1. About the identification of lithium batteries
We often see the following signs on lithium batteries:
850mAh 25C 2S1P
What does each mean?
(1) Refers to the battery capacity, that is, the battery can be discharged for 1 hour when the battery is fully charged and discharged at 850mA.
(2) 25C refers to the maximum discharge rate, that is, the maximum discharge current that the battery can reach is 25*850mA=21250mA. We know that the discharge current of the battery will change with the load. 25C represents the upper limit of the discharge current, not the normal discharge current.
(3) 2S1P means that the battery is composed of two 3.7V series connected in series, one in parallel, and the total voltage is 7.4V. By connecting the batteries in parallel, the voltage can be kept constant and the discharge current can be increased, but the battery life will not be prolonged.
What should we refer to in the process of choosing a battery, I think 3 points should be considered:
(1) The amount of voltage, which is related to the input of the system;
(2) Power, which is how many mAh, is related to the continuous working time of the system battery;
(3) Discharge rate, when the voltage is determined, this directly determines whether the maximum power that the battery can output meets the power requirements of the system
2. An example on how to choose the lithium battery power
Suppose you want to power the TMS320C6713, refer to the Datasheet to find the power supply current:
First make a simple estimate of the power consumption of the chip,
When CVDD=1.2V and the main clock is 200MHz, the power supply current of the TMS320C6713 core is 565mA, and the IO port power supply DVDD=3.3V, the current is 75mA; of course, according to the peripherals driven by IO, there may be other power consumption, Not counting here.
Power P = 1.2V * 565mA + 3.3V * 75mA = 925.5 mW
If using 2 lithium batteries for power supply, the capacity of the lithium battery is 3000mAh, then the total energy of the lithium battery is fully charged (can not be over-discharged, assuming the minimum discharge voltage is 5V) is E = (7.4-5)V * 3000mAh = 25920 J
Using this battery, the total power supply time can reach Time = E / P = 7.8 h, which means that it can continue to supply power for 7.8 hours. This is only a theoretical value for reference. The actual power supply time is shorter than this, because the film must still exist. many other power consumption.
In this way, we can fix the input voltage according to the duration of use we need, and reverse the calculation to select the required battery capacity.
Finally, by the way, I will note an IC that can be used to design a lithium battery charging circuit: LD6275 bq24010 , for future reference.