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Tools To Replace Car Battery – Avoid this common mistake

Batteries don’t last more than three to four years and less if your winter temps get well below freezing for months on end. Luckily changing your battery is one of the easier car repairs you can DIY with just a little know-how and a few tools.

Tools you need to replace car battery, include:

  • Keep Alive Memory tool (KAM)
  • Wrenches (13 or 10 mm are common)
  • Wire brush & sandpaper

In this post, you’ll learn what tools you need to successfully change a car battery, how to avoid a common, costly mistake, and how to nail the whole procedure like a pro.

Tools You’ll Need

Changing the car battery isn’t a big deal. However, you will need a few tools to get the job done right. These tools aren’t expensive, but a tool like the KAM tool plays a pivotal part in the process.

What’s a KAM tool?


As you know modern cars use a ton of computers to manage complex systems. Your car’s computer power supply must not be interrupted, and that, of course, presents a problem when you need to remove the battery. The solution is a temporary power supply.

A KAM tool is a simple device that supplies your car’s control units with power while your battery is disconnected.

It uses an external power source, like a spare battery or a donor vehicle. It’s obviously important to connect the KAM tool to your car before removing the battery terminals.

A good KAM tool will allow you to scavenge power from a donor vehicle by connecting directly to its battery or through the vehicle’s 12-volt PowerPoint. The power is supplied to your car through the DLC (Data Link Connector), which you can find below the driver’s steering column.

I may have made this sound complicated, but it’s really not. Anyway, here’s a link to Amazon where you can check out price and availability on a pretty decent KAM tool for the money.

What wrenches do I need?

You’ll need a wrench to loosen both battery terminals, and they’ll be the same size, but you’ll also need a wrench to loosen the battery hold-down bracket, and it may be a different size.

An adjustable wrench is a useful tool for loosening the battery terminal bolts, but they can be a little cumbersome. An exact wrench will be a lot easier to operate.

The battery hold-down bracket usually comes in two main flavors – it runs across the top of the battery or is mounted at the base of the battery.

Across the top of the battery type bracket is easy to unfasten even with a wrench. But if you have a battery bracket at the base of the battery, you may need to use a long socket extension and ratchet to unfasten; space is limited.

Common battery terminal and hold-down bracket faster sizes include:

  • 10 mm
  • 11 mm
  • 12 mm
  • 13 mm

Wire brush & sandpaper

A wire brush and some rough grit sandpaper (P80) will be needed to help remove corrosion and contaminants from the battery terminals. Corrosion will prevent the flow of voltage from the battery to the car and also, inversely, prevent your car’s alternator from charging the battery.

Can I Change Car Battery Myself?


I’m a mechanic, and by nature, we love the can-do attitude. I say yes, go for it; you can swap out your own battery. Why pay a mechanic (hourly labor rate $120) for something you can easily do?

Although this procedure is simple, don’t be fooled, there’s plenty of opportunity for things to turn to s&*t. Three mistakes are common:

  1. Disconnecting the battery and losing computer settings
  2. Shorting the positive battery terminal
  3. Fitting the battery terminals back-ways

Losing computer settings


Your car is a network of computers all communicating with each other using high and low voltages, not unlike Morse code if you like. The engine, transmission, braking, traction control, electric steering, HVAC, power windows, MMI, etc., all rely on control units (computers).

As your car ages, your car’s control modules cleverly optimize various parameters to allow for wear and tear, for example. These are known as learned settings. The control units need battery power to operate and to retain information about your car’s settings.

When a battery is disconnected, the learned settings are lost. When a car loses its learned settings, it will often run erratically until it re-calibrates.

In many instances, the car will eventually relearn (re-calibrates) new settings, and in other cases, only a trip to the dealer and a date with the workshop diagnostic computer will put things right. The latter costs money, obviously. Check out the bidirectional scan tool I recommend here on the Auto electrical repair tools page.

To avoid your battery replacing mission going pare-shaped, it’s best to employ a Keep Alive Memory tool or KAM.

Shorting positive battery terminal

When removing battery terminals, it’s good practice to remove the negative (black), also known as the ground (“-” symbol) terminal, first. Removing the positive (Red) terminal first risks contacting the metal wrench with the chassis ground, which shorts the battery.

And although your car’s electrical system is protected by fuses, it’s a complex, sensitive system that is expensive to repair.

But a far more serious risk is an explosion. All batteries vent combustible gases, and shorting the terminal causes a spark that could ignite the gases causing the battery to explode. Follow the fitting battery guide below, and the job will move like butter.

Fitting battery terminals back-ways

Battery terminal check

This is a common one, and the outcome can be mixed. In most cases, the car’s electrical system has built-in circuit breakers such as fuses, but that said, it is possible to destroy expensive control modules.

Also common – connecting the batteries incorrectly when boost starting. This can be equally damaging to the car’s circuitry. I wrote a little more on the subject below, how to avoid it, and how to fix it if it does happen.

Car Battery Terminals Wrong Side

Accidentally fitting the battery terminals on the wrong posts is an easy mistake to make. As said, in most cases, no major damage is caused, but it is possible to fry circuits and control units. Mistakenly reversing the polarity of a cars battery commonly occurs when:

Unmarked battery terminals
  1. Fitting the battery, the terminals are connected to the wrong battery post
  2. Fitting the battery, it is placed in its cradle back-ways
  3. Jump-starting, the cables are incorrectly connected

1 Terminals fitted incorrectly

This happens when the new battery is fitted; the fitter pulls the battery cables to fit the wrong side of the battery. In most cases, it isn’t possible to fit the battery wires to the wrong battery post, as the battery cables are firmly secured to the chassis using clips and aren’t long enough to stretch to the opposite side of the battery post.

The root of the problem is more commonly caused by fitting the battery to the car back-ways (see below).

2 Battery fitted back-ways

Your car battery is particular to your car; it has the correct power output to crank over your motor. A larger engine will require a larger battery, no surprise there. But your car battery is also configured to fit your car’s wiring harness orientation.

A car battery comes in many different dimensions. For a car battery to fit your car, it obviously needs to be the correct height, width, and length. But where the poles are positioned on the battery is important too.

For this reason, car batteries are coded, which identifies the dimensions and the location of and polarity of the battery posts. When buying a new battery, the parts store can access all this information by simply knowing your make and model.

Connecting the battery terminals’ backways is commonly associated with fitting a new battery. It happens this way because the owner was sold the wrong battery and would only fit in the car one way or because the battery was fitted to the battery cradle back-ways.

And by back-ways, I mean the positive pole was placed geographically close to where the negative pole should be and vice versa. This then allows the fitter with a stretch to place the battery terminals on the incorrect battery posts.

Here are a few tips to help avoid this situation from developing.

  • It’s always a great idea to take a pic on your phone before dismantling the old battery, just as a reference for later.
  • Check your battery codes, or measure the batteries and note the location and polarity of the posts.
  • If you’re struggling to get the cable to reach across, then stop and check your battery post identification, you may be fitting the posts back-ways
  • The black negative battery terminal is removed first and fitted last.
  • Battery cables are usually color coded, Red cable positive and black cable negative (ground).
  • The positive cable connects to the starter motor and fuse box. The negative is bolted to chassis ground (frame of vehicle).
Car ground strap
Black Negative battery cable connects to chassis ground

3 Jumper cable incorrectly fitted

Connecting the booster cables’ backways is completely understandable. You’re running late; you’re in a flap it’s easy to get it wrong. Getting it wrong will sometimes cause problems, though.

Step One:

Using the picture above as a guide, put the jumper cables on in sequences 1, 2, 3, and 4.

Step Two:

Start the donor car and allow it to idle for a few minutes (it helps charge your battery).

Step Three:

Start your car and leave the cables attached and your car idling for a few minutes.

Step Four:

Now with your engine running, remove the cables in reverse order 4, 3, 2, and 1.

If a clicking sound continues and the battery cables and jumper cables are making good contact, check out “Car won’t start click noise”.

What happens if you fit terminals backways?

Checking fuse

Fitting the battery terminals’ backways usually causes the main fuse to blow. The main fuse is obviously the biggest fuse in your car, and it’s designed to protect the circuit from overload and potential fire. So when a main fuse blows, you know something major has happened. Blowing the main fuse will cause total power loss.

Attempting to attach the battery booster cables causes a similar but much less serious condition. The symptoms commonly associated with this type of event include power in the system, but the engine won’t crank.

Energy is designed to flow through your car’s circuits in one direction, and some circuits are fitted with diodes to prevent directional flow. Connecting the battery in reverse may cause the energy to flow upstream, which can cause control module damage and also fry the battery.

How to fix fitting the cables back-ways

Wear glasses and gloves and disconnect the battery immediately. If the battery is leaking and fizzing over, use extreme caution; leaving the battery connected risks fire.

Test your battery for damage by initially checking the voltage. If it’s leak free and at least 12.5 volts, go ahead and fit it correctly.

Attempt to start your car; if you have no power or the ignition lights come on, but the engine won’t crank, check your fuses.

Car fuse box

Is Replacing A Car Battery Dangerous?

mechanics disposable gloves

Replacing a car battery can be dangerous. A car battery is filled with acid that produces potentially explosive gases. A vent tube is a common approach to venting the battery vapors away from potential ignition sources.

A faulty battery is the riskiest; a damaged or leaking casing may cause uncontrolled gassing. An ignition source such as simply attempting to boost start your car could cause the battery to explode.

As you can imagine, an exploding car battery could be devastating for those involved. Even a healthy battery vents potentially explosive oxygen and hydrogen.

Wear protective glasses and gloves and avoid potential ignition sources when working around batteries.

Potential ignition sources include:

  • Boost starting direct on the battery terminals
  • Shorting the battery
  • Smoking near the battery
  • Soldering near battery

If your battery is wet, it means it is leaking acid. Acid is nasty and burns like hell. Use glasses, gloves, and long sleeves, washing with lots of cold water if it gets on the skin.

Mix baking soda and water, and pour over spilled acid to neutralize. A leaking battery needs to be disposed of safely; check your local treatment facility. Be mindful that a leaking battery will burn plastic, carpet, fabric trim, and paint. Following simple precautions and battery replacement is quite safe.

Is It OK To Touch Car Battery Terminals?

A regular car battery terminal carries 12-15 volts and is not dangerous to touch. Electric vehicle (EV) terminals however are not OK to touch. A Tesla, for example, runs around 400 volts, and the voltage across the human body should not exceed 100 volts.

Electric vehicle terminals, however, are very well insulated, and the insulation is marked by a bright color, common colors include orange, blue, and yellow. Inadvertent contact with an electric vehicle’s battery or terminals isn’t possible. Several covers and safety shields need to be removed.

How To Change Car Battery

This is a simple procedure, and although it looks long when written down, it shouldn’t take more than fifteen to twenty minutes to execute. You’ll have no trouble so long as you follow the steps.

Note: I may refer to the black battery cable as the “Negative” or the “Ground.”

You’ll need to use caution around leaking acid; it will burn the eyes and skin and anything else it touches. Batteries are heavy but usually have a useful carry handle, allowing the battery to fall and risk damaging the new unit.

Locating your battery in some cars may be a challenge, so if yours is not under the hood, check your driver’s manual or google your model. The usual places include, under the hood, in the trunk, under the rear seat, and under the passenger seat.

If your battery is fitted inside your car, it will be fitted with a vent tube; it’s important to refit the tube to the new battery. If your battery is fitted under the hood, it likely won’t have a vent tube.

Battery vent tube
Battery Vent Tube

Your battery may also have a battery sensor fitted on the negative terminal. When a sensor is fitted to your battery, it means the PCM (Power-train Control Module) employs battery monitoring software.

Ideally, the PCM should be notified of a battery change, but this does require a trip to the dealer. It is possible to code your own battery, and I’ve listed the bidirectional tool here on the Auto electrical repair tools page.

Your car will still run, drive and operate without coding the battery but be aware not coding the new battery to the car risks overcharging the battery and shorting its working life.

Battery sensor
Battery Sensor

Step 1

Battery code
  • Check that your new battery is identical, check the code or check the dimensions and remember to check the pole location and polarity.
  • Clearly identify your positive and negative terminals. The positive battery cable runs from the red plus (+) sign on the battery to the starter motor and fuse box. The negative battery cable runs from the black negative (-) sign on the battery to the chassis ground.
  • Take a picture of you old battery in place, just in case you need to reference it later.

Step 2

Keep alive memory tool
  • Fit KAM to your car’s diagnostic socket (under the steering column) and connect to an external battery or a 12-volt power point of another vehicle. Verify it’s supplying power by checking the integrated light.
  • Turn all consumers off, lights, radio, heater, etc.

Step 3

Battery bracket
  • Use a wrench or socket set to remove the battery bracket. Your bracket may be over the top or at the base. Volvo is clearly concerned about battery movement; they fitted both…. lucky me!

Step 4

Negative terminal
  • Using a wrench, remove the negative ground cable first.

Step 5

Positive battery terminal
  • Now remove the positive red cable, don’t allow the cable to rest on or come in contact with the chassis ground. Rest it on or wrap it in a cloth and set it aside.

Step 6

Remove battery
  • Remove vent tube if fitted.
  • Remove battery.

Step 7

Battery post identification
Plus Symbol Identifies Positive battery post
  • Fit new battery and check battery poles are in the correct location.
  • Clean the red positive terminal before fitting and tightening.

Step 8

Negative battery terminal
Minus Symbol Identifies The Negative Post
  • Clean the black negative ground cable before fitting and tightening.

Step 9

Battery bracket
  • Fit battery bracket and tighten.
  • Fit vent tube if applicable.
  • Some copper grease on the terminals helps prevent corrosion in the future.

Step 10

  • Remove the KAM.
  • Start the engine and test drive.

Are Brand New Batteries Fully Charged?

New battery voltage

New batteries are charged, but depending on long they are stored and the ambient temperatures, they may not be fully charged. It’s not unusual for a new battery to need a little boost. However, there should be plenty of life in the new battery to start your vehicle. A short drive will take care of the top-up charging.

Ask the parts store attendant to check battery state of charge before buying, or ask the shop to charge the battery before pick-up.

Use a volt meter to check voltage and charge battery if below 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)

Does A New Battery Need To Be Charged?

A new battery generally doesn’t need to be charged before fitting unless below 12.4 volts. While a new battery should be 100% charged, they often aren’t. Charging the battery for about an hour with a standard household-type charger will be enough to fully charge the battery.

Alternatively, the vehicles alternator is more than capable of charging the battery to 100% capacity with a short drive of twenty minutes without heavy consumers such as lights, heater, etc.

Do I Need To Drive Car After Changing Battery?

If your battery is charged above 12.5 volts, a drive cycle isn’t necessary. However, test driving after making any repair is advisable. It’s better to find any potential issues now; there’s never a good time for a breakdown.

A 15-20 minute drive is long enough to bring the engine up-to operating temperature and also fully charge the battery.

What To Do After Installing A New Car Battery?

To get the best out of a new car battery, it’s important to run a few simple checks, especially if you suspect an electrical issue such as a charging fault or parasitic draw.

After installing a new car battery, run these simple checks:

  • Check the battery cables are clean and tight.
  • Check the battery hold bracket secures the battery in place
  • Check the hood stay and hood clear the battery posts
  • Test drive the vehicle for twenty minutes
  • Test alternator is operating correctly, see graphic below (if suspect a charging issue)
  • Test for parasitic draw, see graphic below (if suspect a battery drain)

Will An Idling Car Charge The Battery?

Idling a car engine will charge the battery, however, bringing the engine up to 3000 rpm will increase the rate of charge. The best option for charging a battery is obviously a battery charger. Using the alternator to constantly charge a low battery stresses the alternator and drive belt and will lead to premature failure.

Taking the car on a short drive is the next best option; a 15-20 minute drive with all consumers turned off is enough to charge the battery. If your battery fails to charge, you could have one of three common problems.

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

Can A Car Battery Go Bad In 2 Years?

It is possible for a car battery to go bad in two years. Batteries don’t last as long as they once did. Replacing a battery after three to four years is quite common. But a car battery should, without a doubt, last longer than two years.

Common reasons car batteries fail prematurely:

  • Discharge through non use
  • Overcharging alternator
  • Parasitic power draw
  • Faulty battery

Discharge through non use

Car batteries just don’t like being idle, they’re designed to be in a constant cycle of charge and discharge. Allowing them to discharge fully can kill them. Today’s cars have a constant want for power, and more and more electrical components cause huge demands on the battery.

Most car owners don’t know their car is still using battery power even though it’s turned off. The control modules (computers) require a small amount of power to keep the stored data in their memory.

A good battery is so important in modern cars. It’s the reason many cars fail to start in the long-term airport car park. A healthy battery should have no trouble remaining charged for a least a couple of months without a drive cycle. If your battery can’t maintain over this time frame, check for a battery fault or a battery drain.

Overcharging alternator

An overcharging alternator will cause premature battery failure. A faulty alternator regulator may cause the alternator to overcharge the battery. This isn’t as common as once it was, as many alternators are now controlled by the engine’s control module, which is a ton more reliable.

If you suspect an overcharging condition, go ahead a run the test as per the pic below. A reading above 14 volts with all consumers on means the alternator is doing its job. If however, the voltage is above 14.6 volts with all consumers off, suspect a fault.

Alternator test

Parasitic draw

A parasitic power draw is a constant drain on the battery. The draw may be small or large, small is the more usual type. When a vehicle is parked up for a period of several days without being started, even a small parasitic draw can flatten a battery.

Prolonged cycles of deep discharging may damage the battery. Today’s cars are packed with electrical consumers and control modules all of which are power hungry. As a result parasitic draws are common.

Battery drain

Faulty battery

It is possible for the battery to be faulty which is disappointing. But before condemning the battery, check your battery cables are in good shape. The terminals are clean and tight. Loose, dirty, corroded or damaged cables will exhibit symptoms identical to a flat battery.

If you have a volt meter, go ahead and check the battery voltage and run a crank test as per diagram below. If it checks out OK, run a volt drop test as per below diagram.

Battery test guide
Battery cable volt drop test
Battery cable volt drop test

You can run the same test throughout the circuit to find a point of high resistance.

Will A Car Battery Recharge If You Let It Sit?

A car battery may not recharge if fully discharged. A battery charger will not attempt to charge a battery if it can’t detect a voltage. It is possible to override the battery charger by connecting a good battery using booster cables. This fools the battery charger and the charging process begins.

After an hour, the donor battery and booster cables may be removed and the flat battery should continue to charge. To fully charge a flat battery, be prepared to wait several hours, depending on the amp rating of the charger.

Related Question

Does Revving Engine Charge Battery? Revving the engine at 3000 rpm will increase the alternator output. Higher alternator output will charge the battery in a shorter amount of time.