RE: Which battery lead to disconnect first

[Johnson David-QA2282 <David.R.Johnson@domain.elided>]

OK, while catching up on past digests, I just followed through a long and involved thread on why you should disconnect the negative battery lead first.  Not a single person mentioned the problem I had always believed to be the primary reason this is advised.  However, John Fielding came closest when he posted:

"I found two sites which happened to be USA Fire Departments and each had a catalogue of how to tackle a vehicle after a collision.  Both stated that the negative lead should be disconnected first.  One even went so far as to say "disconnect the negative lead first as this will not produce a spark if current is flowing, whereas the positive lead can produce a spark which can ignite the battery gassing products".

This statement is incorrect, since as a number of digesters pointed out either terminal can give a spark.  However, the warning of the potential for explosion is valid.   The key point which no-one mentioned is that the flammable gas (actually hydrogen, which is about as flammable as they come) is emitted only at the positive terminal of a lead-acid automotive battery.  Thus, removing the negative cable reduces the risk of explosion significantly by keeping the spark away from the gas.  This fact remains true even for positive-ground vehicles.

It is important to note that the advice also applies to connecting or disconnecting jumper cables or a battery charger (anytime you complete a circuit at the battery).  Always connect positive first, since the circuit is not completed until the negative is attached, reducing the potential for a hydrogen explosion.  A friend of mine learned this the hard way, splitting the case of a just-charged battery and spraying acid on himself.  Fortunately it missed his eyes, but I gather it was not a pleasant experience.  A good reason to wear safety goggles and not to lean over the battery as you work...

Now, a number of other advantages were proposed as to why one should work on the negative terminal.  So perhaps the advice is given for a combination of reasons in favor of negative over positive.  Either way it doesn't really matter, but awareness of the hydrogen outgassing is nonetheless a valuable thing for anyone dealing with lead-acid batteries.  I suspect that the advent of modern sealed batteries may make this advice obsolete, but I don't know for sure that they don't still seep some H2.

AFAIK the details above are accurate, but if anyone knows different please correct me as this is safety related stuff.

Cheers,

Dave J.
1987 Milano Verde
1964 2600 Sprint
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