Skip to Content

Car Battery Dead, Won’t Unlock? Use Secret Lock!

A flat battery is always irritating, but then your day gets even worse when you find yourself locked out of your car, what a pain in the jacksie. Don’t panic there’s a simple solution and here it comes…

Follow these 3 steps to unlock your car when the battery is dead.

  1. Remove the hidden key (Key-less entry)
  2. Remove the hidden door handle lock cover
  3. Place key in the lock and unlock manually

In this post you’ll learn how to access your car without the use of tools, you’ll learn how to boost start your car and how to test the battery. I’ll also show you how to access your car if you’ve misplaced the hidden key.

Car Battery Dead Won't Unlock

Key System Overview

Car Key

During the ’00s bladeless key systems were common with some manufacturers. The ignition switch was a fully electronic unit that didn’t require a metal key to operate. Instead, the fob was placed into a look-alike ignition switch and a transponder receiver read the fob code allowing the ignition to turn and the engine to start.

Most regular family cars today use a key fob with a traditional metal key that folds away into the fob and a regular mechanical ignition switch.

Higher-end latest-gen vehicles use a smart key and employ what they call key-less entry technology.

All these systems require a fully charged battery in both the key fob and the car to operate successfully. As you know, when the battery is flat nothing works.

Gaining Access To Your Car

To access your car, we’ll need to do three things – 1 Successfully locate the metal key, 2 Successfully locate the door lock and 3 Successfully operate the door lock.

If your key is a flip-out regular key, great! You are ahead of the game, you can jump ahead to step 2.

Step 1 Locating your key

For those that don’t have a flip-out key, locate the hidden key in your fob. All key fobs will contain a hidden key. Usually, there’s a discrete slide button or cutaway that reveals the flat metal blade key. Google your model car and you’ll quickly find how to access the key.

Step 2 Locating your door key lock

Locating the door lock isn’t too difficult either. The door lock is hidden behind the door handle cover. Removing the cover or an access cover will allow access to the lock. Some pull-out door handles, reveal the lock when the handle is held out.

The door handle cover may or may not have a lock symbol discretely embossed. It’s not uncommon for the door lock to be fitted and sometimes only to the driver or passenger front door only.

Most manufacturers machine a point on the key blade so that the user can use it as a tool to gently pry up the plastic door cover. If in doubt, google your model for access point and technique.

Step 3 Operating the door lock

You know how this works, but be prepared if your key fob battery is flat, your car alarm will sound when the door is opened. Placing the key/fob in the ignition will turn the alarm off.

Also worth noting, it’s very likely the door lock is very stiff as it’s probably never been used. A shot of WD40 will do wonders, but do be careful not to damage the key especially as it’s needed for the ignition switch.

Accessing The Car Without The Hidden Key

If you have misplaced the hidden key blade and still have the fob and the fob light works when you hit the key, accessing is still possible but a lot more work.

This process involves getting power into your cars system which is difficult because everything is locked including the hood and trunk. It is possible to open the hood by popping the catch, but it’s difficult. Instead, follow the process below, but you’ll need tools and time.

Tools you’ll need:

  • Jack
  • Axle stands
  • Torx/screwdriver/small socket set
  • Charged donor battery
  • Jump leads
  • Helper

You can check out all these tools on the Mechanics tools page.

Technique as follows:

  • Jack the front of the car and support with axle stands
  • Remove splash guards
  • Locate the starter motor
  • Fit ground cable to charged donor battery and to car chassis or engine casing
  • Fit battery positive cable to metal shank of long screwdriver – (A power probe makes this whole process a ton easier, check it out here on the Auto electrical tools page)
  • Place the screwdriver / power probe on the starter power feed (cap may need to be removed)
  • Holding the screwdriver on the starter feed, have a helper connect the positive jumper to the donor battery positive feed (careful not to ground the screwdriver on the chassis or engine)
  • Now have helper press the unlock key fob button while holding power to the starter
  • The vehicle should now unlock

How To Boost Start Your Car

Boost starting is the fastest way to get your car up and running, but the battery may be faulty, so she may not stay running.

To boost start, attach the cables in order 1, 2, 3, and 4. Start the vehicle and remove cables 4, 3, 2, and 1. The final cable at 4 is attached to the ground (GRD). A ground is any clean non-painted metal on the chassis or engine.

See battery testing below. Your car may have a battery parasitic drain and if so this problem will persist, a battery maintainer will help keep the battery topped up but only masks the problem.

If your battery keeps running flat, you may have one of 3 common problems.

  1. Faulty battery
  2. Faulty alternator
  3. Parasitic drain

1 Battery testing

Battery testing

To test the battery first run a volt check and then run a crank test. You’ll need a voltmeter to run these tests.

Check out the test kit, charger, and all the tools I use including wiring diagrams and workshop manuals here on the Auto electrical repair tools page. Attach a DVOM or voltmeter and simply read the state of charge. Load testing however is a better test of the battery’s ability to supply steady voltage, see crank test below. (note: battery must be charged above 12.5 volts)

  • 12.7 – 13.2 volts is 100% charged
  • 12.4 volts is 75% charged
  • 12.2 volts is 50% charged
  • 12.0 volts is 25% charged
  • 0 – 11.9 volts is Discharged (Flat)
Battery cranking test diagram

If you need to charge the battery before testing it, the little NOCO boost/charger pack is about the best I’ve seen and I’m a mechanic for over twenty-five years.

It’s small enough to fit in a glove box and powerful enough to start a diesel engine, seriously! Anyway, you can check it out here on the Auto electrical tools page. Check out new batteries on the Amazon link below.

Amazon Car Batteries

2 Alternator testing

If you suspect an alternator fault, check out this post it covers testing your alternator – “How long to charge battery driving?”

3 Parasitic battery drain

A parasitic drain is a voltage that is draining from your battery as your vehicle sits idle. It is often caused by a faulty component like a radio or alarm etc. After-market accessories are a common source of parasitic drain.

Battery drain

Follow the test above to check for a drain. It is important not to open doors etc. during the test as this will wake up computer modules and cause incorrect diagnoses. An electrical short is another common cause and you can read about that right here “Car fuse keeps blowing”.

Related Question

What to do if the door lock won’t turn? If your car door lock won’t turn, remove the key and check for damage or debris. Using WD40, spray into the door lock and onto the blade of the key. Work the key in and out of the lock and side to side without applying force. Turn the key left and right alternately without using excessive force until the lock opens.