Vauxhall Vivaro occasionally, the starter only turns the engine over slowly and rapidly flattens the ( new ) battery.

I bought the bus 6 months ago and have had this problem several times. I replaced the battery 3 months ago, but the fault remains. I have checked all the wiring for damage, tightness of connections, etc. and found no problems. Jump-starting the bus with a second battery, or from another car seems to work, but the jump leads get warm. I'm worried about damaging the new battery, or getting stuck away from home, so I now carry a spare battery all the time.