We now design electronic products, many times also use lithium battery power, the same as cell phones or tablets with lithium battery power, familiar with the knowledge of lithium battery capacity, perhaps the use and design of lithium battery power supply, including the design of battery chargers, it is necessary.
Now many people concerned about the discharge of lithium batteries, have the experience that when the lithium battery from the full voltage of 4.2V discharge to 3.7V, a long time, but once the 3.7V will discharge very quickly, yes, indeed.
Here are some information about some lithium batteries, to give you a summary.
First of all, the battery discharge platform, which refers to the fully charged lithium battery in the discharge, the battery voltage change state.
Battery constant current discharge, the battery voltage to go through three processes, namely, decline, stabilization, and then decline, in these three processes, the stabilization period is the longest. The longer the stabilization time, the higher the discharge platform of the battery. The high and low discharge platform is closely related to the battery manufacturing process. It is because the market positioning of each lithium battery manufacturer is different, the technical process means different, their control of the discharge platform is different, the quality is also very different.
In general, a 18650 lithium battery full voltage 4.2V, when discharged with a current of 1C to 3.7V, put 60 minutes, then we say the battery capacity is 2200mAh, during this time According to the characteristics of rechargeable batteries, make a graph as follows, you can better understand the battery capacity and voltage and current time and discharge platform relationship：
Li-ion battery battery capacity and discharge platform understanding diagram
Capacity (C) = discharge current × battery discharge platform time
For an 18650 lithium battery with a capacity of 2200mAh, 1C discharging to 3.7V takes 1 hour.
Capacity(C)=2200mA×1hour=2200mAh
Then the question arises, for a good lithium battery, generally in our product testing will be in 3.7V after the voltage drop very quickly, then in a short period of time to put the power is very little.
On the contrary, the bad battery in 4.2V to 3.7V discharge, the voltage drops very quickly, and after 3.7V the voltage drops very slowly, this kind of battery is not good performance, and the general capacity is very low. Then the discharge platform of good lithium batteries is 3.7V.
In general, under constant voltage conditions, charged to a voltage of 4.2V, the current is less than 0.01C stop charging, and then set aside for 10 minutes, at any rate of discharge current, discharge to 3.7V, the length of time experienced by the battery discharge is an important indicator of the battery is good or bad.
However, do not just pursue a high platform, sometimes the platform voltage is high, but the capacity is reduced, because, under different multiplier conditions, the platform voltage is different, therefore, the platform should be considered from many sides. Both high capacity and long duration at the specified voltage is considered a really good battery.
What is the discharge rate again?
Discharge rate F: [1/hour], meaning "N hours charge (discharge) rate, often just say "number", not to say the unit; F is also called "N hours charge (discharge) rate", F = 1/(N hours), can generally be calculated as follows: I = 0.1 X [1/(N hours)] X C
For example, the battery capacity of 2200mAh, charged with a current of 0.1C, equivalent to the clear: the battery charging current I = 0.1 [1/hour] X 2200 [mAh] = 220 [mA]
With the help of the above diagram of understanding the capacity and discharge platform of lithium batteries, can better understand the reasoning behind the capacity of the rechargeable battery and the discharge platform of the battery, which can also be said to measure the working time of the battery high power, the same two battery capacity is the same, fully charged, assuming that at the same time from 4.2V to 3.7V, but a long time, a short time, that is, a long time battery platform high, that is, high voltage The time working time is long, such as these two batteries used in cell phones, standby time is the same, but the two phones together to call, the platform time long battery notice time will be long, the platform time short battery talk time will be short Two, for this figure, another meaning, it is also very meaningful to understand the power management monitoring of lithium batteries.
For example, at present, there are two general methods for monitoring the power of rechargeable batteries.
Measuring the battery voltage method. When the charger detects the battery voltage while charging, it is considered full when the voltage reaches the specified voltage value. Such as lithium batteries are considered full when the voltage is charged to 4.2V. Detect the voltage of the voltmeter accuracy to achieve plus or minus 1% accuracy. Because the lithium battery overcharge to damage. If you want to monitor the battery charging and discharging, there are very mature circuit on the book can be referred to, if only to measure it, then it is good to do, according to the capacity of the calculation of the load resistance, with a standard discharge current discharge, then test the voltage, as long as it can reach or close to the nominal discharge time on the line.
Measuring battery voltage this method, there are many shortcomings. For example, for batteries made by different manufacturers, the relationship between the open circuit voltage and capacity varies, etc. The advantage is that the design cost is relatively low.
There is also a common method to monitor the remaining capacity is, in some places where more accurate battery capacity is required, to know the remaining battery capacity with estimating the battery usage time, by measuring the net charge flowing into/out of the battery to estimate the remaining battery capacity. The total current flowing into/out of the battery is integrated, that is, the area under the curve in the graph is found, and the number of net charges obtained is the remaining capacity.
This method of calculating battery capacity is currently considered to be a more accurate method of calculating battery power. Of course the design cost is also relatively high.
