96 cadillac eldorado etc battery not staying charged when the car is started the battery goes up to 14 when driving and press brake voltage drops 11-12(car may even die) and to 13+ if u give it gas

That is definitely wrong, not normal. Voltage should stay at 14 volts or near that until the battery is close to fully charged, then will slide down to about 13.5 volts, and stay there while engine is running. Also, a fully charged battery should show about 12.6 volts at a minimum, especially a new battery.
I don't know what is going on with your system. Take it back to Merc- were they aware of how it is acting, voltage-wise? Doesn't sound like it. One small possibility: is your alternator belt not tight enough?

First step have the battery tested, a bad cell can cause a internal drain and kill the battery, if the batt is ok then check for interior lights like visors that stay on or interior light that come on by themselves at night due to defective door switches.

if battery keeps running down. check the voltage across the batery posts while car is off.should be around 12.5 to13 volts. then with tester hooked up to battery, start the car. if alternator is ok, it while read about 14 volts give or take a few volts. if the gharging is low its a bad alternator or the drive belt.

First put your battery on a slow charge 6-8 hrs.then check the batteries voltage it should have at least 12-12.8 volts if it does'nt then you need a new battery.If has 12 volts after its charged and goes dead after its installed in the car then you need a new alternato.

For a battery to go dead on you like that there must be something draining the battery while the vehicle is switched off. It could be something like a trunk light which is not switching off or a short across some circuit that does not show up on any light. The first and easiest test is to disconnect the battery negative cable whenever you park the vehicle overnight etc. and see if the battery still runs down.
As a more positive step and if this does not affect the alarm, disconnect the positive lead from the battery and wire a bulb like a tail light bulb, between the terminal and the removed lead. The bulb should light up. Make sure that everything on the vehicle is switched off, the bulb should still be lit. Remove and replace the fuses one by one until the bulb goes out. That is the circuit that is running your battery flat. Check all the components on that circuit until you get the light to stay out and your problem should be fixed.
Check your charging system. With the engine running at about 2000 rpm, a volt meter across the battery terminals should read around 14 to 14.5 volts and drop to about 13.8 volts with the engine at idle. If this does not happen, the charging system may be defective. Even with lights on the voltage should climb, but will drop lower at idle.

The battery discharge rate must be checked along with the charging current to confirm the working of the alternator.
Please check the link below for tips.

http://www.fixya.com/cars/r5824824-draining_car_battery_charge

These lights will come on if your alternator isn't charging fully. With a multi meter test your voltage at the battery with the engine running. The voltage should be above 14 volts. If it is below 14 you will need to replace the alternator.

Sounds like you have a bare wire somewhere that is touching metal and grounding out. Probably a starter cable. Try unhooking the battery, check the voltage of the battery, then hook the cable back up, watching for sparks from the cable. After you get it hooked back up, check the voltage again. If you see a small spark, it is normal. If you see a big spark, you have a wire that is touching something it is not supposed to.

Our 96 eldorado used to do the same thing. After trying everything and several mechanics, we decided to take it to a cadillac dealer. They said it was the battery that was not generating enough power to operate some switch. Changed the battery - voila - no more problem! Hope this helps you.

shot in the dark but you may want to check your shocks