Q: Can I use my 12 volt power pack, via the cigarette-lighter socket, to supply power while I change batteries? If the lighter socket is key-activated, can I turn on the key while I change the battery?

A: Keeping a vehicle’s memories intact during battery disconnection is highly advisable but not essential. In addition to the clock and radio settings, many vehicles also maintain memory for power-seat position, multiple driver entry and exit functions, and other operator preferences. Newer vehicles also maintain adaptive learning data for idle speed, engine and transmission performance tuning, as well as diagnostic trouble codes and test results for emission system monitoring.

Should all memories be lost due to a battery disconnection or discharge, the adaptive data and test results will be re-established automatically, perhaps within a few days. I should add that some vehicles are rather stubborn about re-establishing the emission-test results in a timely fashion, and incomplete test results can result in an emissions test deferral or failure. Re-entering clock and radio settings, seat positions and other operator preferences can be a nuisance.

If the audio system utilizes a security code, this may become the most challenging obstacle to overcome if the battery is disconnected or discharged and the security code is not available for reinsertion. Be sure an audio system security code, if used, is recorded in several places for future reference.

There are a variety of ways to maintain vehicle memories while the battery is disconnected. If the cigarette-lighter socket is active with the ignition switch in the “off” position, a portable power supply may be plugged in to maintain vehicle memories while the battery is disconnected. Inexpensive memory savers containing a household 9 volt battery may be plugged into the cigarette-lighter socket, if it’s active. When using these, it’s important to keep all doors closed and the hood light disconnected to avoid rapidly discharging the tiny battery.

Professional-grade memory savers often plug into the vehicle’s data link connector, which is active at all times. A battery-jumper pack may also be connected to the battery’s positive terminal and engine metal. This is sometimes a clumsy arrangement, as the battery-jumper pack can wiggle loose and increase the risk of an accidental terminal grounding, as I’ll describe shortly. If a vehicle’s cigarette lighter socket is not active with the ignition switch in the “off” position, this may be the simplest and most direct method to be used by a home mechanic. I would advise strongly against leaving the ignition switch on, as an accident with the cable terminals could result in significant and expensive vehicle damage.

When a battery-jumper pack is used as a memory saver, one must be very careful to not allow the disconnected positive battery cable terminal to touch vehicle metal or the negative terminal. Care must also be taken to avoid allowing the wrench you are using to release the cable terminal from touching the body or other metal items. I’d recommend wrapping a rag or towel around the positive cable terminal as soon as it is disconnected, and tuck the bundle out of the way with a small bungee cord.