CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Changing batteries

I asked yesterday about applying voltage to the battery cables before removing them from the old battery to install a new one. Thought I heard one time on here that a small 9 volt battery would be enough to keep the computer & electronics ok while making the battery switch from old to new. A few fellows answered ,but didn,t think that would work. I would like Tester to reply as I feel he is one of the best on this site. Not that a lot of the other fellows on this site don,t know plenty about cars, but I always feel that Tester really knows for sure. PS. I could hook up a 9 volt & a 1.5 volt to get 10.5 volts if 9 volts is not enough. Waiting for Tester,s reply. Thanks in advance. Honda Bill

Honda Bill:
Two points:

  1. You have a thread that you opened yesterday with this exact same question. Opening up a new thread less than 12 hours later because you didn’t like the answers you got in your first thread is an action often frowned upon in this forum.

  2. There are numerous skilled contributors that make this forum great. Tester is one of those contributors, but no one knows everything. If we didn’t learn from each other, this forum would not work.

Please continue the conversation in your original thread. And if you’re really not satisfied that you understand the answer yet, it behooves you to ask it again in a way that is open to all.

Buy two cheap 6 volt lantern batteries. Duct-tape them into a single battery, connecting the positive post of one to the negative of the other with a short jumper. You now have a 12 volt dry cell with enough capacity to “hold” your cars electronics while you change the battery…

Wear glasses.
Alkaline batteries can explode when attempt to charge them, which you are dong when you tie them across a car battery.
For safety’s sake, I’d want to have a diode between the alkaline battery’s + and the car’s + terminal with the bar going towards the car to block current from flowing into alkaline battery while you’re fiddling with the connections.

For those who may be electrically challenged I believe a device using a 9 volt battery can be had at many auto part stores.

The ‘‘memory saver’’ that uses a 9v battery plugs into a power point/cig lighter socket.
You can buy one or you can build one.

And the memory saver with a nine volt battery adapter that plugs into the power point was like $10. I also have one that plugs into the odbc, and an adapter I run from a 12 volt power supply, as I am not sure how long the nine volt battery will last.

If the back up power is ran through the lighter, then I believe the ignition should at least be on acc. One has to check before this attempt to make sure at what position the lighter is “Hot”.

On a different note, since you can run your battery down at any point, or a mechanic could disconnect it to work on the car, you might as well start looking for the radio code from now and save it somewhere safe. I have mine written all over my manual and also saved it to my cell. I do not believe whoever steals my radio is going to return it because he/she does not have the code.

Ignition does not have to be set to acc, that would be unnecessary, and overkill. They are both on the same circuit, in most cars, that is why I prefer the obd plug in, as some cars it is different, but the majority the power point will work.

For whomever disagreed with my post:

Not that I normally care about disagrees except when they are dangerous, and in this case they are: do be careful putting non-chargeable batteries across another higher voltage source, especially one that can do a decent amount of current like a car battery.
Those setting maintainer toys likely have a diode in them to make sure that doesn’t happen. That’s what makes them safe.

Believe it or not: simple batteries can explode. Just because you have not seen it happen, does not mean it does not happen. I had never seen it happen and it almost stopped me from seeing anything else from happening when it did happen as I have an eye doctor’s bill to prove it.
Disagree all you want but be very careful - better safe than sorry. Wear glasses.

Yes, Tester is good and so is OK44** but you have answers from some very qualified folks. I bought one of those things for $10 but have never used it yet. It does give instructions though and you need to use a new 9v battery and you don’t have very long to accomplish the task. I always record the radio code in both my owners manual and in my maintenance book so I have it if I need it, but losing power can screw up the engine management memory too.

" do be careful putting non-chargeable batteries across another higher voltage source, especially one that can do a decent amount of current like a car battery."

Two new 6 volt lantern batteries will show you 13.5-14.0 volts on the meter. THEY are the higher voltage. But even if the lead-acid battery in the car is brand new and is coming right off a charger at 14.5 volts, even THEN, the 1 to 1.5 volt difference is NOT enough to push enough amps to cause the battery to explode…You would have to put 17 or 18 volts across the lantern battery to put it in distress and have a problem. It’s just not going to happen.

One of those 9 volt gizmos does not have the voltage or capacity to get the job done reliably. I used a 12V dry cell like I described for YEARS in my shop without any problems…Other shops/mechanics would use a motorcycle battery to accomplish the same thing. It’s just a matter of what’s at hand…

The radio code thing has pretty much faded into history…Car radios are simply not worth stealing…Disabling the OWNERS radio caused the car makers too many problems…Batteries can once again be safely disconnected…

Please do wear glasses, Caddy.
Two six volt cells across a car battery has less of a potential for doing anything than a 9V but one time is all it’ll take.