My Lithium ion battery is reducing in capacity but takes longer to charge - why?

One question that I often get asked is “As it ages, my Lithium ion battery reduces in capacity but takes longer to charge  - why is this?”

It does seem strange doesn’t it? If the battery capacity is reduced then it is reasonable to think that the charge time would reduce by a proportional amount – after all, Lithium ion batteries are very efficient during charge aren’t they?

Well the answer is to do with battery resistance and the method with which a Lithium ion battery is charged. Lithium ion batteries are charged with what is called ‘CV/CC’ which standard for ‘Constant Voltage’ / ‘Constant Current’ – the battery is first charged with a constant current until the voltage reaches a pre-determined point (this is usually 4.20V for most Lithium Ion cells) and then the voltage is held constant and the current is allowed to taper down. Charge is terminated when the current tapers down to a fixed level. The majority of the charge is integrated during the constant current portion of the charge and less is input while the current is tapering down.

As a Lithium ion battery is cycled (charged and discharged) its available capacity reduces which results in shorter device runtime. At the same time, the internal resistance increases which can be seen as a reduced voltage during discharge, especially at higher load currents. The increase in internal resistance also increases the voltage drop across the cell during charge which means that the time taken too reach the CV (constant voltage) point is reduced, so less capacity is integrated and the remaining charge is input during the less efficient current tapering portion.

 The chart below shows the charge performance of a typical lithium ion battery over 300 cycles. The results are represented as a percentage of the performance on cycle 1. When the battery is new the charge capacity is 100%. After 100 cycles the capacity has dropped to 95% of its initial capacity but the charge time has increased to 104% because the  time taken to reach the CV point has fallen to 90% of the initial time. By the time 300 cycles has been reached the capacity has fallen to 86% and the charge time has risen to 112% due to a CV time of 71%.

 

Different cells may behave differently depending on type and application but this clearly shows why an older Lithium ion battery will have a reduced capacity but take longer to charge than when it was new.

It is worth remembering that temperature also alters the internal resistance of a Lithium ion battery. Colder batteries have a higher resistance so can also take longer to charge than warmer ones. These factors should be considered when designing a battery and charger system.

If you want to discuss this topic further or have a new battery / charger project in the planning please drop me an email.


Happy New Year!
Neil Oliver