Someone came here the other day looking for the answer to this question - which battery terminal should you connect first. It's a great question. We're talking about automotive batteries, and whether you connect the positive terminal or the negative terminal first.
You've got the hood open to your car, you've just done some electrical work of some kind and the batteries disconnected. Now it's time to reconnect them, and you want to make sure you continue to work with the best and safest practices.
The Short Answer
The short answer? Connect the positive first and the negative second. This way you eliminate the risk of creating a short while you're connecting them. If you want to be sure then read below for the caveats - there are situations where this doesn't apply and/or you can damage equipment. In the meantime, for most of us, here's an explanation of why:
Short Circuits in Car Batteries are BAD!
Imagine you've connected the negative first, and now you're on to the positive side. You slip the battery clamp over the positive terminal. Maybe there's a tiny spark as your radio memory draws that tiny bit of power. But no worries, that little spark has so little current running through it that nothing happens. Next you take your half-inch wrench or 12 mm or whatever it is wherever you live. You start tightening the clamp tightly to the post. As you turn the wrench around it comes close to the body of the car and suddenly the wrench slips off the nut and slams against the body while still touching the the positive terminal.
Don't try this at home kids! If a circuit is completed (it might not due to the insulative properties of the car paint) but if the circuit is completed you'll have a great spark show on your hands, and the point of greatest heat will loose. You could melt a hole in your car body, you could melt the battery post (most likely being the softest metal), you could melt and deform your wrench, and of course you could burn yourself.
For most of us, I think we can agree this situation sucks. If you had not connected the negative terminal first, then the body would not be connected to the battery system, and the short would not have been possible.
Connection Sequence of a Car Battery
The car body is connected to the negative terminal, the negative terminal is connected to the positive terminal (and in between has harnessed the electrical potential in the battery), the positive terminal is connected to the electrical system, and each item in the system is connected to the car body. This is a complete circuit.
In the short circuit example above you actually "shortened the circuit". You cut out the electrical system, and just connected the battery straight to the body. If the negative terminal was not connected, then the circuit would have remained incomplete, and you would not have had a short.
Clear? If not, ask in the comments section below and I'll try to clarify. And the moral is connect the positive first, then connect the negative. Go ahead and slip with the wrench all you want when connecting the negative - it's already connected to the car body, and you won't create a short circuit.
The Caveats
Here are a few reasons why you might connect the positive terminal first rather than the negative terminal.
- You've got a positive ground vehicle. Like an older Series Land Rover - yes you know who you are! The electrical systems in those cars are backwards to most of the rest of the cars in the world. Yes, the English had their reasons. If you've got a positive grounded car, then just do it in the opposite way.
- You've got equipment that needs the ground to be connected last. There are some pieces of specialized electronics (poorly made I'll admit) that need to be connected in a certain order or they blow internal fuses. If you've got them, you probably know all about it and you also probably curse yourself for not spending a bit more on a better engineering product!
Any more? Leave a comment below if you know of any!
Hi, I recently purchased an old 1957 Oliver tractor. I went to replace the battery and noticed that the ground wire was connected to the positive (+) terminal of the battery and the hot wire was connected to the negative terminal (-) of the battery. I’ve never seen this before. I’m concerned how to hook up the new battery. Any help?
Danny, you’ve got a positive ground tractor! This was normal on old Oliver tractors. It is similar to various british vehicles such as the older Land Rover. It’s not a problem – reverse the instructions in this article.
Connect the negative terminal first which runs to the tractor electrical system, and connect the positive terminal last, which as you say is connected to the tractor ground.
Have fun, sounds like you’ve got a great old machine!
Thanks Robin, Someone else told me that Oliver did this with certain tractors that they used a 6 volt system in, but the battery looks like a 12 volt. I hope i’m not going to throw a 12 volt battery on a 6 volt system. Any way of telling the difference? Or does it even matter? Thanks, Danny
Do not install a 12v battery onto a 6 volt system! You need to figure out the voltage of the existing battery. Get a voltmeter and put the negative wire from the voltmeter to the negative of the existing battery, and put the positive to the positive on the battery. The reading should be somewhere between 9 and 12 volts for a 12 volt battery and somewhere under 6 volts for a 6 volt battery, depending on the charge left.
Hope this helps!
Hey Robin- I recently purchased two AGM batteries and a manual charger. First time I went to charge a battery, I accidentally put the negative on the positive. Luckily the charger wasn’t plugged in. It did spark, however. Do you think I damaged the battery.
Thanks
I am sorry. To be more clear, I put the positive end of the charger to the negative of the battery ant the negative end of the charger to the positive end of the battery and that is when it sparked.
Morgan, I doubt it. If you didn’t generate enough heat to melt the battery post, you couldn’t have damaged the battery. Besides, the charger wasn’t even plugged in.
Thanks for the reply. Hey – I really enjoyed the story about how you guys had to repair the hole in the battery! Sometimes things happen that are horrible at the time, but end up being something that makes the trip memorable (as long as no one gets permanently hurt lol). Love the website. Keep up the good work.
Morgan
I have a 36 volt system on my golf cart. I am trying to reconnect the batteries. When I try to put on my last jumper wire I get fireworks (sparks). What am I doing wrong?
im sure there can be good arguement for EITHER WAY, sorry, i was a auto mechanic for 6 years and worked in industrial electrical (ac and dc) for 3. it was the position of Ford in one city, and chevrolet in another if you do red first, you can risk grounding the battery on ANYTHING metal under hood threw the negative cable. so hook black first, that way the circuit can ONLY be completed when you attach the positive to the terminal.
hi.. nice and informative column. i just want to ask that i connected the positive first but as i tried to tighten the negative, it sparked twice.. what would be the reason for that?
It could be because there is a power draw on the system such as a stereo (internal clock and memory).
Hi,
What happens if you accidentally series positive terminal to positive and then, negative terminal to negative instead of ground? would you damage any of the cars?
Thanks Robin
I have 1 unit of 12volts battery @ 100amp and 6 units of 2 volts @ 200amps each. I can connect either 12v or 24v, but I have to move cables around each time, this is not very comfortable. I also a 12volts solar panel and a 24volts charger. How can I connect them such that I get a high breed configuration, where either the solar board or the electric charger can work without my having to move cables around. I also want both the solar panel and the electric charger to work at the same time if need be. Thanks.
sir,may I know the minimum ampere-hour(of a 12volt battery) required to supply a inverter of 70watt output.
please answer me as earlier as possible
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for 3hrs back up.how much battery rating should be used.
I am cleaning up a 99 Dodge Ram 1500 with AM Amp and head unit. Both the power to the amp and what seems to be the ground for the head unit are connected to the positive terminal. That doesn’t seem right, but I haven’t had any problems except for a short in the ignition off circuit. Any thoughts?