Hydrogen

I've had a 6V battery blow up in my face before,when I was about 17--had a '63 VW Beetle and the battery would never start it half the time,and being a broke teen,I never had enough cash to buy a good battery..

--I got tired of pushing it to start it,so I found a weak 12V battery someone replaced from a truck, and left it sitting in the rear floor behind the passenger seat,with jumpers hooked up,all set to go--left the seat cushion up so I could get at the VW battery,and the protective metal lid over the battery off too-.

One day I went to start it up and had to jump the 6V battery,and when I connected the jumpers to it,one of the clamps touched the other,an arc went "snap" and the next thing I heard was a loud BOOM ,and felt stuff spray in my face.....

I leaped out of the car and into a snowbank,and grabbed handfulls of snow and rubbed it on my face,and held some on my eyes--luckily I was wearing glasses and none got right in my eyes,only on my eyelids..my face was beet red for a few days and it hurt,but after I went to the ER,a doctor said putting the snow on the affected areas right away,was probably the best thing I could have done under the circumstances...luckily a friend with me wasn't in the car,and was able to call for help and get me a ride to the ER..

When you jump a dead 6V battery with 12V,you basically get 18V at the dead battery's terminals,and it'll start gassing instantly...you could smell that acid stench every time I jumped the 6V battery...
I'd been warned in shop class how batteries can become mini bombs,but had only seen one blow up before,when sparks from a cutting torch flew near a table where several batteries were being charged in an old friends shop once..(and it scared the crap out of us too!)..

After that episode,a friends father showed me how to drill into the 12V battery's case in the right spot,and tap into the lead cell connector inside,so I could use just 3 cells out of the 6 to get 6 volts from the 12V battery...it worked slick!..

When you jump a dead 6V battery with 12V,you basically get 18V at the dead battery's terminals

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You would only get 18 volts if you hooked the two batteries up in a serial configuration, which would actually turn the starter even faster and pose no danger except to the windings of the starter motor if they weren't up to the task.

When you jump across a 6 volt battery with a 12 volt battery you get somewhere between 6 and 12 volts as the 12 volt battery is trying to apply its charge and the 6 volt battery is drawing it down some with its internal resistance, and that current through the internal resistance of the 6 volt battery is what heats up the electrolyte solution and causes the hydrogen gas to boil off.

When jumping a 6 volt vehicle with a 12 volt battery it is best to turn the key on, but not to start and make sure that the transmission is in either neutral or park, and then jump the 12 volt battery directly to the terminals on the starter motor, as that keeps the two batteries isolated from each other since the starter relay is not engaged.

Tom-machinery-idiot said:

Remove those battery caps while charging.

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Removing the caps may stop pressure from building ( the battery should be vented )
but,
removing the caps will do nothing to stop hydrogen from exploding.

The only solution is ventilation,,, dilute the hydrogen with LOTS of air,,,,

I had a nephew blow a battery blow in his face, I had to take him to a hospital,,,

Not fun,, he was lucky, no permanent damage,, other than his pride,,,,

Some of you will get this...,.

CADplans said:

Removing the caps may stop pressure from building ( the battery should be vented )
but,
removing the caps will do nothing to stop hydrogen from exploding.

The only solution is ventilation,,, dilute the hydrogen with LOTS of air,,,,

I had a nephew blow a battery blow in his face, I had to take him to a hospital,,,

Not fun,, he was lucky, no permanent damage,, other than his pride,,,,

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With the caps removed, the pressure containment would have been a fraction during ignition of what it was and it would have vented quickly enough to avoid blowing the end out of the plastic case. At the same time, if I had the foresight to have removed the caps during charging, the hydrogen gas accumulation would probably be far too diluted to ignite, or at least be more than a harmless flash. Either way, lesson learned. Luckily there was a hose handy and sulfuric acid is easily solved with plain water.