Disconnect positive terminal first when disconnect battery?

[hkomori]

Hello guys, I watched Porsche related youtube and in the show electronics expert explained that disconnect negative cable first can lead to the failure of internal ICs. So they recommended to remove positive cable first to protect all the circuits in the car. I think industry norm is always disconnect negative first. But everything other than cars such as computers home electronics like USB connection, it is designed to disconnect positive first then negative. Also, google and all search engine and AI says to do negative first. Can someone enlighten this story? My knowledge is totally like a child when it comes to this area. Thx in advance.

 

[geoffbland]

I don't know why they told you to disconnect positive terminal first?

The car has a negative grounded to the chassis so you disconnect that so you can't short out the disconnected positive by accident by having it touch somewhere in the car accidentally.

The same is true for any household appliance that has a metal grounded case.

Edit: for computers and your USB, the voltages and amps are much smaller and although they are often negative grounded the PCB is usually insulated so it doesn't matter. On a USB plug the data pins (which are pulled high/positive) are disconnected first before the power and ground. This is so data transfers stop first before it could get "noise" from the disconnecting power.

Edit 2: I did some digging. It appears that on modern cars - especially Porsches - many modules stay live when the negative battery terminal is disconnected (capacitors and internal batteries). But with the negative terminal removed they are no longer grounded so in theory a charge has nowhere to ground to. This is called back-feeding. I'm no expert on this but I would expect a well designed car module to deal with this and cope but as everything is built to a cost these days I guess there's always a risk of a car IC killing itself. FWIW the safe is thing to do is to leave the car turned off for 15 minutes to allow the car to "sleep" and all modules to shut down before disconnecting the battery.

 

[Honcho]

On another note, if you're doing battery work, I recently swapped mine out and got an o2 sensor heater and limited telematics warning. I cleared the sensor code and it hasn't come back since. Limited telematics was fixed by disconnecting the battery for an hour or so as advised by others on this forum.

 

[hkomori]

I don't know why they told you to disconnect positive terminal first?

The car has a negative grounded to the chassis so you disconnect that so you can't short out the disconnected positive by accident by having it touch somewhere in the car accidentally.

The same is true for any household appliance that has a metal grounded case.

Edit: for computers and your USB, the voltages and amps are much smaller and although they are often negative grounded the PCB is usually insulated so it doesn't matter. On a USB plug the data pins (which are pulled high/positive) are disconnected first before the power and ground. This is so data transfers stop first before it could get "noise" from the disconnecting power.

Edit 2: I did some digging. It appears that on modern cars - especially Porsches - many modules stay live when the negative battery terminal is disconnected (capacitors and internal batteries). But with the negative terminal removed they are no longer grounded so in theory a charge has nowhere to ground to. This is called back-feeding. I'm no expert on this but I would expect a well designed car module to deal with this and cope but as everything is built to a cost these days I guess there's always a risk of a car IC killing itself. FWIW the safe is thing to do is to leave the car turned off for 15 minutes to allow the car to "sleep" and all modules to shut down before disconnecting the battery.

Thank you for your reply. I got this information in Youtube channel of Japan which link is below. I hope by switching English audio you can understand. Around 4:40, the expert shows and explains all the behind theory including potential capacitor and communication issues between the circuits in the car system after negative disconnected. Also reason why car makers recommend to disconnect negative first although internally they disconnect positive first according to him. Eventually this expert explains none of the potential issues can happen if positive terminal disconnect first. I was wondering if I can trust this information. Appreciate your explanation.

Youtube