Best way to disable 12v battery.

That would generally be the way to go on a negative ground system. Presumably you were given the logic behind why this is a good procedure to do, rather than just a rule to be blindly followed.

It just so happens that on the Prius, there is a handy quick release plug on the positive connector block that can easily be pulled to remove power from the car without any tools. As it is a quick release, it generally can be accomplished with out any sparks or drama.

Either way works, and personally, I find disconnecting the positive to be quicker and cleaner. But each to their own.

 

Really only reason to disconnect the negative cable first (in negative ground system) is to reduce the change of short circuit.

When you first remove negative cable if your tool happens to touch anything conductive at the same time (except the positive battery terminal) nothing happens as it’s all just a ground. If you would disconnect the positive battery terminal first and you tool happened to touch anything conductive it’s a short circuit and bad things happen! Those are physic so they don’t change.

But if you need no tools to disconnect then it doesn’t really matter. There are lots of cars that have quick disconnect at the battery, many of them at the positive terminal.

 

On our '12 v any Toyota FSM instructions note to disconnect the negative cable. Never even heard of the positive quick disconnect until reading your post!

 

You were trained correctly for a negative ground (earth) system, which is almost universal.

The reason the negative is removed first is that it OPENS the battery circuit, meaning no electrons can flow after the ground wire is removed. Why is that important? Because if you accidentally touch the wrench to metal while removing the negative - nothing happens. If you THEN remove the positive and again, touch the wrench to metal, because the circuit was OPENED (by previously disconnecting the ground cable), again nothing will happen.

As others have posted, these have a handy B+ disconnect for the 12v, no tools required.

 

The reason the negative is disconnected first goes back to the big, lead-acid batteries typically seen in cars. When using jumper cables, there is a spark when the first cable disconnected, whether it is the positive or negative. If it is the positive cable, there is a slight danger of igniting hydrogen fumes generated by the battery. Since the negative is connected at some remote metal fitting, a spark there is much safer. That is why you are not supposed to use the negative terminal as a connection point, but to find a bolt, etc., somewhere else.

 

As for sparks, unless there is an electrical draw on the battery, no spark should occur. With virtually all modern vehicles, the computers are connected to battery 24/7/365, it is turning of the ignition to "ON" that sends the "WAKE UP!" message to them. This is one of the primary reasons batteries can go dead after ~2 weeks, something is ALWAYS on. You should NEVER EVER get a spark from the 12v B- to anything metal on the vehicle, if so, there is a serious wiring problem in the battery ground circuit.

You should not get hydrogen gas build up unless the battery was recently charged, but caution is always the best bet.

 

I d u n n o, if I recall correctly, every time I hook up the negative cable I'll see a little spark.

 

While I agree with 05PreeUs, my observation is the same as Mendel, which I find surprising as I expect the current should only be in the realm of low mA's.

 

Hook one up on something with no electronics, like an OLD tractor = no spark

 

Sparking is a function of voltage difference. 12 volts is not a lot of potential difference. If the battery is properly grounded to the engine block or chassis, and everything is really turned off, there won't be a spark since the jumper cable is at the same voltage potential as the engine block, i.e., -12v. Other than "old tractor" types of vehicles, this is rarely the case, so there can be a spark. Assuming you are jumping a dead battery, there will be hydrogen generated since the dead battery will aggressively recharge itself as soon as the cables are connected. Hence, the remote negative 12 volt connection.

 

Yeah I think that's it: there's always a low amount of voltage draw on a modern car, for memories (go to flash drives??) and whatever. It's measurable.

 

It’s not about the amount of current draw. All those control modules on modern cars have capacitors in them to protect them from voltage spikes from the alternator (dc-dc converter in this case) and to stop them creating voltage spikes. That small current draw will drain those capacitors in minutes when battery has been disconnected. When cable is reconnected capacitors will be charged in fraction of a second creating a spark.

 

I realize this a six year old thread, but I was surprised to see a distinct spark when I just disconnected the ground on my 12v battery. I measured the voltage -- 12v running between the negative terminal on my battery and the chassis of the car. I then unplugged the big white positive plug on the 12v battery -- still measured 12v between the B- and the chassis. It was only when I unplugged the little white plug that the voltage went away.

So here's my question: what does that little plug feed? Is this a problematic short, or just the computer continuously being fed, as suggested above?

Thanks. This is freaking me out a little.

 

Equal current flows through the neg terminal as through the positive. Small white is Inverter/Converter remote monitoring of 12v charging at battery

 

Hmm. Hopefully you hadn’t left the car in READY?

 

Nope. I'm taking comfort in the "inverter/convertor monitoring of 12v battery." It IS a bit alarming, though.

 

Nope. They're all doing it the wrong way. BTW, there's a way to do it without messing up all your presets and the ecm.