Pressure control valve for large-format battery systems

Paul Boughton
The number of electric cars on the world's roads is continually rising. Moreover, car manufacturers are launching an increasing number of hybrid vehicles on the market. In industrial applications too, lithium batteries and other electrochemical energy storage mechanisms are gaining in importance, for example as back-up systems for evening out peaks and troughs in network supply and for the temporary storage of energy from renewable sources.
Regardless of whether they are used in mobile or stationary applications, such battery systems have to meet the highest safety requirements. Freudenberg Sealing Technologies has now developed a brand new pressure control valve for the housing of large-format lithium battery systems. This valve ensures that in the event of overcharge or a short circuit in an individual battery cell, the resulting flammable gas can be released from the battery housing quickly and in a targeted manner, thereby ensuring that the housing is not damaged in any way. To this end, the valve opens, releasing the pressure from the housing.
Compared with conventional pressure-relief systems - such as rupture disks or solenoid valves, which are controlled by pressure sensors - this product from Freudenberg offers a variety of advantages, above all maximum functional reliability. Freudenberg, the Technology Specialist, has already produced and extensively tested its first small series. The elastomer component has been designed to withstand temperatures between -40 and 80°C and functions reliably even when soiled.
The pressure control valve consistently seals the battery system against standard pressure fluctuations of ±0.2 bar – the kind of fluctuations that occur when a car travels up or down a mountain or in and out of a tunnel. In other words, such pressure fluctuations do not trigger the valve, which complies with the stipulated IP rating for automobiles, IP 67. On the other hand, critical inner excess pressure levels of approximately 0.5 bar or more, which are caused by damage to either one or more battery cells, immediately trigger the safety mechanism.
The innovative detail of the valve is the asymmetric cross section of its sealing cap, which is securely attached to the battery housing by a rebound strap. The slight projecting end on the cap side facing away from the fastener ensures that the valve opens at this very point and no other. The radial angle can be adjusted to set the "opening pressure" to between 0.4 and 0.8 bar.
With its simplicity and maximum functional reliability, the pressure control valve reduces the weight and costs of large-format battery systems. It is made of 70EPDM. This material is chemically resistant to leaking electrolyte and does not swell (or at least only negligibly). With the help of protective components, the valve is also capable of withstanding the impact of stones or cleaning processes that involve jets of water. For applications where higher thermal resistance levels are required, versions in fluororubber or silicone rubber are also conceivable.
The elastomer valve has two other advantages: batteries featuring the Freudenberg valve are suitable for transportation by air. To this end, the valve can be opened manually so that its release function is not triggered by the rapid drop in ambient pressure during the aircraft's ascent. Secondly, should damage occur, the opened valve is not damaged and can be reattached. This protects the inside of the battery from environmental influences (water in particular) and facilitates the safe transportation of the battery for recycling.
A 'permeation-open' component can also be attached to the pressure control valve to ensure that "normal" pressure differentials can be compensated. To this end, for example, functionalized water-repellent and oil-repellent non-wovens can be integrated into the valve cover or can be added as separate components. In the event of a rapid increase in excess pressure in case of damage, this version of the valve also reacts quickly. In this case, the component would act simultaneously as a pressure compensation element and a pressure control valve.
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