Why is the voltage of lithium ion battery 3.7V?

Now we design electronic products that sometimes are charged by the lithium battery. Mobile phone or a tablet is charged by lithium ion battery. Trying to know more about battery capacity may is good for the usage and design of lithium ion battery, including design battery charger.

Now a lot of people focus on lithium battery discharge, have such experience, was when lithium battery is full of discharge voltage of 4.2 V to 3.7 V, the time is very long, but once the 3.7 V discharge soon, yes, indeed. Thumb through some of lithium-ion batteries below some information, for everyone to do a summary.

One, said the first battery discharge platform, is refers to the charge of lithium battery during discharge, battery voltage change of the state. Constant exile electric batteries and battery voltage have three processes, namely descent, stability, and then, in the process of the three, is the longest plateau. Stability and the longer that the higher the battery discharge platform is. The discharge platform height, and it is closely related to battery manufacturing process. Because of the various lithium battery manufacturer market positioning is different, different technology means, the control of discharge platform is different, the quality also has very big difference.

Generally, section 18650 of the lithium battery voltage of 4.2 V, when use 1 c discharge current into the 3.7 V, with 60 minutes, so we can say the use of the battery capacity is 2200 mAh, based on the characteristics of rechargeable batteries in this period of time to make a figure as follows, you can better understand the battery capacity and voltage current relationship between time and discharge platform:

Lithium batteries battery capacity and discharge platform understand schematic diagram

Capacity (C) = x battery discharge current discharge platform

For 1 day capacity of 2200 mah18650 lithium batteries, 1 c discharge to 3.7 V 1 hours,

Capacity (C) = 2200 ma * 1 hour = 2200 mAh

So the question becomes, for a better lithium-ion batteries, generally when we do the product test soon after the 3.7 V voltage drop, then put in a short time there will be less power. Instead of bad battery discharge from 4.2 V to 4.2 V, the voltage drop quickly, and after the 3.7 V voltage drop is very slow, this battery is the bad performance of the general capacity is also very low. So a good lithium battery discharge platform is 3.7 V.

As a general rule, under the condition of constant pressure, charge to voltage of 4.2 V, current is less than 0.01 C stop charging, then put aside for 10 minutes, at any rate of the discharge flow, discharge and 3.7 V, the battery discharge is living through a length of time, is one of the important indicators to measure the battery is good or bad. Don't blindly pursue the high platform, however, sometimes platform high voltage and capacity declined, because, under the condition of different ratio, platform voltage is different, therefore, the platform issues should be considering. High capacity and long time is within a specified voltage in order to be really good battery.

What is the rate of discharge?

Discharge rate F: [1] when /, which means "N hour (put) electricity rate, often just say" number ", rather than the unit; Also known as "F N hours filling (put) electricity rate", F = 1 / (N hours),

Generally can be calculated this way: I = 0.1 X [1 / (N)] XC

For example, when battery capacity is 2200 mAh, the charging current of 0.1 C, equivalent to clear: the battery charging current (I) = 0.1 [1] when/X2200 (mAh) = 220 [ma]

With the aid of the lithium batteries battery capacity and discharge platform above understanding schematic diagram, can better understand the rechargeable battery capacity and battery discharge platform, also can saying is to measure high power battery working time, two battery capacity is the same, the same after a full charge assumptions at the same time from 4.2 V to 3.7 V, but a long time, a time is short, time is long battery high platform, is the time of high voltage long working hours, such as the two batteries for cellphones, standby time is the same, but the two phone call together, platform time long battery notice, platform time short battery time is short

Second, to this figure, another meaning, to understand the lithium battery power management monitoring is also very meaningful.

At present, for example, was carried out on the rechargeable battery power monitoring method, generally there are two.

Measure the battery voltage method. When the charger in the charging test battery voltage, when voltage when voltage value that is full of reach regulation. Such as lithium battery charging voltage to 4.2 V is considered highly. Detect the voltage of the voltage meter precision to achieve the accuracy of plus or minus 1%.For lithium battery charging to damage. If you want to monitor the battery charge and discharge, the book has a very mature circuit can refer to, if the only measure that is right, according to calculate the load resistance capacity, using standard discharge, the discharge current to detect the voltage at this moment, as long as can achieve or close to the nominal discharge time.

Measure the battery voltage this way, there are a lot of shortage to the battery of different manufacturers, for example, the relationship between the open circuit voltage and capacity of each are not identical, and so on. The advantage is low design cost.

The common method to monitor of SOC is to know the residual capacity of battery in places where require accurate battery capacity so as to estimate the battery use time. Measure the inflows/outflows capacity of the battery to estimate the residual capacity of battery. Add up the integral of total inflow/outflow current, which means to measure the area under the graph. The number you get is the remaining capacity. This capacity calculation method is considered to be more accurate calculation method of battery, but the design cost is relatively high.

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