Charging Li-ion batteries

The most correct way to charge lithium batteries is to charge in two stages. This method is used by Sony in all its chargers. Despite the more complex charge controller, it provides a more complete charge of li-ion batteries, without reducing their service life. Here we are talking about a two-stage charge profile of lithium batteries, abbreviated as CC/CV (constant current, constant voltage). There are also options with impulse and step currents, but in this article they are not considered.

1. At the first stage , a constant charge current should be provided. The magnitude of the current is 0.2-0.5C. For accelerated charging, an increase in current up to 0.5-1.0С is allowed (where C is the battery capacity).
For example, for a battery with a capacity of 3000 mA / h, the nominal charge current at the first stage is 600–1500 mA, and the accelerated charge current may lie within 1.5–3A.
To ensure a constant charging current of a given value, the charger (charger) circuit must be able to raise the voltage at the battery terminals. In fact, in the first stage, the charger works like a classic current stabilizer. At the moment when the voltage on the battery rises to 4.2 volts, the battery will gain approximately 70-80% of its capacity (the specific value of the capacity will depend on the charging current: with accelerated charging it will be slightly less, at nominal – a little more). This moment is the end of the first stage of the charge and serves as a signal for the transition to the second (and last) stage.
2. The second stage of the charge is the charge of the battery with a constant voltage, but gradually decreasing (falling) current.
At this stage, the charger maintains a voltage of 4.15–4.25 volts on the battery and monitors the current value.
As you type capacity, the charging current will decrease. As soon as its value decreases to 0.05-0.01С, the charging process is considered to be finished.

During the second stage of the charge, the battery manages to gain about 0.1-0.15 more of its capacity. The total battery charge in this way reaches 90-95%, which is an excellent indicator.
We considered two main stages of charge. However, coverage of the issue of charging lithium batteries would be incomplete if one more charge stage was not mentioned – the so-called. precharge
Preliminary charge stage (precharge) – this stage is used only for deeply discharged batteries (below 2.5 V) to bring them to normal operating mode.
At this stage, the charge is provided by a constant low current until the voltage on the battery reaches 2.8 V.
The preliminary stage is necessary to prevent swelling and depressurization (or even an explosion with a fire) of damaged batteries, for example, having an internal short circuit between the electrodes. If a large charge current is immediately passed through such a battery, it will inevitably lead to its warming up, and then how lucky.
Another advantage of precharge is preheating of the battery, which is important when charging at low ambient temperatures (in an unheated room during the cold season).
Intellectual charging should be able to control the battery voltage during the preliminary charge stage and, if the voltage does not rise for a long time, make a conclusion about the battery failure.
All stages of the lithium-ion battery charge (including the pre-charge stage) are schematically depicted in this graph:


Exceeding the rated charging voltage by 0.15V can shorten the battery life time by half. Decreasing the charge voltage by 0.1 volts reduces the capacity of a charged battery by about 10%, but significantly extends its service life. The voltage of a fully charged battery after removing it from the charger is 4.1-4.15 volts.

1. How can I charge a li-ion battery (for example, 18650 or whatever)?
The current will depend on how quickly you would like to charge it and can lie in the range from 0.2 to 1C.
For example, for a battery of size 18650 with a capacity of 3400 mA / h, the minimum charge current is 680 mA and the maximum charge is 3400 mA.
2. How long does it take to charge, for example, the same 18650 rechargeable batteries?
The charge time directly depends on the charge current and is calculated by the formula:
T = C / I hours.
For example, the charge time of our battery with a capacity of 3400 mA / h with a current of 1A will be about 3.5 hours.
3. How to charge the lithium-polymer battery?
Any lithium batteries charge the same. It does not matter whether it is lithium-polymer or lithium-ion. For us, consumers, there is no difference.

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