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There are three reasons why a battery will go flat.
1. A battery which has come to the end of its service life.
2. A fault with the vehicle charging system.
3. A drain on the battery. Something on the car which is not switching off, or is being accidentally left on.
A battery drain can be measured with an ammeter. Every battery has an amp/hour rating (a/hr). This is the battery's discharge rating. For example if it's a 50a/hr battery it means you can draw off 50 amps for one hour, one amp for 50 hours, or multiples thereof. But you must bear in mind that this is assuming the battery is in perfect condition and has a 100% state of charge (SOC). You must also bear in mind that in this example the battery will be discharged right down to nothing. A battery sufficiently discharged to prevent it from starting an engine will be well before you get it right down to nothing. This may or may not be useful for you to know, but it's intended to give you the idea.
In layman's terms a discharge of 0.75 amps overnight will be enough to prevent the engine from being started the next morning. An interior lamp or boot lamp would draw this amount of current.
You need to get your mechanic to connect an ammeter and then lock the car up as normal. After a short while everything should be switched off and the current reading should be very near zero. Any current then showing on the ammeter is a drain on the battery, but remember that some components such as control units, clock, radio etc need to consume some battery power, albeit a tiny amount to retain their settings. If you have a drain of greater than 0.1A (100mA) then it's very likely you will have a flat battery after a week of non use.
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