Using a Battery Code Saver?


Hi John,

Regarding your comments with Tim Webster on Radio 2UE in Sydney last Sunday: You suggested connecting a 12-volt battery via the cigarette lighter to avoid shutdown of the car's electrical/security system, thus avoiding potentially time-consuming and expensive re-coding of various components.

Does that work for any car, for example my Subaru Forester? My mechanic recently did some work on my engine which required temporary removal of the battery. After that I had to go to Subaru to have the computer scanned and recalibrated (and the radio reset). That cost me another $100. It's a real drag.

My mechanic is also anxious to know your answer as it's constantly a problem for him too. Do you have to rig up the wiring connections for the cigarette lighter yourself or can you buy something?

Appreciate your answer. And I'm sure lots more would too!




Firstly, Dennis, thanks for listening. I really appreciate it. It's why I'm there.

Secondly, this works on any car. Think of it like this. The body of the car is connected to the negative pole of the battery - so all the exposed metal bits are an extension of the negative pole. All the wires are positive, more or less. So the car is a huge network of transmission lines with power coming out of the battery, going to the fuse box, then flowing to various devices (like the overhead light, for example, when the switch is on) and then back to the battery via the body and the negative terminal. 

The cigarette lighter typically consumes power when it is pushed in. The centre terminal is the positive one, and the barrel is the negative. Basically, a memory code saver incorporates a battery that simply plugs into the cigarette lighter, with the positive terminal connected to the centre terminal and the negative terminal connected to the outside (barrel) terminal.

Actually it's slightly more complex than this: the battery on the memory saver is a lot smaller than the car's battery. The last thing you want is the car's big battery blowing the small battery up by driving the current flow backwards through the small battery. This is why the memory saver incorporates a diode on the positive side. A diode is essentially a one-way valve for electricity, which does not permit the flow to reverse.


This is pretty much all you need to do the job - plus another 12-volt battery to provide the electricity

This is pretty much all you need to do the job - plus another 12-volt battery to provide the electricity

You can buy one of these off the rack from eBay for about $50.

So, you've got your tiny battery in your memory saver hooked up to your cigarette lighter. You've got the cigarette lighter activated (some cars need the ignition in 'ACC' setting to energize the lighter). The diode stops the car battery from blowing the small battery up. You disconnect the car battery, which is the small battery's cue to supply power to the car's electrical 'grid'. Most of the systems are powered down, because the car is off, so it's not all that big a current drain. The diode lets the current flow out of the small battery (the right way) to power up the grid.

When the new battery is in place, the diode senses it trying to pump current up the bum of the small battery, which the diode does not allow (because that would be a violation). All you do then is remove the battery saver, and the car's radio and computer has no idea it has even been disconnected.


It's pretty easy to make your own memory saver. It's also pretty easy to blow yourself up if you get it wrong. And the time you'd need to acquire and assemble the parts is ridiculous, unless you really, really want to. You need a temporary 12-volt battery like this one, and a couple of crimp-on terminals to suit. You also need some black wire and some red wire, and a diode like this one. And some alligator clips like these. So, you solder the diode into red wire in the right direction to allow current to flow only out of the battery's positive terminal. You put the battery crimp-on connector on the right end of the red wire, and the alligator clip on the other end. And you marry up the same thing on the black wire (with no need to worry about the diode, and which end gets which connector.

You can test it's working by connecting the wires to the battery and ensuring there's 12 volts between the alligator clips, using a multimeter. If you don't know how to do this, you shouldn't be messing with electricity. Except, possibly, light switches and power points.

So, you plug the black wire into the negative terminal of the temporary battery, and the red wire into the positive. Clamp the red alligator clip to the positive battery lead under the bonnet, and clamp the black one to the negative lead. Change the battery, taking care not to earth the positive lead anywhere on the body because it is still active. (you can put it in something like a stubby holder to do this.) Take care not to dislodge the alligator clips because this will kill the temporary supply to the car's 'grid' and you'll need to re-code everything.

Re-connect the new battery, and the electronics will have no idea you've fooled them into thinking the main battery was ever missing.


Really. It is. Apart from the solution above, you could just buy this:

$12.95 from Jaycar Electronics.   Full specifications here  .

$12.95 from Jaycar Electronics. Full specifications here.

You need to find your car's onboard diagnostics port (OBDII port). It's under the dash near the steering wheel, usually.

Hook it up to one of these:

$39.95, also from Jaycar Electronics.   Full specifications here

$39.95, also from Jaycar Electronics. Full specifications here

So, for $53 you've got yourself a fully-functioning plug-in solution, provided you can find that OBDII port under the dash.

Lastly, if you want the cheapest solution, this will work:

It's less than $20 and posts from Adelaide, or from auto parts stores. It'll have a diode (mentioned above) built in, up the top of the battery compartment. Click the image for the details

It's less than $20 and posts from Adelaide, or from auto parts stores. It'll have a diode (mentioned above) built in, up the top of the battery compartment. Click the image for the details

Apparently 9V DC is enough to power up the car's computer systems temporarily - or at least keep them thinking they're still connected. I found this video, which has awful production values, and is three times longer than necessary, but gets the point across adequately, and as far as I can see he doesn't get anything wrong (except call himself 'heymargaretlook'):

If you go with the cheap option, just make sure you have everything in the car turned off before you start the job. That's very important because the 9V battery will deplete itself rapidly if you give it anything serious to run - even the interior light. And I'd be using a fresh one from the start, as well as minimising the time the proper cranking battery was disconnected.


These (below) are less than $100 online - and perhaps every mechanic should have one. They incorporate things home-made devices don't, like internal charging and overload protection:


This alternate power source approach is fine for changing the battery. It's not fine for other jobs. Mechanical work often requires the battery to be disconnected for important health and safety reasons, perhaps so you don't start an electrical fire or burn out some component or other by bridging 12 volts through it using a spanner, or something. If you do this with the battery disconnected and the temporary battery fitted, the risk you are trying to mitigate by disconnecting the battery still pertains.

You could also easily hurt or kill yourself in some situations by leaving the car's electrical system effectively powered on.

For example, if you are fitting an accessory under the dash, looking for a suitably positive wire, if you inadvertently bridge 12-volts to the airbag deployment wire, the airbag will deploy. This could kill you. Sometimes 'disconnect the battery' means 'disconnect the battery'...

You have been warned.

See my earlier post: Disconnecting your car's battery and using a memory saver.