Re: [VAC] battery hazard

["Dr. Gerald N. Johnson, P.E." <geraldj@xxxxxxxx>]

As a consulting electrical engineer (no, I don't do house wiring!) I've
have served as an expert witness and investigated a couple exploded
batteries.

When either charging or rapidly discharging (as with a short), the water
of the electrolyte is dissociated into hydrogen and oxygen in the exact
ratio to give the best bang. 2 parts hydrogen to one part oxygen. But
hydrogen will burn in air from a minimum of a few percent up to about 90
percent hydrogen. Its not like gasoline that needs to be between 12 and
16:1 (about) to burn. And hydrogen being the smallest molecule is very
hard to hold in containers, it can slip between the molecules in some
materials...

Years ago my dad blew up a car battery while using it to provide
ignition power for a model B Ford engine running an arc welding
generator. The charging generator tended to overcharge the battery so it
was "boiling" freely filling the chambers with that stoichiometric
mixture. Apparently a welding spark fell through a vent cap hole and the
battery went off with a bang. But since the B engine banged at irregular
intervals under load he wasn't concerned at all. But he did get
concerned in a few minutes when he felt a breeze from the cooling fan on
his bare leg. It was attention catching because he was wearing bib
overalls over pants. He stopped welding, and checked and he was only
wearing the left side of them, the right side of the right leg was gone.
Eaten by the spray of battery acid. After a quick shower, he used some
friction tape to hold the side of the battery on, put it back in the car
and got to Sears just before they closed on a Saturday afternoon and
they honored the guarantee.

In one of the cases for hire, a man was checking the battery in his
daughter's car. And while he was wiggling a battery cap loose the
battery blew the top off an sprayed his face with acid. I don't know how
much damage it did, but enough that a lawsuit was begun and settled.

What had happened there was that due to vibration, some of the active
material from one plate had transferred to the other plate and the local
action of that shorted minicell had lowered the water level so it wasn't
touching the foreign plate material. When he shook the battery a wave of
battery acid hit that shorted cell and when the wave went away the spark
at the interruption ignited the perfect mixture.

Look at any new battery, it will give a list of precautions like using
safety goggles or a full face mask, and caring about ventilation while
working with a battery.

I tried to use a battery in a basement for ham radio equipment one time,
but the fumes from the battery (and those fumes do tend to have some
sulfuric acid with them though they are chemically generated hydrogen
and oxygen) ate the connecting cables and the world protecting circuit
breaker I had bolted to a battery terminal.

In a trailer battery where the current is never going to be nearly as
high as for cranking an engine, in my opinion (and the National
Electrical Code, section 551 on RV electric wiring) the battery should
be connected to the world through a current limiting fuse or circuit
breaker. Probably about 50 amps, certainly no larger than 100 amps. This
is covered in section 551-10. In Table 551-10(e)(1) it lists the fuse or
circuit breaker sizes according to wire size. 30 amps for #10, 20 amps
for #12, 15 amps for #14, 8 amps for #16 and 6 amps for #18. Remember
that a fully charged battery can supply well over 1000 amps when
shorted. That easily can heat a piece of 2 or larger wire hot enough to
burn off the insulation and ignite adjacent wood pieces.

That National Electrical Code says in section 551-10(d):

"Battery Installation. Storage batteries subject to the provision of the
Code shall be securely attached to the vehicle and installed in an area
vaportight to the interior and ventilated directly to the exterior of
the vehicle. Where batteries are installed in a compartment, the
compartment shall be ventilated with openings having an minimum area of
1.7 in. (squared) at both the top and at the bottom. Where compartment
doors are equipped for ventilation, the openings shall be within 2 in.
of the top and bottom. Batteries shall not be installed in a compartment
containing spark- or flame-producing equipment, except that they shall
be permitted to be installed in the engine generator compartment if the
only charging source is from the engine generator."

And a battery bouncing around loose will cause loose connections
externally AND internally and lead to a much shorter battery life.

One other very important battery handling precaution. Always disconnect
the ground first, and connect it last. That's because its way too easy
to hit other objects with the wrench and since MOST of those are
grounded, there's no consequence when loosening or tightening the ground
clamp bolt. Then with the battery disconnected from ground the hazard of
bumping the wrench against the grounded body parts when working on the
ungrounded terminal (in modern equipment, the positive) is removed. Its
still not a good idea to get the wrench from post to post. That high
current can weld the wrench in place, or if it fails to weld the sparks
are a potential source for igniting the gases caused by the heavy
current.

Bill Scott, the liquid space shuttle fuel is hydrogen and oxygen. It
burns very well and has a high energy density. If it wasn't so hard to
contain and so easily ignited it would be super engine fuel. And it
might be used for more engine fuel in the future as the storage problem
is licked. Its renewable by using electricity from solar panels to make
it and it works very well in environmentally friendly fuel cells. Fuel
cells work better with hydrogen than any other fuel.

Gerald J.