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Electric Model Basics Lecture (2) - Basic knowledge of lithium polymer batteries

From the column model AERO alliance

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In today's aircraft model applications, lithium batteries have become quite popular. There are many types of lithium batteries, such as lithium polymer batteries (Li-Po), lithium ion batteries (Li-ion), and lithium iron phosphate batteries (Li-Fe). In the model, we usually use lithium polymer battery (Li-Po) for power and high voltage receiving, etc. Lithium iron phosphate battery (Li-Fe) is used as CDI igniter, receiving power, remote control power, etc. Today, we take lithium polymer batteries as an example to introduce various parameters on the packaging of lithium batteries and how to use them correctly.

The full name of the lithium polymer battery is Lithium–Polymer Battery, abbreviated as Li-Po.

Lithium polymer battery has the characteristics of low memory effect, low self-discharge characteristics, light weight, high capacity and high voltage. It is very suitable for use in aviation models and electric models that need to be lightweight, which solves the problem of battery overweight and discharge for many years. The problem of insufficient capacity.

Representation

S / P

S indicates the serial connection of the battery, and P indicates the parallel connection of the batteries (Parallel).

Generally, the voltage used by electric aviation model power plants is mostly higher than 7.2V, so lithium polymer batteries are often connected in series by several batteries, and the single chip is marked with a voltage of 3.7V. For example, if 2S is marked, it means that two lithium batteries are connected in series to form 7.4V voltage (3.7V×2=7.4V), and 3S means that three lithium battery cells are connected in series to form 11.1V voltage (3.7V×3=11.1V). ,So on and so forth. In addition, there is a way of marking "3S2P", which means that three lithium battery cells are connected in series and two groups are connected in parallel, which means that two sets of 3S series lithium battery cells are connected in parallel, and two sets of 3S/2200mAh batteries are used. After parallel connection, the capacity is added and the voltage is constant, so the battery is 3S2P/4400mAh.

Due to the excellent manufacturing technology of the battery and the cost reduction, most of them directly produce high-capacity batteries, and it is rare to see batteries connected in series and then in parallel.

C number - discharge rate

The C number of a lithium battery refers to the discharge rate of the battery, which is an abbreviation of Capacity. For example, the designation 25C indicates a discharge rate with a current of 25 times the indicated capacity. For example, the capacity is marked 2200mAh, and the discharge rate is 25C. The approximate calculation method is: 2.2A×25C=55A, which means that the battery can be continuously discharged at 55A.

More cautious battery manufacturers will indicate both continuous discharge capacity and instantaneous discharge capacity. For example, the battery writes a maximum discharge of 25C (about 55A), and a discharge rate of 50C (about 110A) can be instantaneously erupted, but this momentary explosion cannot continue. More than 5 seconds, otherwise it will damage the battery. The higher the C number of the battery, the higher the cost of manufacturing and raw materials, so the price and weight will increase.

The illustration shows a Dap DUPU battery with a discharge rate of up to 60C.

Model friends can calculate the current according to the power demand of the aircraft to select the battery with the appropriate discharge rate.

mAh - capacity

It is easier to understand that the capacity unit should be taken apart from the three words of mAh:

m=mili, one thousandth of a unit (millimeter)

A=Ampere, Chinese is ampere, representing the unit of current

h=hour, hour

For example, the capacity is 2200mAh, which is equivalent to 2.2Ah, which means that if it is discharged at 2.2A, it can be kept for 1 hour. If only 1.1A is used, it can be discharged for 2 hours. According to this, the larger the capacity, the same current. The longer the discharge time is.

Because of the large capacity of lead-acid batteries for automobiles, the indication of the capacity is mostly written as 22A/h (22Ah is equal to 10*2200mAh). In fact, this kind of label is easier to understand, just because the unit for calculating the battery capacity of the charger and other equipment used by the model is more detailed, so it is more appropriate to use one thousandth of an ampere.

V - voltage

V is the abbreviation of Volt, Chinese is volt, which is the unit of voltage. The single-chip standard voltage of a lithium polymer battery is 3.7V, so if the battery is 11.1V, it can be easily calculated that it is a battery pack that is connected in series with three batteries (11.1÷3.7=3), that is, 3S. Usually the battery will indicate 11.1V or a few S to indicate the total voltage.

Dap battery indicating 14.8V (4S) 2200mAh 25C

Voltage range

The lithium battery has a marked voltage of 3.7V (volts), a charge cut-off voltage of 4.2V, and a minimum discharge cut-off voltage limit of 2.8V. Below 2.8V is over-discharge, almost impossible to repair, above 4.2V is overcharged, there is a danger of burning.

charging method

Lithium-poly battery charging mode uses constant voltage and variable current. Because the voltage requirements are quite accurate, it is necessary to use a dedicated lithium battery balancing charger. The quality of the charger will affect the charging accuracy, which will affect the battery life.

recharging current

The charging current of the lithium poly battery is 0.7C, for example, the charging current of 2200mAh (2.2Ah) is 1.5A-1.6A (2.2Ah*0.7C=1.54A).

At present, most lithium polymer batteries can be charged at 3 to 5C. Although there is the advantage of fast charging, according to the chemical reaction characteristics of lithium poly battery, fast charging will reduce the battery life, so in the case of sufficient time, it is recommended to 0.7C. Rate charging.

Weight-weight

The weight is usually not marked on the sticker of the battery, but it will be reflected on the factory's catalogue list, so that the model friends can match different aircraft. The weight of the battery is related to the overall model weight. In principle, the larger the capacity and the larger the discharge rate, the heavier the weight.

If you take two different brands, but the battery with the same voltage, capacity and discharge C number, the weight will not be the same. Excluding the weight of the wire, the raw materials used in the battery, and the processing technology are also factors that reduce the volume and weight, but the current battery level is not much different, so the weight reduction is actually limited.

Lithium polymer battery maintenance

Avoid direct sunlight, humidity, and high temperature. Choose dry and cool place. It is recommended to use fireproof bag or explosion-proof box for storage.

Do not store at full power for more than 48 hours

Fully charged lithium polymer batteries are quite active due to internal chemical reactions, so when stored in a fully charged state for too long, the lithium poly battery will age itself and reduce the discharge capacity. The best storage voltage is kept at a single piece of 3.85V.

Lithium polymer battery use precautions

Never over discharge

Since the aviation model battery has a load in use, the voltage will be lower than the measured value when it is still placed. Therefore, when the measured voltage is 2.8~3V after the load stops, it may have caused the battery to be over-discharged during use. The safest way to use is that the measured voltage after use is above 3.55V. Usually, the real-time voltage alarm can be set at about 3.5V.

Do not put the battery in the car

Direct sunlight inside the car will make the temperature inside the car as high as 50 ~ 60 degrees Celsius, lithium poly battery in this environment is prone to damage, and even cause danger! !

Do not leave the lithium battery when charging

In foreign countries, there was a model airplane player who charged the lithium battery in the car. Because no one was monitoring the abnormality while charging, the vehicle burned when the battery burned, and the loss was heavy. The battery fire and other events are believed to be familiar to everyone, so remember to keep the guards during charging. When an accident occurs, immediately turn off the power and prepare a dry powder fire extinguisher to extinguish the fire.

The chemical characteristics of lithium batteries are quite active. They are also quite sensitive in the case of high discharge characteristics. In the process of charging or discharging, if the operation is not carefully controlled, there is a possibility of battery failure or burning accidents. The current charger and electronic governor do a good job of protecting the lithium battery. Unless it is inferior to the lithium battery, charger or damaged battery, the correct specification is safe.