High duty use and fast charging of power tool battery packs: A simulation based study to improve cooling strategies

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In the fast growing, but also highly competitive market of battery-powered power tools short charging times are essential to achieve a high customer acceptance. To prevent loss of performance and rapid charging capability due to cell aging the knowledge of proper use conditions in everyday practice is important.
In this study, we will present a model of an 18V power tool battery which was developed to be able to evaluate different application scenarios of practical relevance such as high duty use and subsequent fast charging. The simulation comprises battery models of commercially available 21700 cells as well as heat transfer models to highlight the influence of different cooling systems. It allows to quickly asses different scenarios and main influencing parameters and thus to develop new cool down and fast charging strategies adjusted to different cooling systems and areas of application.
One scenario of high interest for power tool manufacturers are use-charging cycles. During high duty use phases, the battery packs can reach critical temperatures that make downtimes before fast charging indispensable. From a user’s perspective, this cooling step significantly prolongs the total charging times. We investigated different cooling systems such as phase change material to reduce these downtimes. Specifically, the following use case was applied: The fully charged battery pack is discharged with 25A/cell (step A, resembles e.g. working with an angle grinder). After cooling down to an acceptable temperature to start charging (step B, cool down step), a charging step (6A, step C) follows. The case study comprises four different cooling systems: (I) Non-convective air (reference), (II +III) two high heat transfer polymer and (III) latent heat storage materials (using phase change for cooling). In doing so, the effect of various material properties (e.g. thermal conductivity, latent heat) as well as operation conditions (e.g. ambient temperature) were investigated.
By reviewing different scenarios and applying a key performance indicator optimization, a reduction of the total charging time can be demonstrated. Compared to the reference scenario, shortening of the use-charging cycle of 32% can be achieved using the investigated phase change material for passive cell cooling. By absorbing a significant amount of heat, the phase change material is additionally able to reduce the maximum temperature reached during use phase by 23°C.

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