Keeping settings while replacing battery

Time to replace the OEM battery on my 2010 Escort. I realize that if I disconnect the battery, all the on-board computers, PCM, radio, etc will loose their memory.

So, can I keep power flowing by connecting a charger to the battery cables behind the terminals? Then I can (carefully!) remove the terminals from the battery while the charger maintains power to the electronics…

I also have an easy way to run a 12v float charger into the auto accessory socket so I don’t have to actually clip onto the battery cables themselves.

–> Will this work, or do I risk blowing up the electronics since the charger output may not be the even current from an actual battery?

I’ve also heard that hooking up a 9v transistor radio battery up to the accessory socket will preserve the settings.

–> Will a tiny 9v battery maintain the settings for an hour while I run off to the store?


Second thought, directly connecting a 9v battery won’t work. It would need circuitry (diode, current limiting resistor?) to protect it from being literately blow to bits by the main battery during the time both are connected together. Very bad idea.

But what about the 12v charger?

A nine volt battery will work. The modules/computers/radio just need 5 VDC to keep their memories alive.

You can go to a local parts store and get a cheap 9 volt memory saver and plug into a cigarette lighter/power port that has constant voltage.

I forgot to add. If there’s an underhood light unplug it or remove the bulb before disconnecting the battery. And don’t open the doors while the memory saver is plugged in. If any lights come on with that nine volt battery, it’ll kill the battery and the memories will be lost.


I have a 12 volt one, with a powerpoint plug that then plugs into obdc. I used an old 12 volt ac adapter and attached a powerpoint with spade clips, paying attention to polarity of course. I had one of the 9 volt ones that plug into a power point, It works fine, but I am not sure of battery life, and you need to make sure the power point has continuity when the ignition is off.

Results using the cigarette lighter socket/float charger:

When I disconnected the battery, the idiot light on the charger went out. I assume it decided the load (dome light, etc.) wasn’t the same as a battery.

When I installed the new battery, the idiot light came back on, and the idiot observed…

  1. The radio presets, including Sirius, lasted the 45 minutes to do the R&R plus trip to the store.

  2. The clock lost its setting, and I assume the PCM et. al. did too.

  3. I did the engine relearning procedure (1 min idle, no A/C, then 1 min w/AC, then 10 mins road driving). The shifting was improved as soon as I started driving. Previously, there had been a little slip around 2-3 or 3-4.

Perhaps the power on reset/retraining is a good as having the dealer “re-program the transmission” for $100 ??? !!!

I believe I shall get one of the cords for the OBDC socket for when the next time comes around.

I haven’t used it yet, but I just recently bought a “memory saver” made by Lisle (bought at I went through replacing batteries on 2 different cars … the older one wasn’t much of a problem, but I still haven’t got all the settings restored on the newer one.

General question: Assuming

  1. you don’t have one of the few vehicles that needs to be towed to the dealer if the battery is disconnected,
  2. and assuming you don’t care about resetting your clock and radio presets,

why is it important to preserve power?

How much of a concern is it for the ECU to have to “re-learn” its settings?
I didn’t think it was a big concern? What am I missing?


Saving the radio settings, especially the Sirius authorization, IS important. Especially since it’s my spouse’s car…


“You just replaced the battery in my Chrysler and the engine died as I was pulling out into the street”
“Oh, your car just needs to re-learn the idle control. Just drive the car around the block for a few minutes and it should stop dying.”

“You just replaced the battery in my Honda/Volvo/Chevy/VW/etc and now the radio doesn’t work. It wont come on at all, it just says CODE on the display.”
“Oh, losing battery power has put it in anti-theft mode. Just enter the 5-digit code to restore operation. It’s in the owner’s manual or on a little card in the owner’s package.”
“I don’t have either of those.”
“Well I can’t help you then. You can take your car to the dealer and see if they can get it for you”…alternately…“OK, there’s a procedure for us to obtain the code for you. Have a seat and we should get you back on the road in an hour or so.”
“I can’t sit here for an hour, I have to pick up my kids…”

“You just replaced the battery in my Toyota and now the sliding doors and/or sunroof don’t work”
“OK. Those systems need to be initialized after losing battery power. We’ll be glad to do that for you, the cost is $45 plus tax.”

“You replaced the battery in my car this morning. This afternoon I spent 45 minutes in line at the emissions test station and then they said they couldn’t even test my car because it wasn’t ready.”
“OK. Drive your car around some more and try again.”

I’m not making these up.

Are any of these scenarios acceptable to either the customer or the shop?

It would seem to me that the simplest thing would be to hook a battery charger to the battery terminals before disconnecting them, assuming you own one. Am I missing something?


There is a “quality of power” issue that hasn’t been directly addressed.

A charger can put out a rapidly varying voltage that averages the 12.6v needed to charge the battery. This current isn’t as “smooth” as that from a battery, and it might bother the electronics. This the basis of my original question.

On the other hand, the output of the alternator is a similar “rough” current, and the electronics is prepared to deal with it. But this filtering might be done so that current provided via the accessory socket doesn’t reach it.

Further, the battery by it’s nature acts as a filter, so attaching a charger across the terminals can ameliorate problems in the charger. But with the battery removed, the electronics might not like seeing the charger instead of the battery while I’m off to the store.

Being uninformed on how the new automobile electronic systems are actually implemented, and just guessing at what dangers might or might not exist, I was hoping to hear from an expert.

Blowing up the electronics can get very expensive!

Thanks @asemaster. Good points, especially from the view of a paying customer.

Read this.


Whenever I replace the battery in my Accord I get the antitheft CODE on my dial. I did not have the code, but I did know that Honda will provide the code for you if you answer a few questions to verify ownership. Possibly other manufacturers do the same thing. I had no problem unlocking the radio and after about an hour I still had AF/FM settings. I had to reprogram the XM channels though. I wrote the codes on an index card so that I always have them for the next battery 4 years from now.

I bought a 9 volt device to maintain the settings on my 2011 Toyota Sienna while I cleaned the battery cables. It plugged into a power outlet. However, the clock did not maintain the correct time. The next time I cleaned the battery terminals, I forgot to turn the key on, so no power would flow through the power outlet. The clock reset itself to 12:00, but the radio maintained its settings.

I’d avoid using a battery charger to supply power to the ECM when the car battery is removed. I think you mentioned it yourself, the engineer who designed the battery charger circuitry assumes it will be connected to a car battery. A car battery is – from an electronic circuit’s perspective – a huge capacitor. When the battery is removed, that capacitor is not longer in the circuit, and what the battery charger’s output does without the capacitor is uncertain. Voltage spikes and oscillations could damage the ECM, which is something you definitely don’t want. It’s sort of like removing the suspension springs and shock absorber from a car and trying to decide what will happen on the next bump. Hard to say.

Best to use either an already proven method (best if suggested by the manufacturer of the car) or a commercial device made for this purpose. Whether or not 9 volts is sufficient to maintain all the memories will vary car to car I expect. Since the normal mode is a 12 volt battery power throughout, there’s no particular reason to expect 9 volts will be enough voltage to sustain all the different memories involved.

I considered other alternatives but I simply bought the memory saver @Tester mentioned. I think I paid $10 for it and worked fine. You do need to follow the instructions though. Use a new 9v battery, shut off all accessories and lights, and so on, and have all your prep work done ahead of time to minimize the downtime. Also you have to check to make sure the lighter outlet is on when the ignition is off or not. Just follow the directions is all.

To do this with the minimum of new equipment, why not buy the battery, then

  1. Connect the positive post of the new battery to the positive cable of the car, negative to negative.
  2. (Carefully!) disconnect old battery, leaving new battery powering car.
  3. Put new battery in car. Once each clamp physically touches the post, slip the jumper off the bottom of the post, so the battery can be clamped down.

(To make this MUCH easier, substitute a battery with both top and side posts.)

Somehow, the car’s computer lives through a substantial voltage spike with each and every start, so perhaps it isn’t as fragile as supposed.


“There is a “quality of power” issue that hasn’t been directly addressed”

You’re right. If you ever “looked” at the current coming out of a regular battery charger with a labscope you would see it’s very “dirty” and inconsistent. I’ve seen 16 or more volts coming out of a battery charger, and it’s a noisy 16 volts at that. Many electronic modules are sensitive to voltage spikes. There are special battery chargers/maintainers out there for use when reflashing or reprogramming modules that require the key on for extended periods.

Alternator output is regulated, and while a small amount of AC voltage (ripple) is present, it’s not enough to do any harm. But a system connected to a charger without the battery in circuit to act as a buffer may cause some problems. I’m not sure though because I’ve never tried.

We always plug into the OBD data link via a connector and jumper pack when changing batteries.