Thread Tools
Old 01-07-2012, 06:04 AM
Guest
Join Date: Dec 2003
Location: Athens, Greece
Posts: 3,354
Has anybody tried a "car memory saver"?

I am talking about this device: http://amazon.com/CTA-Computer-M.../dp/B000GKD5Q0

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?
Old 01-07-2012, 06:18 AM
Robot Mod in Beta Testing
Moderator
Join Date: Mar 2001
Location: Pennsylvania
Posts: 21,558
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.
Old 01-07-2012, 06:35 AM
Guest
Join Date: Apr 2010
Posts: 1,803
These will work

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.
Old 01-07-2012, 08:43 AM
Charter Member
Join Date: Mar 2000
Posts: 7,849
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.

Last edited by yabob; 01-07-2012 at 08:44 AM.
Old 01-07-2012, 08:59 AM
Guest
Join Date: Jul 2009
Posts: 12,056
power ports are active with the key to accessory.

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.
Old 01-07-2012, 09:47 AM
Member
Join Date: Aug 1999
Location: Alabama
Posts: 14,542
Quote:
Originally Posted by yabob View Post
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.
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.

Last edited by scr4; 01-07-2012 at 09:48 AM.
Old 01-07-2012, 10:37 AM
Guest
Join Date: Oct 2009
Location: CentralArkansas
Posts: 23,783
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?

Last edited by aceplace57; 01-07-2012 at 10:38 AM.
Old 01-07-2012, 10:48 AM
Guest
Join Date: Jul 2009
Posts: 12,056
it provides enough power to keep the electronic memory alive.

you need to keep any positive electrical wires/cables out of contact with the rest of the system.
Old 01-07-2012, 01:03 PM
Member
Join Date: Mar 2002
Location: KCMO
Posts: 11,216
Quote:
Originally Posted by Dog80 View Post
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...
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.

Quote:
...connect this device and then replace the battery. Is this going to fry anything?
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.
Old 01-08-2012, 05:32 PM
Guest
Join Date: Mar 2003
Location: Minneapolis, MN
Posts: 14,216
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?
Old 01-08-2012, 06:54 PM
Member
Join Date: Mar 2002
Location: KCMO
Posts: 11,216
Quote:
Originally Posted by [email protected] View Post
The most common times that my car battery has been disconnected has been when it was in the repair shop.
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.

Quote:
Wouldn't a device like this tend to interfere with the diagnostic tests the mechanics are running with their automotive computer testing system?
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.
Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 06:31 AM.

Send questions for Cecil Adams to: [email protected]

Send comments about this website to:

Terms of Use / Privacy Policy

Advertise on the Straight Dope!
(Your direct line to thousands of the smartest, hippest people on the planet, plus a few total dipsticks.)

Copyright 2018 STM Reader, LLC.

Copyright © 2017
Best Topics: funny cadence army muddy fish bump under bruise raven says nevermore teletubbies doll recall sweatpants in public swat team masks peroxide vs bleach non offensive jokes pukka sahib definition boot camp nicknames studio boards tiny green flies how long was a tour of duty in vietnam injecting a tick with hydrogen peroxide morons i'm surrounded by morons what happens when you miss your vein shooting up guinness glass 06 13 air conditioners for crank out windows who has the best coleslaw value of used tvs space shuttle columbia crew remains starting a delicatessen business 10 pence coin value in dollars what does tab taste like