Using Lithium-ion Batteries Safely

Just a few years ago, smoking lithium-ion batteries in Boeing’s flagship Dreamliner aircraft did little to allay fears that these very useful power sources can be explosively unpredictable. However, when their limitations are understood, they are the ideal for powering a wide range of electronic and electrical products.

Li-ion batteries were first commercialised in the early 1990s. They are still a popular choice for handheld electronics today, in large part because their energy density is very high, typically twice that of predecessor NiCd technology. For a given capacity, Li-ion batteries can therefore be much smaller and lighter. They have a similar discharge characteristic to NiCd, but each cell has a much higher voltage, typically 3.6V. This means batteries for small devices like mobile phones can use a single-cell battery pack – a NiCd battery pack would need three 1.2-V cells in series.

Li-ion batteries also boast a lack of memory; they don’t need ‘priming’ when new, and no periodic discharge is required to improve the lifetime. They also hardly self-discharge: a major advantage over NiCd.

On the downside, the lifetime of Li-ion batteries today is typically two to three years, whether they are in use or not. Proper storage (controlled temperature, partial charge) can help, and new chemistries are being developed all the time to try and extend their life. Li-ion batteries also have very specific requirements when it comes to charging and discharging to prevent overheating and ensure safety. However, with careful control of charging voltages and currents, they are completely safe to use.

Overcharging in particular can cause damage to the cell, or it can become dangerously unstable. With the right control circuit to carefully regulate voltage and current, this can easily be avoided.

 

Proper Charging Profile

Let’s take a closer look at a battery charging control IC for Li-ion chemistries. The LM3658 from National Semiconductor (now part of TI) can safely maintain a single-cell Li-ion battery operating from an AC wall adapter or a USB power source.

Li-ion batteries are safely charged using a regulated, constant current, then once the final voltage is reached (typically 4.2V), the charger supplies enough current to maintain the final voltage, to prevent over-charging (see LM3658 charging graph below). This is called a constant current-constant voltage (CC/CV) regime. A good charging circuit will make the transition between the constant current and constant voltage phases as smoothly as possible to prevent damage to the battery.

 

Figure 1: Charging cycle of the LM3658

 

For Li-ion batteries, the ideal charging current is equal to 1C, where C is the amount of current the battery can supply for one hour (i.e. for a 750mAh battery, C is 750mA). The LM3658 can supply battery-charging currents up to 1000mA. Charging at any higher current than 1C can damage the battery. Charging at a lower current is possible, perhaps when plugged into a USB source, but it should still be held constant. What happens in this case is that the charging circuit spends a lot longer in the constant current phase, and the cell is almost completely charged by the time the constant voltage phase is reached.

If the battery is very flat to start with, a pre-qualification mode applies a very small current of 50mA initially to stop the cell overheating, until the battery reaches its full voltage. Then the CC/CV regime can begin.

Once the battery is charged, a top-up cycle starts (‘top-off’ mode), which helps make sure the battery reaches its full capacity without over-charging. Overcharging by more than 1% can result in failure, but undercharging by 1% results in reduced capacity (undercharging by only 100mV, which is within 2.4% for a 4.2-V battery, represents a 10% reduction in capacity). Charging during this mode therefore has to be carefully controlled to ensure the battery is always charged to within 1% of capacity.

Following top-up, charging stops and a maintenance mode monitors voltage until it drops sufficiently to trigger the charge cycle again.

Several built-in timers check that all parts of the charging cycle are not taking longer than it should, as that could indicate a fault, and lead to dangerous over-charging.

The IC also has a temperature sensor to guard against temperatures outside the battery’s safe range. An external fuse is typically also used in the battery pack, which breaks if the battery is ever exposed to a theoretical very high temperature (say, 100°C), after which point, the battery is unusable.

See all battery charge controller IC available from RS here.