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Changing a car battery without losing the codes

According to consumer reports Walmart has good inexpensive batteries. The Walmart mechanic claims that even if their memory saver wouldn’t work with my 2003 Lexus LS430, the battery could still be changed by them without losing all the memory codes, by the mechanic first starting the car and then while the car is running, taking the old battery out and installing the new battery. The guy says that once the car is running the alternator takes over, power is not lost (codes are not lost), and even though the battery is removed the circuit is still not broken since the alternator is completing the circuit and supplying the power to run the car. I can find nothing to support this method on the internet or Youtube. Is what the Walmart guy says is correct and/or will it cause damage to the car?

Never disconnect a battery while the engine is running. This can cause voltage spikes that can damage computers and modules.

Here’s a link that addresses what to do and what not to do when replacing a battery on modern vehicles.


Wow. I can’t believe you’re getting this advice.
I remember last doing this on a 1963 Dodge Valiant (when I was too stupid to realize how I could blow the then-primitive electronics).

The battery acts as a huge capacitor to even out the natural voltage spikes that your alternator will produce. Without the battery in the circuit to absorb those normal spikes, they will end up causing harm to the numerous sensitive electronics you have.

This could be a very costly mistake to you.
Run, don’t walk, from this Walmart mechanic.

Tester is right. This is just a “work around” solution that the Walmart service technicians are using. It can damage your vehicle. The proper way to replace the battery is to connect a “memory saver” or 12VDC directly to the battery terminals before removing the old battery. Make sure the new battery is fully charged before beginning the R&R procedure.

What everyone else said.

And don’t take your car to Walmart. Do you really want the underpaid Blue Vest Brigade screwing around with your car?

Very, very bad idea.
You can change the battery by using one of those devices that plug into the cigarette lighter. Plug it in, open the hood and close the doors. Wait about twenty minutes and then change the battery out.

Yeah. Walmart isn’t known for there automotive excellence. DO NOT DISCONNECT YOUR BATTERY WITH THE CAR RUNNING!!! This is very very bad. Here’s what you can do. With your car off, put jumper cables on to the terminals of your car and connect them to the terminals of another car. Disconnect your battery and install the new one. You should have had power provided constantly throughout the transaction.


I get it. I suspected something was wrong with this plan. Thanks for the responses.

In the days before the alternator replaced the generator it was not a good idea to disconnect the battery while the engine was running as the generator or voltage regulator or both could be damaged. There were specific instructions in the owner’s manual of my 1954 Buick as to how the battery was to be replaced. When the new battery was installed, the positive cable was to be connected first. With all accessories off and the doors closed (so the dome light would be off), one was to touch the negative cable against its post for just a second and then move the clamp away from the battery post. Then immediately scratch the clamp against the terminal again. If there was no spark, reattach the terminal. If there was a spark, allow the clock to run down and repeat the process. This was to make certain that the clock was fully wound. Clocks in those days were spring driven and were rewound about every two minutes.

Even more than 50 years ago, the device that plugs into the cigarette lighter to maintain the memory codes would have been useful.

Yes, and one slip of the wrench while removing or installing the positive battery cable and you will be spot welding.

I would love to be a WalMart mechanic. I like giving people bad advice on thie board and by working for WalMart, I could practice what I preach!

What “codes” are we talking about loosing?, radio presets?

That and the presets for the seat and steering column adjustments for both drivers. I think a Lexus 450 might have those.

I liked the battery commercial where the non-skilled guys say “we’ll make it fit” and you see these two idiots beating hell out of the car to force the battery in.

Ever pulled a battery out of a late 90’s GM “G” van? what a job. There was one business jet I worked on in the 80’s that you put the battery on your head and stood up to raise it into its hole. On this plane the breaker panel for the rear cabin was over the toilet,so much fun. I am 6’1 230 and it was no fun sitting in the cockpit of even a larger jet like a Citation III to work on the panel if someone else was also in the cockpit, these planes are for smaller people. It seems that not a lot of thought is given to battery placement in more than one type of transportation.

Airplanes, electricity and toilets seem to run together. I never landed an entire plane, just various parts of them. Crew entry doors land hard.

Just to let you know, my 2000 Ford Explorer has a back-up system built in. I’ve have the battery replaced twice since I owned it, and never lost my radio presets or memory seat settings. I would think a 2003 Lexus would also have such a system.

A 9V battery like those used in smoke detectors is enough to keep from losing the stored radio presets, etc. You can hook it up to the the battery terminals before you remove the car’s battery, securely so it stays connected while the battery cables are being handled. You can also buy or make an apparatus to plug into the cig lighter - check wiring diagram to know which is positive and which negative. This has worked for me. Remember that in some cars the cig lighter is not energized with the ignition off.

My car cigarette lighter is on all the time. Am i to believe that if i plug the 9 v thingy in the cigarette lighter receptacle,and THEN disconnect the car battery is the right way to do it?

What do you do for a car that needs the ignition on?

The back-up 9 volt battery will keep computers and modules Keep Alive Memories alive. This is because these electronic components only need 5 volts to retain the KAM.

Only use this back-up 9 volt battery system to replace the battery. As it only lasts so long. When I use it, I place a sign on the drivers door window to remind me not to open the door while this 9 volt back-up system is connected. Because if you open the door, the body control module awakens and tries to turn on the interior lights. This then kills the 9 volt battery and you lose the back-up system.

If you need to turn the ignition on, make sure the main battery is connected. Because you’ll kill that 9 volt battery as soon as the door is opened.


I have a truck that the cig.lighter is “hot” all the time,so i assume all i have to do is connect the “memory saver” into the cig lighter, and remove the 12 volt battery,correct?

The car has a cig. lighter that needs the key turned to probably the accessory position,then remove the 12v battery,correct?