I am having issues with my xsara picasso 1.6i petrol 2001. When I try and start the car it turns over but struggles to start. It sounds like that the battery is dead as the instrument panels dims and it turns over slowly, but after about 10 seconds it does start. I tested the battery with a battery tester and it states that the output is good. I've taken a fairly new battery off my other car and the results are the same although the battery from the almera is lightly smaller than that for the picasso. Once the car starts, if I turn the engin off and restart it, it starts with no problems. It seems to only happen when the car has been left over night.

Any advice on what could be causing this would be appreciated.

Thanks