Modern car batteries are very reliable, but leaving the lights on will soon result in a flat battery. To start a car with a flat battery you need a set of good quality jump leads and someone with a car with a battery that is in good condition.
When purchasing a pair of jump leads, go for those which have heavy duty cables and sturdy clamps at each end. Never use jump leads that are damaged as these can overheat and catch fire. Never try to jump start a battery that is leaking or appears to be damaged as it could explode.
Procedure for using jump leads
The first step is to position the other car near to the car with the flat battery so that the batteries are close, but the vehicles are not touching. The ignition and electrical equipment on both cars should be switched off and both vehicles should have the handbrake on and be in neutral. Now open the bonnets.
One end of the red jump lead should be connected to the flat battery's positive terminal. The lead must not touch any vehicle metal. Attach the red lead's other end to the boosting battery's positive terminal.
Take the black lead and connect one end to the boosting battery's negative terminal. The black lead's other end should be connected to a metal bracket or bolt on the engine block of the car with the flat battery. The lead must be a good distance from the battery.
Ensure that the jump leads will not touch any the moving parts of either of the engines. Wait three minutes for the voltages to equalise. Now start the engine of the booster car and let it run at a fast idle speed. Next, start the engine of the car with the flat battery and check that the engine runs properly.
Stop the engine of the booster vehicle. On the car with the flat battery, switch on the heater, blower, heated rear window and lights, as this will prevent voltage surges. If you are concerned about this, an alternative approach is to turn off both engines after leaving the car with the flat battery running for ten minutes. The jump leads can now be disconnected in the reverse order to connection, being careful not to let the clips touch each other or any metal. Once this is done you can switch off everything and close the bonnets.
You should now be able to drive your car away. It takes time for the alternator to fully charge the battery, so try not to stop the engine too soon, or stall the engine.