Battery users often wonder why an aged battery takes so long to charge when compared with a newer one. In this article, we will try to simplify this for the common reader using some simple analogies. To elaborate this we have used the graphs by Battery University.

Li-ion batteries do, in fact, take longer to charge as they age, even when there is little to fill. This is referred to as “old-man syndrome.” A new Li-ion battery with a capacity of 100 percent and an older battery yielding just 82 percent of its capacity are compared in Graph 1 to show how long they take to charge. The charging time for each is about 150 minutes.

Graph 1: Charging of New and aged Li-ion batteries.

Each battery pack charges for about 150 minutes. When fully charged, the new battery has a capacity of 1,400 mAh (100%) whereas the old battery can only reach 1,150 mAh (82%).

The voltage increases dramatically during Li-ion charging, much like when you use a rubber band to pull a weight. Graph 2 shows that the new pack has a far higher “hunger” and can take in more “food” before reaching the 4.20V/cell voltage limit than the old Li-ion, which reaches V Limit in Stage 1 after just about 60 minutes. To use a rubber band metaphor, the new battery has less slack than the old one, thus it can take more charge before it becomes saturated. The older cell reaches 4.20V/cell in 60 minutes while the new Li-ion battery requires 90 minutes to fully charge.

Graph 2: Charge times of a new and aged Li-ion in Stage 1

As the current trails from the fully regulated current to around 0.05C to trigger-ready mode, Graph 3 shows the various saturation periods in Stage 2. On a brand-new battery, trailing is brief; on an older pack, it is longer. This explains why an older Li-ion battery with less capacity takes longer to charge. An example would be a teenage athlete finishing a sprint with little to no slowing down, as opposed to an older person who runs out of breath and starts to walk, increasing the time it takes to get there.

Graph 3: Saturation times of new and aged Li-ion in Stage 2  before switching to ready

The new battery lasts longer and has a shorter current trail. Lithium-ion batteries frequently experience the decrease of charge transfer capacity as they age. This results from the electrodes developing passive materials that obstruct the flow of free electrons. As a result, the electrodes’ porosity is reduced, their surface area is shrunk, their ionic conductivity is decreased, and their migration resistance is increased. Ageing is a permanent occurrence that cannot be stopped.

The charge signature gives important Li-ion health indicators. A good battery absorbs the majority of the Stage 1 charge before it reaches 4.20V/cell, and the Stage 2 lagging is minimal. Li-ion batteries that “lack hunger” are partially charged; batteries that have unusually long lagging periods have poor capacity, high internal resistance, and/or high self-discharge.

Stage 1 and Stage 2 can be compared using algorithms that take capacity and state-of-charge into account. Setting acceptance limits makes it possible to spot anomalies like limited capacity and increased self-discharge. Chargers with algorithms that will perform these diagnostics are being created by various researchers across the world.