I have read that disconnecting a battery causes all sorts of serious problems on a modern car. https://dannysengineportal.com/car-battery/

a) Is this article only talking about when the engine is running and the ignition is on?

b) Is it talking about when you remove both negative and positive cables?

c) Will these be a problem if you remove negative cable only? I imagine its not the negative cable since you often have to do that when working on the car.

if b how do mechanics change battery batteries, do they always use a memory saver as suggested in the article?

2 Answers 2


The article does note the first point -

Regardless of the year, make or model of vehicle NEVER disconnect the car battery while the engine is running or the ignition key is on.

The alternator has a certain amount of energy in its magnetics, and even though the regulator will switch off when the output voltage rises, there needs to be a path for that energy to dissipate, if it can't go to a low impedance load, ideally the battery, it can generate a damaging spike. Some alternators include protection against this "load dump", and that protection may be elsewhere in the system, but if not, electronics can get damaged by it.

Removing either cable opens the circuit, the negative one is the first you disconnect and the last you disconnect to avoid the risk of a tool shorting between the positive clamp as you fasten it and the bodywork.

The warnings about loss of the memory contents are true, but the ECM has a default set of values which is good enough to start using the vehicle - this is what happens when the vehicle first drives off the production line, so it must be functional to a reasonable degree. It can often take 100 miles or so for the full table of fuel trims to be fully updated, and there's a readiness monitor for emissions testing that requires a number of engine start/warmup cycles before it will report that the emissions system is functioning when it gets plugged into diagnostics for an emissions test.

THe settings that identify different modules to each other are generally in non-volatile memory, and it's not likely that the AC or radio will stop working, though.

  • The highlighted quote is important, but (IMO) that should be just common sense- though I suppose you might be working on the car and "forget" the ignition was left switched on. The rest of the linked article is mostly scaremongering. For example on my own car you have to reset the date and time for the clock when you disconnect the battery, but that's it - everything else works fine.
    – alephzero
    Oct 9, 2019 at 20:01

Phil G is pretty much spot on. A battery is essentially a big voltage regulator, effectively sinking (or sourcing) lots of amps to keep the voltage right around 13.6V. As for emissions learning. The default tuning parameters should be pretty close. Trims should only be fine tuning and a couple of miles driving should be enough to get things there. Now some manufactures require major resets of computers when a battery is changed. It has to do with how they handle the charging. BMW claims that failure to do so with result in limited battery life. We gone 50 years without having to do so and BMW wants you to pay the shop rate to reprogram/reset your computer to learn the new battery.... I guess don't buy BMW.

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