Rick's Free Auto Repair Advice

Replace a car battery

Replace a car battery

A note about providing backup power when you replace a car battery

Any time you disconnect battery cables from your car battery, the computers in your vehicle will lost their volatile adaptive memory. When you start the vehicle with the new battery, it may not start well or run well because it’s running off of factory programming. It can take up to 10 cold starts and drives to operating temperature for the computers to relearn how to run properly.

There’s one easy way to avoid this; provide backup power to the vehicle BEFORE disconnecting the battery terminals.

How to provide backup power when you replace a car battery

You’ll need three things: a jumper pack, a special OBDII cable, and electrician’s tape.

jumper pack

Get this ES5000 1,500 amp peak power jumper pack for about $125

Solar ESA30 OBDII Memory Saver Cable

Solar ESA30 OBDII Memory Saver Cable

Here’s the procedure:

1) Connect the D-shaped end of the cable to the OBDII port on your car.
2) With the engine off and keys out of the ignition cylinder, plug the opposite end into the jumper pack and turn it on.
3) Disconnect the negative battery terminal.
4) Disconnect the positive battery terminal and immediately wrap the entire terminal end with several layers of electrician’s tape. With backup power applied, the positive terminal is powered and if it touches any metal component, it will blow out the fuse in the jumper pack and depower your car’s computer and modules. For extra safety, shove a disposal plastic cup over the end of the positive terminal and tape it to the cable.
5) Remove the old battery and clean the battery tray and negative terminal
6) Clean the posts on the new battery.
7) Replace the car battery with a new one
8) Remove the tape, clean the positive terminal and install on the positive post
9) Install the negative battery terminal.

If your vehicle has start/stop, there’s an extra step

Start/stop vehicles are usually equipped with an enhanced flooded battery (EFB) or an absorbed glass mat (AGM) battery. As these batteries age, the ECM adapts its recharging routine to account for aging lead plates in the battery. When you replace the battery, you MUST tell the ECM that you’ve installed a new battery OR it will continue to recharge it using the old routine. That can damage the new battery in as little as six months!

Some vehicles automatically conduct a self relearn while others required a scan tool reprogram. You must check this out ahead of time to find out what your vehicle requires.

If your vehicle requires scan tool reprogramming, get it to to a shop for reprogramming ASAP after replacing the battery. If you skip this step, you can destroy the new battery.

Replacing a car battery in an older vehicle with a mechanical throttle body is simple

• Providing backup power as described above is HIGHLY recommended.
• Remove the battery hold down brackets
• Swap in the new battery and reinstall the hold down brackets. Don’t over-tighten the hold down brackets; you can crack the battery case
•Use a battery terminal and post cleaning brush and reconnect the terminals.

Replacing a battery on a late model engine with and electronic throttle body is different

The electronic throttle body is the biggest issue

An electronic throttle body contains a small motor that opens and closes the

electronic throttle body

The gas pedal sends electronic signals to the PCM which then tells the throttle body how far to open

throttle plate to control how much air enter the engine. As carbon accumulates in the throttle body opening, the engine computer compensates for the buildup and establishes a new “home” position for the throttle plate.

WARNING: If you disconnect the battery without providing backup power, the computer will lose this setting. When you connect the new battery, the engine may not start! If it does start, it may run poorly. In some vehicles you have to perform a throttle body relearn procedure to get it to start and run properly.

Supplying backup power prevents this from happening!

Throttle body relearn after you replace a car battery

If you skip the backup power step, you’ll most likely have to perform a throttle body relearn procedure.  Some car makers have a self learning throttle body re-learn procedure. In those vehicles, all you have to do is turn the key to ON and leave it there for 30-secs. without starting the engine. But other car makers have a complicated Hokey-Pokey procedure that requires multiple brake pedal applications, along with several key on/off cycles. Still others require a factory scan tool relearn.

Stability control relearn is also a big problem

Stability control systems compare inputs from the steering angle sensor to the readings from the yaw sensor to figure out if the vehicle is in a skid situation. If you disconnect the battery without providing backup power first, whichever position the wheels are in when you disconnected the power will be the new “straight-ahead” when you connect the new battery. As soon as you drive the car, the computer will detect a steering angle sensor problem and set the check engine light. How do you reset the steering angle? Yup, an expensive scan tool.

Disconnecting the old battery without backup power also causes these other problems:

• The computer resets all its readiness monitors

If you have smog/emissions testing coming up soon, a “readiness monitors not ready” situation will be an automatic fail.

• Ford Failure Mode Effect Management (FMEM) resets

The FMEM stores an adaptive “fail-safe” strategy that substitutes known-good values for sensor data that’s missing.

• ABS module resets to factory default

On some makes and models you need a high-end scan tool to reset

• Climate Control loses memory

Most climate control systems vary air temperature by opening and closing baffles and doors to regulate how much air passes over the heater core or evaporator coil. Those doors are operated by electric motors. When the HVAC controller loses power, it forgets what position the doors were in. Many late-model vehicles require a scan tool re-calibration to return the motors to their home position.

• Body Control Module (BCM) may not work properly

Depending on the condition of the battery before you disconnect it, the BCM may scramble its memory and not operate properly. That will affect lighting, door locks, moon roof, power mirrors and windows.

• Anti-theft reset

Some anti-theft systems require a relearn if they lose power.

• Power accessory memory loss

If you’ve programmed memory seat positions or auto up/down features on your windows, those may be gone if you disconnect power.

• AGM start/stop reset

If you install a new battery on a vehicle equipped with start/stop, EVEN IF YOU DON”T USE THAT FEATURE and you don’t reprogram the computer to tell it you have a new battery installed, you will shorten the life of the new battery.

Keep the computers powered when you replace a car battery

If you don’t own a $3,000 factory scan tool and don’t want to have your car towed to the shop after you replace your car battery, you’d better be prepared to keep all the computers powered up during the swap. Here’s why. All the modules have an initialization point. When you disconnect power, they forget that point and that can cause a no-start, rough idle, sluggish acceleration, traction control or stability control malfunction, as well as loss of settings for your memory power seat, radio, mirrors and anti-pinch power windows.

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©, 2016 Rick Muscoplat

Posted on by Rick Muscoplat


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