Charging the lithium-ion (Li-ion) and lithium-polymer (Li-Pol) battery

The Li-ion charger is a voltage-limiting device similar to the sealed lead acid (SLA) battery charger. The main difference between them is that the charger for li-ion batteries has higher voltage per cell (rated voltage of Li-ion cell is 3,6V in comparison with 2V per SLA battery), tighter voltage tolerance and absence of trickle or float charge at full battery charge.

While the lead acid battery offers some flexibility in terms of voltage cut-off, manufacturers of Li-ion cells are very strict on setting the correct voltage. The charge voltage limit of li-ion cells with graphite electrode is 4.10V/cell, with coke electrode - 4.20V/cell, the tolerance on batteries of both types is +/-0.05V/cell. For newly developed and designed batteries, most probably, the end-of-charge voltage threshold will be another one. Thus, the battery chargers should be adapted to the required charge voltage.

The higher voltage threshold value provides a higher battery capacity. So it is in the interest of manufacturer to select the highest possible voltage threshold without compromising safety. However, this threshold is influenced by the temperature of the battery and is set low enough to allow an elevated charge temperature. User intervention in any Li-ion charger is not recommended.

In battery chargers and analyzers that allow you to change the voltage threshold, the correct setting of this threshold is very important when maintaining any Li-ion type batteries. However, most manufacturers do not indicate the type of Li-ion battery. If the voltage is set incorrectly, the coke battery will give lower capacity, and the graphite will be a little bit overcharged. At moderate temperature, no damage occurs, and a lower discharge voltage will not damage the graphite battery. Below is the table that allows comparing variants of battery elements with coke and graphite electrodes.

Parameters Manufacturing technology
Coke Graphite -1 Graphite -2
Maximum charge voltage 4,20 V 4,10 V 4,20 V
Discharge voltage 2,50 V 3,00 V 2,50 V
Recommended charge current (С - rated capacity) 0,2C 0,2C — 0,5C 0,2C — 0,5C
Temperature increase at charge From 5°C to 8°C From 2°C to 3°C From 2°C to 3°C

The charge time of Li-ion batteries is approximately 3 hours and the battery remains cool during charge. The full charge is achieved after the voltage reaches the upper voltage threshold and the current is reduced to some given low level.

Increasing the charging current in Li-ion charger does not significantly reduce the charge time, especially for coke execution. Although the battery reaches the voltage peak quicker, still longer charge is better. The figure shows the stages of charge of Li-ion battery. Observe the similarity with the SLA charger.

During the basic charging method, the charge ends as soon as the cut-off voltage is reached. Such charger is faster and simpler than a two-stage charger; however, it can fill the battery to about 70% capacity.

A continuous trickle charge is not used because Li-ion can not accept overcharge. A continuous trickle charge would cause plating of metallic lithium and lead to element instability. Instead, a short-term topping charge can be used to compensate for the small self-discharge the battery and its protective circuit consume.

Commercial li-ion batteries contain several built-in protection circuits. Typically, a fuse is triggered if the charge voltage reaches 4.30V/cell or the cell temperature approaches 100°C (212° F). A mechanical pressure switch in each cell interrupts the charge if a safe pressure threshold is exceeded; internal voltage control circuit cuts off the battery at low and high voltage points.

The majority of manufactures of li-ion cells sell them only assembling in battery packs including battery protection devices. This is because of the possible risk of explosion and ignition if the battery is charged in unsafety environment.

A potential problem may arise if battery housings reserved for NiCd and NiMH batteries are adapted to Li-ion cells. Such batteries may be charged on non-resident chargers but turned to be a hazard if the chargers do not have special protection. It is recommended to produce Li-ion battery terminals incompatible with NiCd and NiMH rechargeable batteries.

Non-rechargeable lithium batteries have a significant market share in video cameras, watches, and other small electronic devices. Due to their long service life and high energy density, the lithium batteries are employed in military equipment and emergency devices.

Precautions: Never try to charge a non-rechargeable lithium battery! An attempt to charge these batteries can cause an explosion and ignition that spreads poisonous substances and can cause damage to equipment.
Safety measures: In case of rupture, leaking electrolyte and contact with skin flush with water immediately. If the electrolyte gets in your eyes, flush with running water for 15 minutes and see a doctor immediately.

Additional information:

Li-ion (Li-polymer) batteries initially charge at constant current to 4.2V/cell, then at constant voltage up to charge rate 0,05C. After that, the charge completely stops. The following figure shows the typical characteristics of Li-ion and Li-polymer batteries quick charge depending on the charge current.

 Pic. Typical characteristics of fast charging of Li-ion (Li-polymer) batteries