Getting the best from Lithium Ion batteries

what to do and what not to do

Lithium Ion (Li-ion) rechargeable batteries are all around us – in mobile phones, tablets, PCs and a host of other portable electronic devices. To get the best from them demands a basic understanding of their characteristics, and how they should be stored and charged.

 

Li-ion Characteristics

Lithium-ion batteries have been popular since the 1990s, when their superior properties ousted NiCad as the battery of choice. NiCad had problems with memory; the batteries needed to be primed before use and had to be fully discharged in each cycle to ensure maintain good capacity and lifetime. NiCad also suffered from self-discharge if left for long periods of time. Li-ion fixed these issues and, along with these advantages, brought higher energy density and higher voltage; 3.6V per cell compared to 1.2V for NiCad. The combination of higher energy density and high voltage means the batteries could be much more compact; single Li-ion cells could be used where NiCad previously needed 3 cells to make up the same voltage. This made them very popular and they are still the number one choice for handheld and portable devices today.

 

Storing Li-ion Batteries

Correct storage of Li-ion batteries helps to maximise the batteries’ lifetime to the expected two to three years and maintain full capacity.

While the batteries are safe to be stored for a short time between -20°C and +60°C, for long term storage, -20°C to +25°C is recommended. However, the optimum storage temperature is +15°C; commercial storage of Li-ion batteries therefore requires a controlled temperature environment.

Contrasting with lead-acid batteries (which are stored fully charged), and NiCad batteries (which can be stored fully discharged), Li-ion batteries should be stored partially charged to maintain their capacity. The optimum charge level for storage is 40% in order to minimise ageing, which can result in reduced capacity. Although self-discharge is relatively low, if the batteries are stored for very long periods of time (many months), they will need to be periodically checked and topped up to 40% if required.

The cell voltage during storage should stay between 2.0 and 4.1V. If the voltage drops below 2.0V, copper shunts can form inside the batteries which can speed up self-discharge or cause shorts; subsequent charging of a battery in this state could be dangerous. Li-ion batteries which have dipped below 2.0V for more than a week should be disposed of.

 

Charging Li-ion Batteries

Li-ion batteries have very specific requirements when it comes to charging. Done improperly, charging can lead to overheating, which can lead to explosion or fire, so charging properly is of paramount importance for safety reasons. Using a battery charger specifically designed for Li-ion batteries is essential.

The Li-ion charge cycle is different to how other batteries need to be charged. The most effective, safe charging scheme is called constant current/constant voltage (CC/CV). In the first charging stage, a constant current is applied until a maximum voltage is reached, then in the second stage, enough current is supplied to maintain that voltage. This ensures the battery is not overcharged and will not overheat. In the first stage, the ideal charging current is equal to the amount of current the battery can supply for one hour (ie, for the 2200mAh Enix Li-ion battery pictured below, that would be 2.2A), but do check the manufacturers’ data sheets for confirmation.

Maximum charge voltage for Li-ion batteries is usually 4.2V (you can see the max charge voltage on this Ansmann Li-ion battery pack).

 

When the battery is fully charged, a top-up cycle begins, which maintains the battery to within 1% of full. Overcharging by more than this can damage the battery, while letting the battery charge fall too low before topping up can reduce the battery’s capacity.

Some battery packs may include protection circuitry to guard against charger failure that could lead to improper charging. This Li-ion battery pack from LG contains a protection device which limits the battery voltage to between 4.35V and 2.7V for each cell, for safety. It also limits battery current to 6.7A (+/- 1A). This circuit is not to be used as a form of charge control, merely a safety measure.

 

 

RS stocks Li-ion battery packs from Ansmann, Enix and LG, amongst others, and stores them in the proper environment to ensure long life and capacity.