Lithium-ion Batteries

Caution

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A lithium-ion (or li-ion) battery is a rechargeable battery that uses the intercalation of lithium ions between anode and cathode to generate electricity, and the reverse of this process to recharge.¹ They are nominally around 3.6-3.7 V per cell, with full charge being reached at about 4.2 V and a flat battery dropping to 3.0 V.

Lithium-ion batteries can be considered a family of battery technologies including:

1. Traditional lithium-ion
2. Lithium polymer
3. Lithium-iron phosphate

The below table shows some of the basic properties of lithium-ion batteries:

Some of the basic properties of lithium-ion batteries.

Property	Value
Specific energy	100-265 Wh/kg (0.360-0.954 MJ/kg)1
Energy density	250-693 Wh/L (0.90-2.49 MJ/kg)1
Specific power	250-340 W/kg1

Self Discharge

Lithium-ion batteries experience about 1-5% of self-discharge per month. This means that they lose 1-5% of their remaining energy each month, even when there is no external power draw. Note that it is a percentage of their remaining energy, not a percentage of their maximum energy. Assuming no other circuitry is drawing current (even battery protection ICs!), this means the battery state-of-charge decays exponentially over time.

State of charge over time for a lithium-ion battery starting at 100 %, plotted for 1 %, 3 % and 5 % monthly self-discharge rates. Because each month’s loss is a percentage of the remaining charge, the decay is exponential ($SoC(t) = 100 \cdot (1 - r)^t$).

For a quality battery, the percentage is often quoted around 1-2% per month. You also have to take into account any battery protection circuitry which adds additional discharge current (this is usually a fixed current draw rather than an exponentially decaying drop).²

Lithium-ion batteries have a better self-discharge rate than other common rechargeable chemistries. For example, conventional NiMH (nickel metal-hydride) batteries have a self-discharge rate of about 30% per month!³

Charging

The common way to charge a lithium-ion battery is to follow these steps:

1. The pre-charge stage, which is only applicable if the battery is really flat (less than 2.8V). The battery is charged with a constant-current, usually 10% of current used in the next stage (sometimes called the fast charge).
2. Charge the battery with a constant current (CC), at a rate of around 0.5-1C.⁴
3. Once the battery reaches a certain voltage (typically 3.9-4.2V), we switch from CC charging to CV charging. We maintain this voltage while the current begins to decrease.
4. Stop charging once the current drops to approximately 0.1C.

Caution

It is not healthy to store a lithium-ion battery for a long time at 100% charge. Storing them for a long time at 40-50% capacity appears to provide the best lifetime.⁵

1C is the current required to drain the battery in 1 hour, e.g. for a 500mAh battery, 1C is 500mA. Most battery charger ICs will automatically perform the above steps for you.

Pre-charge can also be confused with the pre-charge step that occurs in the manufacturing process after the cells have been filled with electrolytes.

Protection Circuit Modules (PCMs)

Protection circuit modules (PCMs — not to be confused with Pulse Code Modulation) are circuits designed to protect Lithium batteries (and other chemistries) from overcharge, over-discharge and over current conditions. They normally feature a analogue switch (i.e. two back-to-back MOSFETs) to disconnect the load from the battery cell during a fault condition. They reconnect the load after the fault condition is removed and/or after a certain duration.

As shown in the image below, they are commonly integrated into the battery itself, so that the cell is protected from the moment it leaves the factory floor.

A photo of a small 500 mAh 3.7 V lithium-ion battery with the kapton tape removed and the PCM exposed (right battery). The PCM consists of three components, one PCM IC and two MOSFETs (connected back-to-back to be able to disconnect the cell in both directions).

TI’s BQ Range

Texas Instruments sells a large number of battery charger ICs under the prefix BQ. Some are I2C controllable, others are standalone. Some have hot/cold temperature monitoring and others (typically with a J after the numerical code in the part name) have JEITA temperature monitoring.

Four phases:

1. Battery short
2. Pre-conditioning
3. Constant current (CC)
4. Constant voltage (CV)

Richtek RT9481, RT9485

The RT9485 has an on-board MCU.

Footnotes

1. Wikipedia (2023, Jul 27). Lithium-ion battery. Retrieved 2023-07-31, from https://en.wikipedia.org/wiki/Lithium-ion_battery. ↩ ↩² ↩³ ↩⁴

2. Bonnen Batteries. Lithium Battery Self-Discharge Explained: What Engineers and Buyers Must Know. Retrieved 2026-05-26, from https://www.bonnenbatteries.com/lithium-battery-self-discharge-explained-what-engineers-and-buyers-must-know/. ↩

3. Wikipedia. Self-discharge [wiki]. Retrieved 2026-05-27, from https://en.wikipedia.org/wiki/Self-discharge. ↩

4. Battery University (2021, Oct 25). BU-409: Charging Lithium-ion. Retrieved 2023-08-13, from https://batteryuniversity.com/article/bu-409-charging-lithium-ion. ↩

5. Battery University (2021, Oct 21). BU-702: How to Store Batteries. Retrieved 2023-08-13, from https://batteryuniversity.com/article/bu-702-how-to-store-batteries. ↩