Charging the Lithium Ion Battery

The Li-ion charger is a voltage-limiting device similar to that of the SLA charger. The main differences of the Li-ion charger are higher voltage per cell (nominal cell voltage of 3.6V vs. 2V on the SLA), tighter voltage tolerance and the absence of trickle or float charge at full charge.

Whereas the SLA has some flexibility in terms of voltage cutoff, the manufacturers of the Li-ion cells are very strict about the voltage choice. The voltage threshold of the Li-ion with the graphite electrode is 4.10V whereas the coke electrode is set to 4.20V with a tolerance on both types of +/- 0.05 volts per cell. New Li-ion types are in development that will probably require different voltage limits. Chargers for these batteries will need to be capable of adapting to the correct charge voltages.

Since higher voltage thresholds provide higher capacities, it is in the manufacturer’s best interest to choose the highest voltage threshold possible without affecting safety. This voltage level, however, is affected by temperature and the threshold is set low enough on purpose to tolerate higher temperature charge. Tampering with any Li-ion charger is not recommended.

On chargers and battery analyzers that allow adjustment of the voltage thresholds, the correct setting must be observed when servicing any Li-ion battery. Most battery manufacturers do not specify which version Li-ion is used. If the voltage is set incorrectly, a coke version will yield lower capacity readings and a graphite one will be slightly overcharged. At a moderate temperature, no damage occurs and the lower discharge voltage does not harm the graphite version battery. The table below compares the coke electrode to the graphite electrodes.

  Coke Graphite-1 Graphite-2
Peak charge voltage 4.20V 4.10V 4.20V
End-of-discharge voltage 2.50V 3.00V 2.50V
Recommended Charge Rate 0.2C 0.2C - 0.5C 0.2C - 0.5C
Temperature rise on charge 5 degrees C to 8 degrees C 2 degrees C to 3 degrees C 2 degrees C to 3 degrees C
Typical Manufacturers Sony, Asahi-Toshiba Sanyo, Panansonic, Hitachi Maxell, Saft Asahi - Tobshiba, Pansonic, Moli, Sony

Performance characteristics of Li-ion cells with coke and graphite electrodes

The charge time of the Li-ion is about 3 hours and the battery remains cool during charge. Full charge is attained after the voltage has reached the upper voltage threshold and the current has dropped and leveled off to a low plateau.

Increasing the charge current on a Li-ion charger does not shorten the charge time by much, especially on the coke type. Although the voltage peak is reached quicker with high current, the topping charge will take longer. the figure below shows the charge stages of a Li-ion charger. Observe the similarities with the SLA charger.

Charge Stages of a Li-ion Battery

A more basic charge method terminates the charge as soon as the voltage level is reached. This charger is quicker and simpler than the two-stage charger but can only charge the battery to a 70% capacity level.

No trickle charge can be applied because the Li-ion is unable to absorb any overcharge. Trickle charge could cause plating of the metallic lithium, a condition that would make the cell unstable. Instead, a brief topping charge should be applied from time to time to compensate for the small amount of self-discharge the battery and its protective circuit consume.

Commercial Li-ion batteries contain several built-in protection devices. Typically, a fuse opens if the charge voltage of any cell reaches 4.30V or the cell temperature approaches 100°C (212°F). A pressure switch in each cell stops the charge current if a certain pressure threshold is exceeded; and internal voltage control circuits cut off the battery at low and high voltage points.

Most manufactures do not sell the Li-ion cells by themselves but make them available in a battery pack, complete with protection circuit. This precautionary procedure is understandable when considering the danger of explosion and fire if the battery is charged and discharged beyond its safe limits.

A potential problem can occur if commercial battery packs reserved for NiCd and NiMH chemistries are fitted with the Li-ion cells. Such battery packs may end up on the wrong charger and cause danger if the pack is not properly protected against charging on a wrong charger. Manufacturers are asked to make the charge terminals of the Li-ion types incompatible with their nickel-based cousins.

Non-rechargeable lithium batteries have found a strong market niche in applications such as still cameras, watches and small electronic devices. Because of their long shelf life and high energy density, the Lithium batteries are also used for military applications and emergency response teams.

Warning: Never try to charge a non-rechargeable lithium battery! Attempting to charge these batteries can cause explosion and fire which spreads toxic material that can inflict injury and damage equipment.

Important: In case of rupture, leaking electrolyte or any other cause of skin or eye exposure to the electrolyte, immediately flush with water. If eye exposure, flush with water for 15 minutes and consult physician.

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