The outside of a motorbike engine seen up close near the exhaust

Home Repair And Restoration

Battery Removal - Negative First

By Ren Withnell

01 January 2016

Always disconnect the negative terminal of your battery first - unless you have a positive ground setup on your motorcycle. 

Firstly - what is negative ground - or earth, what does that mean and why do bikes use this system? Simply put - the negative terminal on the battery has a big fat wire connecting it to the frame or the the engine of the motorcycle. This means that pretty much all the metal parts of the motorcycle become negatively charged from the battery. You should by now know that to light a lightbulb you will need both negative and positive connections to the battery. So - rather than having to run 2 wires from a lightbulb all the way to the battery you only need to run 1 wire to the positive terminal and a short wire to any handy metal parts of the bike. 

So simply put if the whole bike is negative then you don't need to have much in the way of negative wires. Clever huh?

There is of course a minor downside. If the positive terminal of the battery touches the frame, or anything metallic such as a spanner connects the positive to the frame then POP! Lots of sparks, scorch marks and electricity. "Oh, I don't need to worry about that, I'll be careful." Call my cynical, call me an experienced fool or call me over cautious but rather than worrying about being careful, just disconnect the negative terminal first. Once the negative wire is off make sure you push it well out the way, they have a nasty habit of accidentally spring dangerously close back to the terminal.

Once the wire is completely clear then it won't matter if you accidentally touch your spanner against the frame while disconnecting the positive. 

So it's a few sparks, what's the problem anyhow? If, like many bikers, you're pedantic about your bike then scorch marks across your nicely painted frame don't looks good. We're always told electricity follows the shortest route. Hmmmm. It should, but I have heard of CDI units, engine computers and digital clocks being fried from a short circuited battery even though these parts were nowhere near the sparks. And finally inflammable liquids. Often times when you're tinkering with your bike you might have the tank off and that might lead to a few drops on the garage floor. Paraffin for cleaning? Rags soaked in thinners? All these things are fine until you spray sparks across them.

So just disconnect the negative wire from the negative terminal first. It's a simple thing to do that causes no hassle at all.

A motorcycle battery still on the bike

Reader's Comments

Ian Soady said :-
.... and of course replace the live first then the earth when reconnecting!

I must confess to occasional brain fade as most British bikes between the early 1950s and mid-60s were positive earth for reasons I can never quite understand, and so are different to modern bikes as well as cars. Earlier ones like my 1931 Sunbeam are generally negative earth (but to keep me on my toes were originally built with all black wiring which I'm replicating with some headaches).

I rebuilt my last restoration, an early 60s Triumph twin, negative earth just to make my life easier.
02/01/2016 10:33:28 UTC
Ren - The Ed said :-
Now you see EVERY motorcycle and car I've ever owned or worked on has been negative ground. I was aware that a very small number of ancient motorcycles were positive ground but I certainly did not know that most Brit bikes were.

Technically I can't think of any advantages either way, can you Ian? Is the positive ground more like to cause furring of contacts or something like that?

Otherwise yes you are quite correct, on the common negative ground motorcycles and cars - reconnect the negative terminal last!

So you do restorations? If you ever feel in the mood I'd love to publish any projects, hints, tips or other juicy information you might have.
02/01/2016 11:53:28 UTC
Ian Soady said :-
Hi Ren.

Yes, I've always had at least one old nail / desirable classic (delete as appropriate) as well as generally a more modern bike. The longest lasting was a Norton Commando that I had for 20 years and used mostly for touring - the first big trip was to Greece and back....

Since early retirement I've been working through the bikes I never had but always fancied - Velo Venom, BSA A10, Matchless G80, Triumph T100 and now the Sunbeam which is the earliest I've ever attempted. Some have only needed freshening up - others have needed more or less nut and bolt restoration. The Sunbeam is in this category although it was sold to me as a running bike.

On the home straight now I think although many a slip.

Sunbeam Model 10 as bought
02/01/2016 12:33:57 UTC
Ian Soady said :-
Oh BTW I found this which gives some (albeit feeble) rationale for the positive earth (or ground as our American cousins would say) layout.

The Sunbeam is in "as bought" condition and looks considerably better than it is.

This is the Commando shortly before I sold it to a bloke from Norway.

02/01/2016 12:40:17 UTC
Ian Soady said :-
Forgot the l,ink:
02/01/2016 12:41:00 UTC
Ren - The Ed said :-
Ian - I've never got into the old bikes thing but I must say there's something about that big tall cylinder and the simplicity that I like about this one. I figure you've got a nice warm dry garage or workshop to work in too. And a lathe. I'm starting to get very jealous now.
02/01/2016 12:44:00 UTC
Ren - The Ed said :-
Ian - it all makes sense! I did suspect that corrosion - or furring - might be something to do with it, but the wrong way around. Positive grounds are less likely to fur up it seems. Then of course the transistor and the generally default stance of the electrical world led us to negative ground.

Great link, thanks for that.
02/01/2016 12:51:47 UTC
Ian Soady said :-
I do have a nice dry (but not always warm!) garage but it was not always thus. I rebuilt my first Commando (an ex-police bike that had seized its gearbox) in the back garden of a terraced house with the bare minimum of tools.

But I now have the luxury of a full set of tools including a bike lift which saves my decrepit knees, a nice place to work (although it could be bigger) and plenty of time.....
02/01/2016 16:15:33 UTC
Ren - The Ed said :-
When I grow up Ian I want a garage or workshop. How old are you?
03/01/2016 10:40:29 UTC
Ian Soady said :-
Old enough to know better. As a clue, I passed my bike test when it consisted of a short run round the block followed by a bloke jumping out in front waving a clipboard. And helmets were still optional.

As they say, growing old is inevitable - growing up is optional!
03/01/2016 12:59:56 UTC
Ren - The Ed said :-
If memory serves me well the helmet laws came in during 1973. The first of the radio to rider tests came sometime in the late 80s I think. So let us say you were 17 in 1973, some 42/43 years ago, that makes you 42 + 17 = 59 - or more.

Well I hope you are well and I hope you still get as much pleasure from riding and tinkering as you ever did. Now...that lathe of yours...could you machine me up some stainless steel brake pistons for the 125?
03/01/2016 17:15:04 UTC
Ian Soady said :-
A little older - 66.......

Still very well but a knee replacement 4 years ago caused me to think carefully about what bikes were suitable and I regretfully parted with the Triumph Tiger 955i I'd had for 8 years as it was getting a bit much. A short ownership of a Suzuki V-strom followed (nice bike but still a bit too heavy and awkward for manhandling) then the current SLR650 - the research for which led me here....... I'm still enjoying riding & restoration as much as ever but I think my days of nipping down to the Alps or Pyrenees and back in a week are just memories.

Brake pistons might be beyond my (and my pre-war Granville lathe's) capabilities. They really need to be surface ground to get the right finish.
04/01/2016 14:35:03 UTC
Ren - The Ed said :-
Hi Ian. You don't need to nip down to the Alps, you've got the time to gently meander down to the Alps and then saunter back.

Surface ground? I've got some emery cloth? Hehe.
05/01/2016 09:17:03 UTC

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