• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Changing out a car battery, but keep ECU powered

mzeitlin3348

mzeitlin3348

See it and Believe
I did something stupid because I wasn't 'thinking'!

My car battery, which lasted 5 years using a battery tender when not in use, developed a bad cell. The voltage dropped from its usual 12.8 volts to 10.6 volts (dead cell).
Amazingly, it still would start the car, but barely. I bought a new battery and decided to use a small UPS 12 volt battery to power the ECU through the OBD port while I change the battery. I want to do this so my ECU settings and emissions monitors don't reset.

Big mistake.

Making the connection using a commercial OBD connector cause the OBD 7.5 amp fuse to blow instantly. I didn't think that connecting the small 12 volt UPS battery would try and charge the depleted 10.6 volt car battery.

What would be a smart and safer way to maintain a car's ECU using the OBD port (since I have the tool) and not encounter wild amp current flows? There will likely always be voltage differences between the car's starting battery and a maintenance battery.
 
Register to hide this ad
I believe the manufacturers have ways to do this that probably varies by brand. Search your vehicle model on the web for battery change and there should be some videos about it. Not dealer method but the shadetree mechanic type swap.
 
Yes, I have one of those, but you still have to supply a secondary voltage source since the main battery is going to be disconnected.
 
If you're fast and can switch batteries quickly, you should be OK as the ECM should have a small onboard battery in it to retain the memory when the battery is undone. Leaving the battery undone for 15 minutes or so will erase any codes or memory in the ECM. Always best to check the service manual first though.
H2oman7.62mm said:
There are a few devices that save the ECU's memory and settings, but I think you have something else going on here.
Click to expand...
 
I'm not sure what the OP is doing or wants. If you are talking about the "Brain" of your car, also the ECU or PCM, you dont need to apply ANY voltage whatsoever to it when you change your battery. If someone told you this, they dont know squat about cars. The ECU/PCM has what amounts to flash memory in it. Meaning that once programmed, it needs nothing to retain the information. Think of a thumb drive. You can put data on it and take it out of your computer and let it sit for years and plug it back in and your data will be right there. If a cars ECU were to lose programming with a simple battery change, all vehicle owners would be aware of this and would have to take their cars into a service center just to change the battery. There would also be devices for sale at any auto parts shop for the DIY crowds. NO mechanic that I have EVER talked to who told me my batttery needed to be changed has EVER said ANYTHING about losing data when I needed a new battery.

Just to give you an idea of where I am coming from, I replaced ECUs in all 3 of my Jeeps. I bought them online and had them programmed with the VIN for each Jeep. They ALL have the factory programming in them. It took several days to get them in the mail. Just an ECU in a box. No battery powering them. I even removed the factory programming from my daily driver and replaced it with custom programming. I have the factory programming sitting on an ODB flash drive sitting on a shelf. Been on that drive for quite a few years now, with no external power source. If I wanted to, I can plug it back into the ODB port and put my Jeep back to factory and it will be just fine.

As for removing stored codes by disconnecting the battery, that is a load of crap. The ECU is a type of flash memory and once a code is set, it will remain there no matter how long you disconnect the battery. The only way to remove the code is to fix what is causing the code and then erase it from the memory with a code reader. My shop messed up my Jeep a few days ago. When I got home, I pulled a code P0500 out of it. I erased that code with my code reader, took my Jeep back to the shop and told them what the code was. They hooked up my Jeep to their industrial many thousands of dollar computer and couldnt find a code anywhere. Codes are stored in three different areas of the ECU. I erased the code and my Jeep didnt throw a new code so they couldnt find and code in my ECU.
 
mzeitlin3348 said:
I did something stupid because I wasn't 'thinking'!

My car battery, which lasted 5 years using a battery tender when not in use, developed a bad cell. The voltage dropped from its usual 12.8 volts to 10.6 volts (dead cell).
Amazingly, it still would start the car, but barely. I bought a new battery and decided to use a small UPS 12 volt battery to power the ECU through the OBD port while I change the battery. I want to do this so my ECU settings and emissions monitors don't reset.

Big mistake.

Making the connection using a commercial OBD connector cause the OBD 7.5 amp fuse to blow instantly. I didn't think that connecting the small 12 volt UPS battery would try and charge the depleted 10.6 volt car battery.

What would be a smart and safer way to maintain a car's ECU using the OBD port (since I have the tool) and not encounter wild amp current flows? There will likely always be voltage differences between the car's starting battery and a maintenance battery.
Click to expand...
They used to sale a service that had a battery in it that plugged into your liter that had just enough voltage to keep the memory in the computer
 
Mercedes has a second smaller battery in addition to the main battery. Mercedes recommends disconnecting the secondary battery before replacing the main battery. I've read posts about electronics getting bricked because the proper procedure was not followed. Coming back to the OP, not sure maintaining power to the ECU is worth the risk.
 
1. Attach jumper cables from the terminals on the existing car battery to the new battery.
2. Disconnect existing battery terminals
3. Remove old battery
4. Install new battery
5. Attach battery terminals
6. Disconnect jumper cables.
7. Be careful
 
You must log in or register to reply here.
Back
Top Bottom