Matt replied: "I really like the dodge mans answer above and hope for your sake he is right, but I am afraid that if he is not and that your regulator on your alternator is just not playing the game. Your battery will recharge its self to a certain degree while left doing nothing over an 8 to 12 hour period. Assuming that your regulator is allowing the battery to keep a charge somewhere in the vicinity of 11volts whilst running, the rest period over night will be enough to start a small well tuned engine. However, after the initial start, as the friction heat within your alternator increases, it is possible that the resistance within the regulator is also increasing, reducing the voltage available to keep your battery topped up and keep your car running at idle speeds. PLEASE keep in mind, before you go out and spend a fortune, that a corroded or broken battery cable will give similar side effects and may be obvious, i.e. a cable that heats up significantly during the first 15 mins of running time and/or has obvious discoloration or melting of the insulation. What you really need to do is look at your engine bay for obvious signs, feel around the insulated cables for abnormal inflexibility or heat while or just after the engine has been running. Borrow a Multimeter or voltage meter and see what charge is being put out by your alternator and what voltage your battery is charged at. A normal alternator with a low charged battery should be putting out in the vicinity of 14 volts and as such your battery when fully charged should hold 14 volts. Your battery should be at a minimum of 11 volts after a normal start and should regain the starting charge lost within the first 15 mins of running time. Hope this help and good luck..."