The care and feeding of your embedded design's Li-ion battery subsystem

Fran Hoffart

June 24, 2008

Fran HoffartJune 24, 2008

Choosing a charger
Although a battery charger has no control over a battery's depth-of-discharge, discharge current and battery temperature, all of which have an effect on battery life, many chargers have features that can increase battery life, sometimes dramatically.

A battery charger's role in extending battery lifetime is mainly determined by the charger's float voltage and charge termination method.

Many Linear Technology Li-ion chargers feature a ±1 percent (or lower) fixed float voltage of 4.2V, but there are some offerings in 4.1V and 4.0V, as well as adjustable float voltages. Table 2 below lists battery chargers that feature a reduced float voltage that can increase battery life when used to charge a 4.2V Li-ion battery.

Table 2: Battery chargers that feature a reduced float voltage that can increase battery life when used to charge a 4.2V Li-ion battery are listed.

Battery chargers that do not offer lower float voltage options are also capable of increasing battery life. Chargers that provide minimum charge current termination methods (C/10 or C/x) can provide a longer battery life by selecting the correct charge current level at which to end the charge cycle.

A C/10 termination level will only bring the battery up to about 92 percent capacity, but there will be an increase in cycle life. A C/5 termination level can double the cycle life although the battery charge capacity drops even further to approximately 85 percent.

Table 3 below contains a number of chargers that provide either C/10 (10 percent current threshold) or C/x (adjustable current threshold) charge termination mode.

Table 3: Battery chargers that provide minimum charge current termination methods (C/10 or C/x), such as listed above, provide a longer battery life by selecting the charge current level at which to end the charge cycle.

Longer runtime or longer battery life?
With present battery technology and without increasing battery size, the answer is no. For maximum runtime, the charger must charge the battery to 100 percent capacity.

This places the battery voltage near the manufacturer's recommended float voltage, which is typically 4.2V ±1 percent. Unfortunately, charging and maintaining the battery near these levels shortens battery life.

One solution is to select a lower float voltage, which prohibits the battery from achieving 100 percent charge, although this would require a higher capacity battery to provide the same runtime. Of course, in many portable products, a larger sized battery may not be an option.

Also, using a C/10 or C/x minimum charge current termination method can have the same effect on battery life as using a lower float voltage. Reducing the float voltage by 100mV will reduce capacity by approximately 15 percent, but can double the cycle life.

At the same time, terminating the charge cycle when the charge current has dropped to 20 percent (C/5) also reduces the capacity by 15 percent and achieves the same doubling of cycle life.

Figure 4: Most Li-ion batteries use either a petroleum-based coke material or graphite. The latter produces a flatter discharge voltage, then drops quickly.

Discharge voltage drop
As expected, during discharge, the battery voltage will slowly drop. The discharge voltage profile vs. time depends on a number of factors, including discharge current, battery temperature, battery age and the type of anode material used in the battery.

Presently, most Li-ion batteries use either a petroleum based coke material or graphite.The voltage profiles for each are shown in Figure 4 above. The more widely used graphite material produces a flatter discharge voltage between 20 percent and 80 percent capacity, then drops quickly near the end, whereas the coke anode has a steeper voltage slope and a lower 2.5V cutoff voltage.

The approximate remaining battery capacity is easier to determine with a coke material by simply measuring the battery voltage.

Parallel connections
For increased capacity, Li-ion cells are often connected in parallel. No special requirements are needed, other than the batteries should be the same chemistry, manufacturer and size.

Series connected cells require more care because cell capacity matching and cell balancing circuitry is often required to assure that each cell reaches the same float voltage and the same level of charge.

Connecting two cells that have individual pack protection circuitry in series is not recommended because a mismatch in capacity can result in one battery reaching the over-voltage limit, thus opening the battery connection. Multicell battery packs should be purchased assembled with the appropriate circuitry from a battery manufacturer.

The lifetime of a Li-ion battery is determined by many factors of which the most important are battery chemistry, depth of discharge, battery temperature and battery capacity termination level.

<>Charging a battery to the manufacturer's suggested 100 percent capacity level will provide the stated number of full charge/ discharge cycles. Applications requiring increased battery lifetime will benefit greatly by selection of a charger that allows charging to less than 100 percent capacity.

This is achieved by selecting a battery charger that features a lower float voltage or one that terminates earlier in the charge cycle.

Fran Hoffart is an Applications Engineer at Linear Technology Corp.

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