ULTRA FAST CHARGING OF LITHIUM-ION BATTERIES

J.René van Beek1 and P.H.L. Notten1,2

1Philips Research Laboratories Prof. Holstlaan 4, 5656AA Eindhoven, The Netherlands

2 Eindhoven University of Technology

Den Dolech 2, 5612 AZ Eindhoven, The Netherlands

The limits for ultra-fast charging Li-ion batteries have been investigated. Minimum charging times have been determined in relation to the charge conditions. The impact of ultra-fast charging on the charge current, the charge voltage, temperature and initial Depth-of-Charge (DOC) has been revealed. Li-ion batteries are usually charged according to the well-known CCCV charging method, which implies limitations of both the maximum charge current Imax (CC-mode) and maximum charge voltage Vmax (CV-mode). In the present research the impact of the maximum charge rate was investigated by omitting the conventional charge current limitation. The as-denoted ‘Maxrate’ experiments were performed by enforcing the cells immediately to the CV-mode by employing high charging currents. Conse-quently, the charging current and hence the charge rate is determined only by the internal impedance of the battery, resulting in a minimum charging time. In Fig.1 the maximum charge rate characteristics of a Li-ion battery with a nominal capacity of 1100 mAh are compared with those of conventional charging. The development of the cell voltage and charging current in the standard CCCV-mode is as usual (dashed lines in Fig.1). The ‘Maxrate’ results are represented by the solid lines; charging starts and proceeds only in the CV-mode, as can be concluded from the constant voltage curve (VMaxrate). The charging current (IMaxrate) drops rapidly, within 1 minute, to a 4 A current level and is followed by a more gradual decrease. The impact of a high charging current on the charge build up seems to be surprisingly small, as can be concluded from the coinciding current curves at the end of charging. The capacity build-up versus time for both charging modes is shown in Fig. 2. The dashed line, representing the standard CCCV charging of an empty cell, results in only 16% DOC after 10 minutes. The top curve depicts the non-linear capacity build-up, when the ‘Maxrate’ is applied to an empty cell (0% DOC): the battery is charged ultra-fast and almost 50% of its capacity is gained within 10 minutes. Fig. 1 indicates clearly the relationship between the current and DoC. The charge current decreases with increasing charging level. Therefore, ‘Maxrate’ experiments have been performed not only with empty cells but also with batteries, having a predetermined initial storage capacity. Fig. 2 shows the capacity grow during the first 10 minutes of those cells for which Maxrate charging starts at 10% DoC and 25% DoC. Although the charging rates decline somewhat, the capacity build-up is still much faster than that for standard charging an empty cell (compare solid and dashed lines). Fig. 3 shows the impact of the maximum charging current on the total charging time of an empty cell towards different charging levels. The graph clearly shows that hardly any time profit is gained with maximum charging currents bigger than 4A for a battery of 1100 mAh. Moreover, the minimal time needed to charge towards a desired DoC can be easily derived from

this figure. For example, it takes less than 7 minutes to charge an empty battery till 30% DoC, when Imax is limited to 3A. In addition, results of charging at different voltages levels and at different temperatures will be presented together with the impact of ultra fast charging on, for example, cycle life. It can be concluded that ultra fast (partial) charging of Li-ion batteries can be easily accomplished leading to significant charging time reduction which in turn lead to obvious benefits for “ real life” applications. Maximum charging currents beyond a 3-4 C-rate do, however, no longer contribute to further charging time reductions.

Figure 1: Charge characteristics of a Sony US18500 cell during standard and Maxrate charging.

0

2

4

6

8

0.0 0.5 1.0

Capacity [mAh]

Ch

arg

e C

urr

ent

[A

]

3.5

3.7

3.9

4.1

4.3

Cel

l Vo

ltag

e [

V]

Vmaxrate

Vstandard

Istandard

Imaxrate

10005000

Figure 3: Impact of Imax on the total charging time of an empty Sony US18500 cell towards different DoC.

0

2

4

6

8

10

0 10 20 30

Charging Time [min]

Ch

arg

e C

urr

ent

[A

]

Imax = 3A, 30% DoC charge --> 6.9 min

30% 40%20%10% DoC

50%

Figure 2: Capacity grow of a Sony US18500 cell during standard and Maxrate charging at 0%, 10% or 25% DoC.

0

200

400

600

0 2 4 6 8 10

Time [min]

Ch

arg

ed C

apac

ity

[m

Ah

]

Standard CCCV

Maxrate 0% DoC

10%

25%