Battery Loses Charge. Short?

If the battery is dying or losing it's charge when the engine is running then the alternator or wiring is defective so it's not charging the battery.

Are you sure the battery is good? Test it for cranking amps? Check specific gravity of electrolyte?

A fully charged battery should start the vehicle - now if a good battery loses its charge then some circuit is discharging the battery when car is not being used. Need to locate the circuit responsible and take action to remedy.

Bad battery

also check that there is nothing left on eg the boot light not going out after boot is closed something maybe drawing power

process of elimination here disconnect one thing at a time till drain stops also disconnect the battery as it wouldnt be the first time i have had a battery with a internal short in the plates in a cell

Normally, when you leave a car with a battery connected in it for about half a month without running the car(leaving it idle), it loses charge and eventually it dies and you have to replace it with a new one or recharge it.
In your case, it is assumed you have been using this battery for quite a long time and this has affected the amount of time it can stay idle without losing charge.
In other words, continuous use if the battery leads to an eventual death or reduction in the battery's performance. If you have been using this battery for a long time, the battery may be coming to the end of its life-span and may need to be replaced with a new one.
Hope this solution was helpful? Please comment for more information and thank you for using fixya.

turn every thing on in park if voltage drops change alt

It sounds like you have a major compent shorted out somewhere. Some thing is causing a discharge. Therefore your altenator is not responding and the car is draining the battery. Is the circuit between the altenator and the battery broken?? Also, you may need to check all the chassis grounds. Sometimes all your problems can stem from just a bad ground.

it sound like your alternater has gone bad. and your trany is bad you will have to replace it

if you have a volt meter, check the voltage at the battery before you start the engine and after the engine is running, before could be 11.5v after should be 13.5 or higher but no higher than 14.5, this will test the altenator, check the belt by pressing on the middle of the belt between the two pulleys the belt deflection should not be more than 1/2", if it is tighten and retest altenator