The purpose of this device is to preserve the memory of a car (ECU, radio codes etc.) while the main 12V battery is disconnected or replaced.
I didn’t buy it, instead I made one myself using scavenged parts from other devices but I haven’t tried it yet.
My main concern is that most cars nowadays disconnect the cigar lighter socket when the ignition is turned off. So in order to change a car battery I will have to turn the ignition on, connect this device and then replace the battery. Is this going to fry anything?
If you are going to make something out of your own parts, you’ll do better to put big alligator clips or some type of wire clamps on the end of it and have it attach to the battery cables.
This device is convenient and will work fine. I change many batteries and never bother to save anything. I know from experience that the initial draw on the system when hooking up can be up to 250ma for as much as 45 seconds and then it should drop down to sleep mode under 30ma. They use the 9VDC battery because it is readily available. I would prefer 12 volts, but the 9 volt will handle it for several hours. And since you deploy this device before disconnecting the battery, you avoid the initial shock that brings the tiny battery down much faster.
If I were going to worry about this while changing a battery, I think I’d just use a couple six volt lantern batteries in series - I generally have a couple lantern type flashlights around. And just clip them to the battery cables, as suggested above.
with power port method has the backup power out of the way though it doesn’t take much to dislodge the plug. also the key could get turned to off and that port loose conductivity.
clipping to the starter battery works but you need big honking clips that aren’t going to fall off when you move the battery terminals. this is more secure with the right materials.
A 12V battery doesn’t put out exactly 12V. If you connect two batteries (the car’s battery and the lantern batteries) in parallel, whichever has a higher voltage will try to charge the other. This can be dangerous or damaging.
It may be OK if the lantern batteries have higher voltage; the internal resistance will limit the current, and I suspect the car’s battery can handle that current. The other direction will be very dangerous; the car’s battery is designed to output a large current, and lantern batteries aren’t designed to be recharged at all, let alone at such high current. You should be able to prevent this situation by using a diode in series, but I’m not making any guarantees.
So this thing is keeping your cars electrical system energized when the car battery cables are off?
What happens if the car battery cables short against something while disconnected? I know a 9V radio battery won’t hurt me. But what about the electronics in the car?
No, as mentioned above probably just to the accessory position. The smart thing to do is check the socket with a test light so you KNOW it’s connected.
If you get the polarity reversed, YES. Use a lighter socket plug to make your device and be SURE you get the polarity right when connecting it.
The most common times that my car battery has been disconnected has been when it was in the repair shop. Wouldn’t a device like this tend to interfere with the diagnostic tests the mechanics are running with their automotive computer testing system?
There are quite a few procedures where the first step in the factory instructions is to disconnect the battery. One example is starter replacement, but there are many, many more.
There aren’t any tests I can think of to do with the battery disconnected, and this is only used when the battery is being disconnected, so no.
