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To change out my automobile battery, some sites say, to save memory devices, I should procure a gadget using a 9 V battery which is hooked up to the car battery cables just before you undo them, so there is no interruption to the computer and accessories when you take out the 12 V car battery.

At the moment of the disconnect of the 12 V battery there is, for a short time, a 9 V battery across the terminals. Is this a safe situation?

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    \$\begingroup\$ Those gadgets probably have a reverse voltage protection diode. \$\endgroup\$
    – Passerby
    Commented Dec 18, 2020 at 2:09
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    \$\begingroup\$ Yes, you could do it safely. Iff you're several meters away and wearing a full face shield so that the resulting shrapnel does not injure you. n.b. shrapnel is named after British Lieutenant General Henry Shrapnel. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Dec 18, 2020 at 2:50
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    \$\begingroup\$ The 9V probably will keep the devices alive. But I don't think the 9V battery will survive being connected to a 12V car battery. I think there is something to this idea, though. Maybe a charger connected through the cigarette lighter. Maybe a 9V battery with a diode in series. Or maybe just another 12V battery (a small one). \$\endgroup\$
    – user57037
    Commented Dec 18, 2020 at 8:37
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    \$\begingroup\$ @Passerby your comment is a good answer but you should change "probably" to "certainly." \$\endgroup\$
    – stretch
    Commented Nov 8, 2021 at 14:50

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As far as the car is concerned, when everything is off, 9V is fine. A car battery that's nearly drained will have that voltage at some point. Obviously the small 9v battery can't supply enough current to actually run any of the car's systems.

Trying to turn on even just the accessories would probably bring the little battery's voltage down noticably. So for sure take the key out before hooking this thing up, don't blink the lights etc.

As far as directly connecting the two batteries together (+ to +, - to -), probably a bad idea unless the 9V has a diode in it to prevent current flowing into it from the higher ~12V of the vastly more powerful car battery. (Under NO circumstances should you hook up "+ to -" and "- to +" !)

The "gadget" would have such a diode and maybe other protections. Careful to hook it up the right way. If it's an alligator-clip job, careful with that too, since all the metal parts of the car are a "-" terminal.

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    \$\begingroup\$ Some cars have all the metal parts connected to the positive terminal! Generally really old ones though. \$\endgroup\$
    – Hearth
    Commented Dec 18, 2020 at 2:35
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Be really careful please. if a 9V battery is connected directly to a car battery, it will probably overheat and possibly explode. You need a diode in series with the 9V battery to prevent current from flowing into it.

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I hooked two 9 V batteries together in series and jump started my 2007 Honda Ridgeline with a bad cell in the car battery.

I used an amplifier power lead with a fuse that ran to the battery, connected the 9 V in series, connected positive to battery, and then quickly tapped the grounds together while turning the engine over. It worked, but I was certain the 9 V batteries were going to explode in the cab with me, but I had no other options available.

I would not recommend doing it. The potential for disaster is too high. On another note, while the wires did get hot, the batteries did not.

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