Roger Bryant:

My understanding is that this is to keep the spark when you connect and disconnect away from the battery to avoid a hydrogen explosion. . .


. . . Batteries do explode. I had one used for traction purposes that appeared flat. I charged it for a few hours and tried again. The battery, luckily in a shielded from me, exploded blowing the top off a cell and spreading acid all around. The cause was a crack in one of the interconnectors (a common problem with low maintenance batteries) which allowed me to charge at a few amps but when I put a load on the connection opened up and sparked.

Hydrogen gas explosion? Someone needs to go back and look at the chemistry. When you continue to drive current backwards through a fully-charged lead acid battery, the water content of the electrolyte is broken into hydrogen and oxygen, which is emitted, giving grounds for caution. A flat battery does not give off hydrogen.


Modern cars manage the battery charging electronically in a much more controlled way than the old voltage regulators used to, and do not continue to pass charging current through a battery once fully charged producing hydrogen. That is the reason why nowadays we rarely need to top up the electrolyte - little generation of hydrogen and oxygen so the water does not get used up.


This sounds a bit like "nanny state" advice. A bit like that about switching off your mobile phone at a petrol station, for instance.


I have fortunately not experienced a battery explosion but I have known of cases of "sudden death". This I presume is due to  failure of an interconnector. In such cases it is wise to take extra precautions (wear goggles for instance). However the choice of how to connect to jump start is not relevant to this.

Roger Bryant:

My understanding is that this is to keep the spark when you connect and disconnect away from the battery to avoid a hydrogen explosion. . .


. . . Batteries do explode. I had one used for traction purposes that appeared flat. I charged it for a few hours and tried again. The battery, luckily in a shielded from me, exploded blowing the top off a cell and spreading acid all around. The cause was a crack in one of the interconnectors (a common problem with low maintenance batteries) which allowed me to charge at a few amps but when I put a load on the connection opened up and sparked.

Hydrogen gas explosion? Someone needs to go back and look at the chemistry. When you continue to drive current backwards through a fully-charged lead acid battery, the water content of the electrolyte is broken into hydrogen and oxygen, which is emitted, giving grounds for caution. A flat battery does not give off hydrogen.


Modern cars manage the battery charging electronically in a much more controlled way than the old voltage regulators used to, and do not continue to pass charging current through a battery once fully charged producing hydrogen. That is the reason why nowadays we rarely need to top up the electrolyte - little generation of hydrogen and oxygen so the water does not get used up.


This sounds a bit like "nanny state" advice. A bit like that about switching off your mobile phone at a petrol station, for instance.


I have fortunately not experienced a battery explosion but I have known of cases of "sudden death". This I presume is due to  failure of an interconnector. In such cases it is wise to take extra precautions (wear goggles for instance). However the choice of how to connect to jump start is not relevant to this.