Brand new battery keeps being drained. I found the interior light switch does not work. Could this be the culprit?

Bought the car used. Has brand new battery. Keeps being drained and car won't start if not driven for greater than 16 hrs. No problem was disclosed. Already checked all the fuses and removed the fuse for the cigeratte lighter that looked bad. So frustrating as I really can't afford a fishing expedition at a dealership.

My car has 137000 miles.
My car has an automatic transmission.

Experienced mechanics share their insights in answering this question :
Even though you have a "new" battery, it does have to be load tested just to be sure. Such is not a problem though as many large battery vendors will do it for free. Once you have confirmed the battery, you want to check the output of the alternator and compare against the Factory Service Manual (typically output should be around 13.8 to 14.5 volts but the FSM spec is the one to rely on). If all that checks out, you have a phantom load on one of the car’s circuits that is drawing a significant amount of current when the car is off. Normally, some battery power is required to keep the car "alive" when it is sitting in your garage (alarm, computer, clock, etc.) but this current draw is so small, at around 50 milliamps, that it won’t discharge the battery "quickly". Basically, to find the phantom load(s), when the car is off, you measure the amp draw from the battery to each and every circuit fed by the battery. If you do this methodically, eventually the circuit that is discharging the battery will show up. That individual circuit(s) is then traced to identify the faulty device and or wiring issue that is letting current flow when it should not flow.

How to Identify and Fix Common car Problems ?

Our sources include academic articles, blog posts, and personal essays from experienced mechanics :

The most common problems however are: Parasitic Drain – If the electrical devices in your car or the lights on your car are left running even when your ignition is off, they will suck the juice out of your battery slowly but dramatically. This is often eerily described as parasitic drain.
Even just leaving a door slightly ajar may be enough to cause the internal lights to illuminate – and if you leave your lights on long enough, your car`s battery will be drained. Luckily, this can be easily fixed with a jump-start, and in most cases, your battery will be just fine afterward.
But when the engine is off, the alternator can`t recharge the battery, allowing little electrical mishaps to drain your battery entirely. The battery strain caused by these electrical whoopsies is known as a parasitic draw.
Faulty Charging

If your charging system isn`t working properly, your car battery can drain even while you`re driving. Many cars power their lights, radio, and other systems from the alternator, which can make the battery drain worse if there`s a charging problem.

If your phone`s battery is draining faster than usual, it either means you`re using a lot of energy or your phone is not using energy efficiently. This could mean you`re overworking your phone by running too many applications, or something is physically wrong with the phone battery itself.
The ignition relay is essentially an electrical device that works as a switch for the power to the ignition system. Its job is to make sure electricity flows from the battery to the different components that need power. A bad relay will drain your battery and make it difficult to start the ignition.
Well, a lot of cars will cut off power to the power ports (what we used to call “cigarette lighters”) when you turn the car off. So it`s not an issue for most people.
LEAVING YOUR KEY CLOSE TO YOUR CAR

Leaving your car key in your car overnight or even on a hook near the car is not recommended. If the fob is too close to your car, it can continue to communicate with it, which could drain the battery.

No a burnt fuse can`t cause battery drain issues. Some fuses don`t get power until the circuit is turned on so don`t be concerned about them. Has your battery been replaced or load tested to make sure it`s good? A weak battery can make it seem like you have a draw.
An effective method to determine whether it`s your battery or alternator that`s gone bad is to connect jumper cables from a running vehicle`s battery to yours. After a few moments, try starting your vehicle. Once it`s started, remove the jumper cables — if your engine stalls, your alternator has probably gone bad.
If your alternator fails when you`re far from home, you can typically drive up to 200 miles on battery power alone. That`s assuming your battery is in good shape and fully charged. Of course, it`s best to get the alternator repaired or replaced as soon as you can.
Open the Settings app. From the list, tap Battery or Battery Usage. It will display the list of apps and the battery life they consume.
A bad starter can, and will drain your battery if you don`t take care of your vehicle regularly. With these tips, however, you can prevent this from happening.
If jump starting gets your engine started and running, but the car will not start again once you turn it off, the battery is probably the problem. In this case, the alternator is doing its job of keeping the battery going once it has been jumped, but the battery can`t hold a charge when the alternator is shut off.

Relevant Questions and Answers :

the most relevant questions and answers related to your specific issue

Brand new battery keeps being drained. I found the interior light switch does not work. Could this be the culprit?
ANSWER : Even though you have a "new" battery, it does have to be load tested just to be sure. Such is not a problem though as many large battery vendors will do it for free. Once you have confirmed the battery, you want to check the output of the alternator and compare against the Factory Service Manual (typically output should be around 13.8 to 14.5 volts but the FSM spec is the one to rely on). If all that checks out, you have a phantom load on one of the car’s circuits that is drawing a significant amount of current when the car is off. Normally, some battery power is required to keep the car "alive" when it is sitting in your garage (alarm, computer, clock, etc.) but this current draw is so small, at around 50 milliamps, that it won’t discharge the battery "quickly". Basically, to find the phantom load(s), when the car is off, you measure the amp draw from the battery to each and every circuit fed by the battery. If you do this methodically, eventually the circuit that is discharging the battery will show up. That individual circuit(s) is then traced to identify the faulty device and or wiring issue that is letting current flow when it should not flow.

Just replaced head gasket, changed crack sensor new plugs. The check engine light is on, but dim, the only other light that the dash is a normal battery indicator. The fuel pump is not running when switch is turned on, the power windows don’t work, it turns over but not firing. It’s like only one part of the system is working. Also the heater fans work even if the switch is off.
ANSWER : I would first look at the diagnostic trouble codes. Those codes are available because the check engine light is on. If the fuel pump is not running (momentarily) at key on, that fault alone will prevent the car from starting as there will be no fuel pressure. Pump output and pressure can be directly tested and you can also check for electrical power to the pump. To find out why the engine will not run, the recommended diagnostic is a no start diagnostic. If you request that service, the responding certified mechanic will get this taken care of for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.

I got a new battery new alternator put in but my battery just gets drained I keep having to buy new batterys
ANSWER : This may be a sign of a failing voltage regulator or potentially a bad grounding problem causing a draw on the system. The voltage regulator is a unit that regulates the charging of the battery by the alternator. Often times this will result in the alternator allowing too much power to be delivered to the battery resulting in damaging wires and prematurely sometimes destroying the battery. In other cases it may result in the alternator not supplying enough power to the battery, resulting in undercharging the battery or not charging it at all. In either case, I would recommend having an expert from YourMechanic come to your location to diagnose your vehicle’s charging system.

The Battery Light is on and the lights and power mirrors are not working – 2002 Mercedes-Benz C320
ANSWER : Hi. If the Battery Light is on, this an indication that either the battery is bad, the alternator is not charging properly, or both of them are bad. Low battery voltage will cause the car to do abnormal things like have no headlights. There are other things that can cause headlights to not work as well, for example, a blown fuse, broken headlight relay, or, the most common cause, blown headlight bulbs.

I suggest you have a certified technician, such as one from YourMechanic, come to your location to determine why the Battery Light is on and suggest the necessary repairs.

Dashboard lights out and something draining new battery
ANSWER : Hello there, thank you for asking about your 1957 Chevrolet Bel Air. Checking for parasitic draw on a battery can be done by inserting an ammeter in series with the negative battery cable, then removing the fuses one by one until the draw drops. Once this happens, you have found your problem circuit. A professional, such as one from YourMechanic, can [find the problematic fuse and replace it for you](https://www.yourmechanic.com/services/fuse-replacement]. In regard to your instrument panel, unfortunately, I do not have the wiring diagram for your vehicle due to the age. The instrument panel will need to be checked for both power and ground to determine where the problem is. Once again, a technician from YourMechanic can inspect your electric problems.

All dashboard lights and tail lights do not work after driving through a lot of snow. Headlights work. And brake lights work.
ANSWER : Hello. We’ll be happy to help you. There are a few things that I recommend checking to discover why your lighting system is malfunctioning. The headlight dimmer switch should be checked. Along with the wiring harness above the right front tire behind the fender well cover. Also
the main fuse under the hood should be inspected because other people with your particular vehicle have had similar lighting issues if that fuse goes bad. It will be a great idea to check the rest of the fuses in that area too while checking the main fuse. Once the fuses are checked inside of the main fuse box then I would check the fuses inside of their interior fuse box. The interior fuse box should be located on the driver side of the dashboard near where your legs would be. If you need more assistance with repairing your vehicle please keep in mind that our organization provides Mobile Mechanics who can help you diagnose and repair your vehicle, all at competitive rates.

Okay so when I turn my lights on at night my brake lights do not work, when I don’t have my lights on they work fine.
ANSWER : Hi there. Based on your inquiry, it’s highly probable that you have a damaged light switch or some electrical fault inside the turn signal relay. However, it’s also possible that the brake switch above the brake pedal could also be the source of your problem. Due to the fact that multiple electrical components might be causing this issue, it’s a smart idea to first have a professional mobile mechanic complete a brake lights are not working inspection, so they can determine what’s causing all of these issues and recommend the right repairs.

Electrical wiring issue with the dash lights.
ANSWER : Check the lights on the dash and look for the wiring. Follow the wires to where they hook up to and check that component to see if it’s working. Most dash lights are hooked into the light switch and on the terminal for the dimer lights rheostat. If you replaced the light switch, then there is a break between the light switch and the dashboard light. If you need further assistance with the dashboard lights not working, then seek out a professional, such as one from Your Mechanic, to help you.