1995 540i where is the battery located

First, I'm not a EW Tech.
* Check your warranty and call for service.

*** The following is for information only.***
* Most power loss and low power stem from blown fuse(s), broken or burnt wires and/or connectors. Locate and replace bad wire.
* Problems with charge or gradual power loss is usually the battery system. Since this is most likely a Lead Acid battery (flooded or Gel) then those need to be checked (load tested) and replaced. Do not expect these batteries to last forever. Most will last about +200 charge cycles. Less if the battery is stressed (deep discharged,High temperatures, Over charged).
* Lead Acid batteries concider a charge cycle to be when ever charging. Even if the charger is removed prior to a full charge and discharged, this is still a charge cycle. Short charging is not recommnded. Top off charge is a cycle charge.
* Follow your MFR charging instructions.
* Check Battery; https://www.youtube.com/watch?v=mgEiNp8l_PE
* Check Fuses; https://www.youtube.com/watch?v=rgK2l6cMCc4

* Review EW36; https://www.youtube.com/watch?v=K_NcgxUgAvE
* you may want to upgrade to Lithium Ion Battery Pack (with BMS) which will have a higher cycle rate (+1000)

Aloha, ukeboy57

Some possibilities, battery--battery connections--generator--generator wiring-- too much parasitic drain with everything off.

Here is charging system diagram. I didn't look for mega fuse, I will if you want it. In the diagram, you can see applicable fuses and fusible links.
If battery is down get it charged up, can't do any testing if battery is down. With a charged battery, everything thing off, use digital multimeter across battery posts, the generic spec for a full charge is around 12.6 volts. Then start vehicle, take second reading across battery posts, the second reading should be higher than the first if charging system is working.

The way to check for an over or under charge is to connect your volt meter to the battery, before starting the vehicle.
You should have a reading of 12.5 to 13 volts. If it's 11.8 like you stated or less, it means battery is not fully charged, or it's bad. Now start the car & check voltage again while running the engine, the voltage should exceed 14+ volts, this means your Alternator is charging, the voltage should drop after battery has been charging for awhile. When you turn the engine off after letting it charge the battery, & if the battery is good, you should now have 12.5 volts to 13 volts.
Now to answer your question, the Voltage Regulator is inside of the Altenator, I believe you xcan take the alternator apart to replace it, or simpler to just replace the complete alternator.

What kind if vehicle is this?

The battery is located in the front trunk of the car, under a cover just below the middle of the windshield. Either remove the battery or disconnect the positive (Red) cable and hook up a battery charger. You can get a battery charger at sears or any auto-parts store. Get one that automatically stops when the battery is fully charged. If you have to charge it after each trip it could be that the battery needs to be replaced or that you need a new alternator.

battery is on the passenger side in the trunk behind the interior panel.

The computer is behind the bumper under the air filter box. You will need to remove the air filter enclosure to access it.

Hello the battery is located under the hood and the driver side. here is a picture of the battery.

Sounds like an alternator (and/or battery) issue...not fuses.

There is without a doubt an issue in your cars charging system. Depending on location there are a few cheap options. First of all I would get the battery charged back up to full power on a lower amp charge. Once charged, start the vehicle up and see if you have the battery light on. If you do, drive it an autozone and have them check the battery and charging system for free. More than likely it's your altenator. I would first check for fuses for the altenator either under hood or in cab. The owners manual will give a break down of fuse locations and what each fuse gives power too. let me know if you need farther assistance