Battery Replacement and Test Guide

Need to replace, load or draw test your battery? We are a team of ASE certified mechanics that will show you how to do the job correctly while saving money, or at least see what you are paying for when taking your car in for repairs.

What Goes Wrong?

A car battery is designed as a chemical reactor which uses lead plates to generate voltage. A battery goes bad when the chemical reaction inside the battery starts to become weak. This is due to the natural degradation of the lead plates in one or more of the batteries 6 cells and can normally happen in about three to four years of the battery's life. 

A battery can fail in one of two ways, the first way is very quickly in others word you go out to your car and no lights will come on and the car is dead. In this case a jump start may not even help because the battery is so far gone it just dumps (pulls in) all the voltage supplied to it.

The second way is the battery can go bad while supplying a slow decline of voltage. This results in slow than normal engine cranking sound that you will notice in the morning or after the car has been sitting for a long period of time. Once you notice this it's up to you to be preventive and change the battery before total failure occurs.

Sometimes a good battery will be dead in the morning because the car has a voltage drain in the electrical system which is called a parasitic draw. This means there is something electrical staying on in the car when its not suppose to be.

How Does it work?

Your car battery is designed to store electrical power which is used to start the engine when the key is turned to the crank position and engage the starter. A battery also stores electrical energy to start the engine and operate accessories in the electrical system throughout the car when the engine is not running.

This battery is a rechargeable unit in the SLI (starting-lighting-ignition) variety which is lead acid based and is the oldest style of rechargeable electrical supplies. These batteries are divided into 6 separate cells. Each cell producing about 2.1 volts which when combined create up to 12.6 volts.

As the state of charge decreases the voltage and amperage it produces also decreases. Electrical power is created through a chemical reaction where positive and negative lead plates are immersed in an electrolyte solution (acid). This voltage is then released through the positive terminal (red) and returned to the negative terminal (black).

A battery is equipped with a positive and negative terminal from which two cables are attached. These cables route positive (red +) power from the terminals to the power distribution center or fuse panel and negative (black - ) power to the engine block and the car frame.

A non "deep cycle" battery must have a rest period or "downtime" to allow the plates inside each cell time to cool. A battery subjected to a complete discharge will fail prematurely unless it is a deep cycle battery. All batteries are hazardous material and must be disposed of at a local parts store or recycle center.

If the battery warning light has been on while you have been driving the car you will need to test the alternator first. This is because the battery might not be the problem if it is not receiving a proper charge from the alternator.

In This Guide

  • Battery Replacement
  • Load testing a battery
  • Troubleshooting an electrical system drain

Reasons for a Dead Battery

  • Battery is more than three years old
  • The alternator is not charging or overcharging the electrical system
  • There is a electrical (parasitic) draw on the system

Let's Get Started

Battery Replacement

When a battery goes bad it can expel acid around the battery, battery box and cables. This acid is corrosive and you don't want to get it on your clothes or skin. It's a good idea to wear rubber gloves like you would use to clean your household oven. Also, put on safety eyewear and old clothing. By sprinkling baking soda over the battery and then rinsing it with water will help neutralize the acid and clean the area before work begins.

Step 1: Remove the Battery Cables

Though appearances may vary slightly for each car the process is virtually the same. The battery location will be under the hood in most cases but some cars may have it in the trunk or under the rear seat. Record the radio stations for re-entry as they will be lost once the electrical power is disconnected.

Using an 8mm or 10mm wrench to loosen the bolt for the negative cable end. You might need to give a twisting motion to help remove the cable. Be careful not to touch the wrench to the positive side of the battery to avoid a short circuit. Always undo the negative side first because the positive side is more prone to shorting to ground via any metal in the area. Then push the cable away to avoid it accidently coming back and reconnecting to the terminal.

Next, undo the positive side cable this will normally be a red color and identified by a positive sign near the terminal connection. Use a wrench or ratchet to loosen the cable end. At this point there is no chance of creating a short circuit because the circuit is no longer live with the disconnection of the negative cable.

Step 2: Remove the Battery

Some manufacturers have a battery cover which now can be removed once the cables have been disconnected. This will also allow you to gain access to the battery hold down bolt. This cover should be cleaned before re-installation with water and soap to remove any acid residue that might be present.

Locate the hold down bolt or J bolt nuts which are either at the top or bottom of the battery mounting box. If the hold down is at the bottom it with only require one bolt to be removed while a J bolt style of mount will require two nuts to be removed. On a bottom bolt style of mount use a long extension and ratchet to help in the removal. These bolt sizes include 8mm and 10mm. There will be a plastic wedge piece that will also need to be removed as well.

After moving the bolts and hardware and while still using gloves grasp the battery and wiggle it slightly to break it loose from its mount. Then hold tight as you lift the unit from the vehicle. Brace your knees and back for this step due to the weight of the battery. Once the battery has been fully removed use a garden hose to fully wash the area free from acid. This will also prevent corrosion of metal pieces and wiring which could be routed below the battery box.

Step 3: Match the New Battery

Now that the old unit has been removed compare it to the new battery while checking the size, terminal position, cold cranking and reserve amps which should meet or exceed the old battery.

Step 4: Install the New Battery

While firmly holding the new battery, gently lower it into position. Be mindful of the terminal orientation so the electrical cables will attach to the corresponding positive and negative cable.

Once the new unit is in place wiggle it to ensure it's down into the box mount then lower the mount wedge and bolt into place. This wedge should only be installed in one direction with the stepped part toward the batteries side wall. Wash these parts with soap and water to remove corrosion before reinstalling them.

After the hold down has been lowered into position tighten the bolt securely. At this point you should see and feel the battery get tight within the battery box.

Step 5: Attach the Cables

Remove both terminal covers of the new battery which are used to prevent damaged and shorting while in shipping. The terminals should now be clean and ready for the positive and negative cable to be installed. Set the cover back into place before the cable installation.

Use a cleaner tool with a twisting motion which will create a fresh metal surface within the cable end to aid the transfer of amperage needed to run the car. Take note this cleaner has a taper fit and will only work correctly in one direction. Complete this operation by cleaning both negative and positive cable ends. A small wire brush will work for this as well.

Install the positive cable first, tighten the nut while holding the end downward to help the connection. To ensure a proper installation wiggle the cable end slightly. Never attach the positive cable to the negative side of the battery or the negative cable to the positive side. This will cause major electrical problems such as shorting out the alternator, main computer or other controllers inside the vehicle. 

Once tight reinstall the positive cable protective cover. This cover helps prevent accidental short circuits when working inside the engine bay.

Finally install the negative cable end onto the battery terminal and tighten while holding downward. Check the cable end for tightness by giving it a twisting motion. Wipe away any dirt or metal shavings from the cable end cleaner and double check your work and you are all set. Once the replacement is complete it's a good idea to check the alternator voltage output using a voltmeter. Also observe the electrical system warning light or gauge this will ensure the operation of the alternator.

Watch the Video!

Watch our video which gives you an idea on what you are in for when replacing your car battery.

Battery Load Test

Need to load test your car battery? We help you test your car battery without the aid of a tester. We use this test in our shop which only takes about 15 minutes. When working with a battery wear protective eyewear and gloves. Stand clear of the battery while the engine is being cranked over.

Step 1: Battery Test and Inspection

Open the car hood to locate the battery and check its condition (some batteries can be located in the trunk or under the rear seat). You are looking for leakage and bulging of the battery sides which are an indication of warped internal plates that can short circuit. You are also looking for corrosion at the positive and negative terminals which can eat away at the metals in the battery cables that will cause a connection problem. To start the battery test turn the headlights on and leave them on for about 15 minutes (do not start the engine).

Step 2: Load Test the Battery

While the headlights are still on crank the engine over and observe the headlight bulb brightness. The bulbs should only dim slightly when using the starter. If the headlights dim way down and go out or you notice the engine is cranked over slowly followed by a machine gun style of clicking noise the battery must be replaced because it cannot supply the voltage needed to operate the car. If the battery is good it will withstand this load test.

Watch the Video!

Here is a video of the car battery load test being performed.

Battery Draw Test

Is your car battery dead overnight? We have created to help save you money by doing the repair yourself or at least see what you are paying for when taking your car in for repairs. We will show you the easy fixes first and then onward to the more complicated electrical problems.

Most Common Electrical Draw Problems

  • Seat switch stuck
  • Glove box light on
  • Trunk light on
  • Dome light on
  • CD Stuck in player
  • ABS relay sticking
  • Side mirror switch stuck
  • Alternator shorted internally
  • BCM or GEM shorted
  • Alarm module shorted
  • Door lock switch stuck
  • Cigarette lighter stuck down
  • Vanity mirror stuck on

Scenarios We Cover

  • My battery is more than three years old
  • The car has a sulfur or rotten egg smell and is dead
  • I am driving the car with the battery warning light on
  • I just installed a new battery and it has lost it's charge
  • I smell something burning and now the engine wont crank over

Now that you have an idea of what could be wrong let's go into detail on how to check for these problems and how to fix them.

Step 1: Load Check Your Battery

If the battery charge warning light was not on light the last time you drove your vehicle the alternator is probably working correctly. If you did notice the battery warning light on the battery is probably down on charge which is not the fault of the battery.

A sulfur or rotten egg like smell is produced when the battery is shorted internally either from age or being overcharged by the alternator. In either case the battery must be replaced and the alternator output voltage checked. As the battery starts to age, three years or more, it will loose its ability to hold a charge overnight which is the first sign the battery is ready for replacement. It is easy to load test your battery even if you don't have a voltmeter or load tester to see if the battery is weak. 

Step 2: Check the Seat Switch

There are several "live" electrical circuits that bypass the ignition switch that can draw electrical power when the key is in the off position. Inspect the electric seat control switches. This switch can become sticky or weak allowing the switch to stay engaged forcing the seat motor to draw power from the battery. To check for this condition observe the operation of the seat control switch. If it doesn't return to it's neutral position or is sticking in one position replace seat the switch with new part which you can get from Amazon, an auto parts store or the dealership.

Step 3: Check the Glove Box Light

In most cases this light is controlled by a small pin switch inside the glove box door frame. If this switch malfunctions or is misaligned it will allow the light to stay on draining the battery. This can be sometimes tough to see in the bright light of day so it's best to check during the evening time. Look for light inside the unit through the small cracks in and around the box door. If the light is on open and inspect the light switch to replace it if needed or make minor adjustments to help it work correctly and turn the light off.

Step 4: Check the Cigarette Lighter

Check the cigarette lighters in both front and rear passenger compartments. Check to see if the lighter is stuck down which is usually accompanied by a fair amount of heat and a burning smell. Pull the lighter upward to stop the electrical draw. In this case the lighter has gone bad and needs replacement.

Step 5: Check the CD Player

When you shut the ignition key off the radio turns off as well. This is not true for the CD player and changer. If a CD gets stuck in the load or eject position it will continue to work the small motor inside the player while draining the electrical system. With the key off while being very quite listen closely to the player, if you hear a motor running a CD is stuck. Try to work the CD either in or out using a small tweezers or flat blade screw driver. If you cannot retrieve the CD pull out the radio fuse until you can replace the unit or have it repaired this will stop the battery draw.

Step 6: Check the Door Lock Switch

The door lock switch is constantly hot or live with electrical power. Sometimes the control switch will get gummed up with dirt or soda which makes the switch stick in the lock or unlock position. This will force the lock actuator in the door to stay on which will draw the battery down. Inspect the lock switch to make sure it returns to its neutral position. If this switch is broken or permanently stuck replacement is required.

Step 7: Check the Mirror Switch

The side view mirror switch continues to get power after the ignition switch is off. This switch like other switches can get gummed up from grime and soda which can hold the switch on forcing the motor to draw power. Check the switch operation to ensure that it returns to its neutral position. If this switch is broken or stuck it will need to be replaced.

Step 8: Check the Vanity Mirror Light

A vanity mirror which is located in the passenger or drivers side sun visor has a light in most cases that can stick on. To check for this problem move the visor slightly downward to observe a light that is still shining even if the visor is in the up position. If this is the case pull the visor down to inspect the small pin switch and repair or replace as needed.

Step 9: Check the ABS Pump Motor

The abs system will stay live when the ignition key is off in most vehicles. This system has a control relay that is integrated into the module or in the fuse panel. If when you have the key off and you hear something running under the hood or frame of the vehicle it's probably this unit. Locate the fuse for the ABS system and remove it from the fuse panel until you can replace the ABS module with a new part.

Step 10: Check the Trunk Light

A trunk light may or may not be controlled by the car's BCM depending on the year and manufacturer of the vehicle. The information we have found on this subject is vague to say the least. The good news is it's extremely simply to check if the trunk light is causing the electrical draw problem though it's best to check in the evening time where observing the light is much easier. Pop the trunk open to observe the light. Then slowly close the lid while watching the light through a small opening near the rear of the trunk lid, the light should go off. If the light stays on replace or adjust the pin control switch or trunk latch.

This section is reserved for the more obscure problems

Step 1: Check the Alternator

The alternator is used to charge the battery while the engine is running. When this unit shorts out it can create a draw on the system. To check for this problem wait until the engine is cold then locate the alternator and feel it with your hand to check for warmth. If the alternator is warm with a possible burning smell there is an internal short and the alternator must be replaced.

Step 2: Check the BCM or GEM

The computer BCM, Alarm and GEM modules can internally short circuit which can cause various electrical systems in the car to stay "live" which will draw electrical power. This problem can be checked by locating the fuse of the system and removing it or removing the controller altogether. If the vehicle starts the next morning you have found the problem.

Step 3: Manual Draw Check

If you can't find the cause of the battery drain a manual draw check of the electrical system will need to be performed. With the ignition key in the off position and the doors closed (driver's side window down) wait 15 minutes. Then disconnect the battery cable on the negative side. A fifteen minute wait time allows the computers to go into "sleep mode" which shuts down most electrical systems.

Attach a test light between the negative battery cable end and the negative battery terminal. The test light should light up dimly or not at all. If the test light is on brightly there is a strong electrical draw in the system. To locate the draw start removing fuses one at a time (doors closed). When the test light goes out the circuit in question has been located. Identify all accessories in a particular circuit by using a wiring schematic and disconnect them one by one until the test light goes off.

Step 4: Check the Headlight Switch

These next set of problems pertain to older vehicles built before the year 2000 in most cases.

If you leave the headlights on overnight the battery will be dead. Check the headlight control switch. If the switch is in the on position turn the switch off and jump start or charge the battery. The battery should re-gain its state of charge after about fifteen minutes of driving.

Step 5: Check the Dome and Interior Lights

Once all of the doors have been closed check the interior and dome lights. These lights should turn off after a short amount of time. If the lights stay on beyond a reasonable amount of time check the interior light bypass switch located at the headlight switch or on the bezel which also controls the brightness of the dash lights. You should feel a small bump while rotating the thumb wheel informing you when the switch is activated or deactivated. All doors including the lift back or hatch have a pin switch or an internal switch located in the latch itself that detects when the door is open. When these switches fail or become misadjusted the interior lights will stay on.

Step 6: Check the Under Hood Light

Some SUV's and trucks have an under hood light that is controlled by a mercury switch which senses the level of the hood. At night look under the vehicle to check for light near the engine bay. To fix this problem replace the light assembly.

Battery Charging

Voltage from the engine's alternator maintains the batteries state of charge. This continuous cycle can produce electricity for three to four years. Test the battery easily without a load tester if you are not sure of the battery's condition.

The most common way to recharge a battery besides the car's alternator is by a process called trickle charging which is performed by using an inexpensive battery charger. Most repair shops have a more powerful and expensive charger that is capable of recharging at a higher rate over a shorter time period. When re-charging a battery use caution. Always connect the charge leads while the charger is off. This will prevent a spark from being generated. A battery is most prone to an explosion after a recharge.

A new battery can go dead and fail prematurely if there is a parasitic draw which will occur overnight. The system should be tested for this condition to ensure a long battery life. If your battery has failed you can jumpstart the car by using jump cables.

Watch the Video!

Battery Recycling

The old battery is completely recyclable. Contact your local recycling station, auto part store or repair garage to dispose of properly. Do not discard batteries into landfills.

Battery Storage

  • Store a battery fully charged in a cool place and off of the floor
  • Recharge the battery every few months
  • Remove all electrical connections from the battery


Never touch a metal object between the two battery terminals to see if it is charged, this can cause it to explode. Use caution when working with batteries because they are filled with acid. Inspect batteries regularly if acid is present replace it and check the charging system. Be extremely careful when handling a battery by using gloves and eye protection.

When the battery is overcharged water in the electrolyte solution is transformed into hydrogen and oxygen. This can cause an explosion if a spark or another source of ignition is present near the battery.

Got Questions?

If you have any car battery questions, please visit our forum. If you need car repair advice our community of ASE certified mechanics is happy to help.

Article first published