C-RATE


The charge and discharge current of a battery is measured in C-Rate. Most portable batteries (with the exception of the SLA) are rated at 1C. A discharge of 1C draws a current equal to the rated capacity. For example, a battery rated at 1000mAh provides 1000mA for one hour if discharged at 1C rate. The same battery discharged at 0.5C would provide 500mA for two hours, or at 2C it would deliver 2000mA for 30 minutes. 1C is often also referred to as a one hour discharge rate; a 0.5C would be a two hour, and a 0.1C a 10 hour discharge rate.

The capacity of a battery is commonly measured with a battery analyzer. If the analyzerís capacity readout is displayed in percentage of the nominal rating, 100% would be shown if 1000mAh can be drawn for one hour from a battery rated at 1000mAh. If the battery only lasts for 30 minutes, 50% is indicated. A new battery often provides more than 100% capacity. In such a case, the battery can endure a longer discharge time than what is specified by the manufacturer.

When discharging a battery with a battery analyzer that allows the setting of different discharge C-Rates, a higher capacity reading is observed if the battery is discharged at a lower C-Rate and verse visa. By discharging the 1000mAh battery at 2C, or 2000mA, the analyzer is scaled to derive at the full capacity in 30 minutes. Theoretically, the capacity reading should be the same as with a slower discharge since the identical amount of energy is dispensed, only over a shorter time. Due to some energy losses that occurs inside the battery and a drop in voltage that causes the battery to reach the low-end voltage cut-off sooner, the derived capacity reading would be lower, say around 95%. Discharging the same battery at 0.5C, or 500mA over two hours, a capacity reading would likely be more than 100%.

The discrepancy in capacity readings with different C-Rates depends, to a large extent, on the internal impedance of the battery. On a standard NiCd with good load current characteristic, the difference in the readings is only a few percentage points. On a battery exhibiting a higher impedance, such as an ultra-high capacity NiCd or NiMH, the difference in capacity readings could swing as much as plus/minus 10 percent. To compensate for the poor capacity readings at higher currents, some manufactures rate their batteries at a C-Rate of 0.5C or lower.

One battery which does not perform well at a 1C discharge rate is the SLA. To obtain a practical capacity reading, manufacturers commonly rate these batteries at 0.05C or 20 hour discharge. Even at this slow discharge rate, it is often difficult to attain 100% capacity. By discharging the SLA at a more practical 5h discharge (0.2C), the capacity readings are lower accordingly. To compensate for the different readings at various discharge currents, manufactures offer a capacity offset.

Applying the capacity offset does not improve the battery performance; it merely adjusts the capacity calculation if discharged at a higher or lower C-Rate than specified. The amount of capacity offset recommended for a given battery is specified by the battery manufacturer.

The Li-ion is suitable for 1C discharge rates but is not as tolerant as the NiCd when delivering load currents that are much above 1C. Li-ion battery packs are typically fused or electronically protected against currents much higher than 1C. The Li-Polymer and the reusable alkaline are only suitable for very light load demands.

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