbol C. Likewise, for the same battery a charge current of 250 mA corresponds to a C-rate of C/2, meaning that this current will increase the state of charge of this battery by 50% in one hour. The C-rate is never negative, so whether it describes a charging or discharging process depends on the context.įor example, for a battery with a capacity of 500 mAh, a discharge rate of 5000 mA (i.e., 5 A) corresponds to a C-rate of 10C, meaning that such a current can discharge 10 such batteries in one hour. While rarely stated explicitly, the unit of the C-rate is h −1, equivalent to stating the battery's capacity to store an electrical charge in unit hour times current in the same unit as the charge or discharge current. The C-rate is defined as the charge or discharge current divided by the battery's capacity to store an electrical charge.
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