Solutions for brazing challenges in battery coolers – Part 1
Leszek Orman, Sebastian Gutmann, Solvay Fluor GmbH, Germany
Thermal management of electric vehicles is faced with challenges, as batteries need to be kept at a defined temperature range between 15°C and 35°C in order to function at peak performance throughout the life of a vehicle. The manufacture of such battery cooling systems involves controlled atmosphere brazing technology using noncorrosive flux. OEMs develop different battery cooler designs that require adapted fluxing methods. In this paper, selected cases of fluxing and brazing challenges in the manufacturing of battery cooling systems will be presented. Brazing experts will provide guidelines for fluxing and brazing of battery coolers.
Batteries currently used in electrical vehicles [EV] can operate within temperature between –30°C and 50°C; however at temperature extremes, their performance is reduced and also battery life time suffers quite significantly. At high temperatures above 70°C, a chain reaction can occur causing destruction of the battery pack. Also during fast charging, batteries must be cooled down. High current going into the battery produces heat that must be removed. EV batteries perform the best at temperature between 20°C and 25°C .
EV batteries can be cooled using air cooling or liquid cooling. Liquid cooling is the method of choice to meet modern cooling requirements.
Air cooling systems use air to cool the battery and exists in the passive and active forms. Passive air cooling uses air from the outdoor or from the cabin to cool the battery. It is usually limited to a few hundred watts of heat dissipation. Active air cooling gets its air intake from an air conditioner, which includes an evaporator and a heater to control the air’s temperature. It is usually limited to 1kW of cooling and can be used to cool or heat the cabin.
Liquid cooling is the most popular cooling technology. It usually uses a liquid coolant such as water mixed with ethylene glycol and a corrosion inhibitor. Components carrying the liquid prevent direct electrical contact between the cells and the liquid coolant.
Like in air cooling, passive and active systems exist. Active liquid cooling is more complex and expensive but provides better performances for propulsion and charging power and the cooling results are much more reliable, thus nowadays, most batteries for EV are liquid cooled using active cooling, despite the fact that the active cooling is more expensive since it includes more components, such as a heat pump, a heat exchanger, a circulating pump, valves, and multiple temperature sensors .
Types of EV battery water coolers
An aluminium component is surrounding the battery cells. The component is filled with liquid coolant, which carries the heat to another location, such as a radiator. There is no direct electrical contact between the cells and the liquid coolant. The cooling component is usually located at the bottom of the car cabin.
Figure 1. Exemplary location of the battery cooling system in an EV 
One type of the cooler design is based on Multiport Extrusion tubes [MPE]. It has been patented by Tesla . In this design the flat MPE is slightly bent around battery cells. Brazing is required only for joining the MPE tube with the collector. Such joint is quite easy to braze and so far no information about brazing problems connected with this type of battery coolers have been known to the authors.
Figure 2. Concept of battery cooler made with MPE tubes 
A second type of the battery cooler design is based on two flat plates usually of large size (up to about 2 meters). The plates are brazed together by some kind of inner dimples or dikes formed in one plate. The dimples/dykes provide resistance to inner pressure of the coolant and make the coolant flow turbulent.
Figure 3. EV battery cooler design 
Figure 4. Different designs of connectors between the two plates of an EV battery cooler 
2. Methodology of brazing fault analysis
Brazing as any other technical process is controlled by several parameters. These are: temperature, flux load and its uniformity, furnace atmosphere, joint geometry, filler metal availability, cleanliness. When each of these parameters is within the optimal limit, 100% successful brazing is guaranteed. Thus, when investigating a brazing fault one should concentrate on checking the values of the above parameters during brazing. This is not a straightforward process since there are no technical means to measure them directly during the brazing process (except the furnace atmosphere). It means that by examining the faulty parts one should define which of the brazing parameter was not met. This examination could be quite simple (just visual observation) or may require using more sophisticate technics like optical and/or electron microscopy. 
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