Car Battery Query

We've had a car sat on the drive without being used for a couple of months. Unsurprisingly the battery was completely dead when we went to start it so I put it on charge. When refitted it still would turn the engine over even with a jump so we bought a new battery. I fitted this tonight and it started first time albeit the first crank was a bit sluggish. I only let it run a few seconds as the belt was squealing but then when I tried again 5 minutes later it wouldn't turn over and sounded flat again. I've chucked it on charge overnight but surely, even if the alternator isn't working, it shouldn't run flat after starting the engine once? Anyone got a suggestion of what the problem might be? The only thing I can think is a dodgy connection / damaged cable but how can I check this?
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Son had similar problem with E46 BMW. Battery would lose charge and had to be jumped. Battery and alternator checked/changed. No different.
Bad earth strap was problem. There may be several around the car.
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Battery conditioner type charger like a Ctek is a good idea if the car is left for long periods. If there is a drain on the battery you could fit a cut off switch to isolate the battery whilst the car is standing. Any radio presets and the clock will die however.
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