JUST A LITTLE ABOUT LITHIUM ION(LI-ION) BATTERIES

Recently we have started to offer lithium-ion batteries, and for all those of you who would like to know what they are, how they work, how were they built, how to make a system with these batteries, this is an article for them. It can be said a lot about lithium-ion batteries, but in this tutorial we have tried to cover the basics.

Lithium-ion battery in 18650 packaging

There are lithium batteries, which are non-rechargeable, and there are lithium-ion / lithium-polymer batteries which are rechargeable. Here, we’re talking about these other, therefore, rechargeable. They are called lithium ion as lithium ions move from the negative electrode to the positive during discharge and vice versa when filling. Between the two electrodes there is a separator (partition that separates them), and everything together is immersed in an electrolyte that facilitates the movement of ions. The difference between the lithium-ion batteries and lithium-polymer is in the electrolyte, so the lithium-polymer battery can be found in a range of participative forms of packaging and in the famous silver plastic (for example, in cell phones), while the lithium-ion batteries have to be in the proper forms of housings to be portected, such as the figure above.

Currently, lithium-ion batteries have best characteristic among all types of rechargeable batteries that are commercially available. Their rated voltage is 3.7V (3.6V), unlike eg. NiMH whose rated voltage is 1.2V, and have at least twice the energy density thereof (the energy density is quantity of energy that may be stored in a volume). In addition, they have a minimal memory effect, so they can begin to be charged and discharged at any state of charge without the battery permanently losing some of its capacity. And it is also a long life battery, meaning it  can survive 500-1000 complete cycles (from completely empty to completely full) and can be created in a fairly large capacity, unlike other types of rechargeable batteries.

Of course, not everything can be positive. This type of battery has its negative side, too. For starters, it can be very dangerous if it is not approached with caution. More about that in the section below. What to watch for, as the designer of some of the system and / or user of the battery, is to never have battery  too empty or too much charged. The battery, according to the charge, changes its voltage, so that when fully charged, has a voltage of 4.2V, while “completely empty” can be considered if there is a voltage of less than 2.5V. Although, the voltage should never be less than 3.0V, because of the risk of losing battery’s capacity. This problem can be solved by electronically adding additional circuits to batteries that can break the circuit when excessive voltage or too low voltage comes to the battery. Such an arrangement is called PCM (Protection Circuit Management, marked green in the picture below). The same circuit ensures that the battery does not give too much power, because it can be a problem, especially in the short-circuits because then the battery is running the maximum current that it can provide. In addition, the battery loses a good deal of its capacity when stored.

Green marked PCM built in lithium-polymer battery

Lithium-ion batteries can not only be connected to a power source and then expected to be charged. They require special charging arrangements to preserve the original battery capacity, and that it could even be fully charged. There are three stages of charging:

• – Preconditioning: does not always happen, only when the battery voltage is below 3V. Being in this state, battery is too drained to get ready to store energy, it must first be brought to the level of 3V. If the battery voltage is greater than or equal to 3 V when connected to the charger, this step is skipped.
• – Constant current: In this step, the battery is charging with constant current. It happens when the voltage is greater than 3V and less than 4.2V. The current that the battery is actually charged with is usually adjustable on the charger which you use to charge the battery. Usually, it is recommended to charge the battery with 0.5C, where C is the battery capacity. For example, if we have a battery of 1000mAh, 0.5C would be 500mA. In such charging “speed”, the battery will end this process after exactly two hours, regardless of capacity and provided that the charger can provide the required power. It can also be rechargeable at higher C-a (1.0c, 2.0c), which is called a quick charge, but that charge reduces the capacity of the battery and it is not advisable to do it often. In general, manufacturers recommend 0.8C or less. Interestingly, it is pointed out that the quick charge reduce the duration of this step, but will significantly extend the duration of the third step in the filling, so the total charge capacity of up to 100% will be about the same. Fast charging will quickly reach capacity of 70% charge (known as Tesla Supercharger), but if we want to achieve 100% charge, we will need almost as much time as we need for charging with weaker currents.
• – Constant voltage: The third and final step in charging is the constant voltage charging. It happens when the battery reached a certain voltage, typically 4.2V. This step complements the battery to its capacity of 100% and is usually quite lengthy. Charging stops when the charging current reaches a certain level, and this value is adjustable os some chargers.

Charging curve of one li-on battery

Special chips are designed for this purpose of charging lithium batteries. In addition to implementing all these algorithms of charging, such specific iCovs have very high precision of mentioned voltage (+ – 50 mV) and current, because the tolerance of the battery to such fluctuations is very small. One such module with charger for lithium batteries in a minimalist version can be found here. This module, for example, doesn’t have PCM – the protection for the battery.

Until now, we haven’t mentioned what does the code “mAh” stands for on the battery. Eg., Our LG battery has 2900mAh or 2.9Ah. This means that our battery could deliver 2.9A for one hour and then it would be completely empty. In this case, discharge would be 1.0c – it can also be measured as the charging – the C mark. For our Samsung battery, for example, it is not advisable to empty it at the 30C because it would very quickly “kill” the battery, ie. reduce its capacity. Therefore,there are batteries designed with specific purposes of rapid discharge. On them you will often find labels such as “20C” or “30C” and you will know that these batteries can be discharged at high currents, and these tags will indicate the maximum current with which you can discharge the battery. You can usually see them on the models with radio management (RC), for example: drones.

Generally, such a battery can be put in the holder without a problem and be used it for whatever you need. Since there is no additional protection to those batteries, like PCM that we mentioned before, if you make a short circuit or pull too much power out of it, you will have a problem. What kind of a problem, see the section below. It is advisable to use the PCM with batteries and often systems that provide for the installation of li-ion batteries on them, have built-in protection. That battery can be used without PCM, but with extra caution.

So, in general, these are the things to be taken into account if there is no protection:
• Do not draw too much power from the battery (Prevent short circuit)
• Do not overcharge the battery (> 4.2V)
• Do not over drain the battery (<3.0V)
• Charge the battery with appropriate charger

Although it may seem fun, it is not when it happens in a place where it should not happen 🙂 Therefore, we must apply the steps of the protection from the previous section!