Charge and discharge rates of a battery are governed by C-rates. The capacity of a battery is commonly rated at 1C, meaning that a fully charged battery rated at 1Ah should provide 1A for one hour. The same battery discharging at 0.5C should provide 500mA for two hours, and at 2C it delivers 2A for 30 minutes. Losses at fast discharges reduce the discharge time and these losses also affect charge times.
A C-rate of 1C is also known as a one-hour discharge; 0.5C or C/2 is a two-hour discharge and 0.2C or C/5 is a 5-hour discharge. Some high-performance batteries can be charged and discharged above 1C with moderate stress. Table 1 illustrates typical times at various C-rates.
C-rate | Time |
---|---|
5C | 12 min |
2C | 30 min |
1C | 1h |
0.5C or C/2 | 2h |
0.2C or C/5 | 5h |
0.1C or C/10 | 10h |
0.05C or C/20 | 20h |
The battery capacity, or the amount of energy a battery can hold, can be measured with a battery analyzer. (See BU-909: Battery Test Equipment) The analyzer discharges the battery at a calibrated current while measuring the time until the end-of-discharge voltage is reached. For lead acid, the end-of-discharge is typically 1.75V/cell, for NiCd/NiMH 1.0V/cell and for Li-ion 3.0V/cell. If a 1Ah battery provides 1A for one hour, an analyzer displaying the results in percentage of the nominal rating will show 100 percent. If the discharge lasts 30 minutes before reaching the end-of-discharge cut-off voltage, then the battery has a capacity of 50 percent. A new battery is sometimes overrated and can produce more than 100 percent capacity; others are underrated and never reach 100 percent, even after priming.
When discharging a battery with a battery analyzer capable of applying different C rates, a higher C rate will produce a lower capacity reading and vice versa. By discharging the 1Ah battery at the faster 2C-rate, or 2A, the battery should ideally deliver the full capacity in 30 minutes. The sum should be the same since the identical amount of energy is dispensed over a shorter time. In reality, internal losses turn some of the energy into heat and lower the resulting capacity to about 95 percent or less. Discharging the same battery at 0.5C, or 500mA over 2 hours, will likely increase the capacity to above 100 percent.
To obtain a reasonably good capacity reading, manufacturers commonly rate alkaline and lead acid batteries at a very low 0.05C, or a 20-hour discharge. Even at this slow discharge rate, lead acid seldom attains a 100 percent capacity as the batteries are overrated. Manufacturers provide capacity offsets to adjust for the discrepancies if discharged at a higher C rate than specified. (See also BU-503: How to Calculate Battery Runtime) Figure 2 illustrates the discharge times of a lead acid battery at various loads expressed in C-rate.
Smaller batteries are rated at a 1C discharge rate. Due to sluggish behavior, lead acid is rated at 0.2C (5h) and 0.05C (20h).
While lead- and nickel-based batteries can be discharged at a high rate, the protection circuit prevents the Li-ion Energy Cell from discharging above 1C. The Power Cell with nickel, manganese and/or phosphate active material can tolerate discharge rates of up to 10C and the current threshold is set higher accordingly.
Comments
I have a lithium phosphate 170ah battery in my campervan that has a lesser rating of 1C. However if i max out my 2000w inverter, its pulling energy from the battery equivalent to nearly a 1c 170amps. Will this damage my battery or is it a case of under rating the battery or just that its new?. The manufacturers of the inverter don't recommend my battery pack with this inverter. Both products are made by renogy. Will i damage the battery if i draw more energy than the C rating??
The energy capacity of any battery is a function of discharge rate. Fundamentally, this is true because there is no such thing as zero internal resistance. It's not that somehow there is less stored energy at higher current more of the available energy gets converted to heat and is there for not available as electrical power at the terminals. Even within lithium cells there are so called power cells that are optimized for high discharge rates and energy cells that are optimized for larger capacity but of necessity at lower rates.
Thank very much for this great site.
I have been searching for this rich information until i was referred by a friend here.
The link for the analyzer test equipment shows it is empty.
Any help on how it looks like and to use it so that i an use it to study my batteries please?
Thank you.
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300F Capacitor에 25V를 축전하여 부하에 공급할때 부하에서 받을수 있는 Wattage는 얼마가 됩니까?
수초동안 축전하여 수초동안 사용하는 경우입니다.
Up/down Inverter로 축전시의 전류/전압의 제어가 들어가고 방전시도 Up/down Inverter로 부하에 필요한 전류/전압으로 변환하여 인가시키려하는데, 300F Capacitor에 어느정도의 에너지를 넣고 사용할수있는지 의문입니다