Connecting an sla direct onto a car battery system will almost certainly lead to premature failure of your sla. These batteries have quite stringent charging requirement and the lack of current or voltage control will lead to failure. There is also the danger that the battery itself might well explode due to excessive gassing resulting in pressure build-up within the battery.
Google "sla charging requirement" and you'll get a host of info.
Any company that manufactures or sells what they describe as sla charging leads for use within a vehicle that contain nothing but a fuse are potentially criminally negligent
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