Date: Fri, 1 Oct 2010 13:24:09 -0700
Reply-To: Alistair Bell <albell@SHAW.CA>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Alistair Bell <albell@SHAW.CA>
Subject: Re: Battery placement alternatives
In-Reply-To: <20101001153637.TRF0H.2019942.imail@eastrmwml30>
Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed
the reaction during regular charge of a wet cell lead acid battery
does not produce oxygen nor hydrogen.
Anode (oxidation):
PbSO4(s) +5H2O(l) <-----> PbO2(s) + 3H3O+(aq) + HSO4- (aq) + 2e-
(1.685V)
Cathode (reduction):
PbSO4(s) + H3O+(aq) + 2e- <-------> Pb(s) + HSO4-(aq) + H2O(l)
(-0.356 V)
you see that no gas produced.
Overcharging will produce, through electrolysis of the water in the
electrolyte, hydrogen and oxygen. You can see this when you charge
with caps off and the charging voltage is high (14-15 volts), little
bubbles fizzing off the plates. During a boost assisted start, I see
a spark risk, but I don't see electrolysis unless the donor
electrical source is high, and or the battery being boosted cannot
accept the electricity fast enough to drive the lead above reaction,
and therefore the electrolyte undergoes electrolysis.
(one thing too watch out for though, is the accidental bridging of
the battery posts. The massive and rapid discharge is dangerous :))
The practice of making your last connection during boosting, to a
ground other than the battery neg terminal, is a prudent step in that
one does not know how the accepting battery will take the charge, or
if the accepting battery headspace is full of hydrogen and oxygen due
to some previous charging malfunction. Or if the accepting battery
has a serious screw up inside and there are internal shorts etc.
What i was trying to convey in my original post, is that the battery
compartment, and the lid seal is not adequate to contain any hydrogen
gas produced. A vent tube, solidly attached to the vent nipple on
battery, then leading out of the van, woudl be more effective (as
Loren pointed out).
I seem to always be arguing chemistry on Fridays.
alistair
On 1-Oct-10, at 12:36 PM, Dave Mcneely wrote:
Alistair, you probably know enough to correct any error that I commit
here. My understanding is that all lead acid batteries produce
hydrogen gas during charging due to electrolysis of water. The
amount of hydrogen depends on the charging rate, but even routine
charging during driving would produce some hydrogen gas. Battery
explosions are most commonly explosions from within the battery, due
to gas accumulating inside, but gas can be present any time charging
is going on. The most common ignition source for a battery explosion
is a spark, which is why jump starting instructions specifically
provide for making the last connection at a point distant from the
battery.
I don't think any of us think that a battery explosion is a likely
event in our vans. Safety precautions are taken to make even
unlikely events less likely.
When one jump starts a vehicle, the donor vehicle is supposed to be
run at faster than normal idle, to make sure the donor battery is not
depleted during the donation. So, it is charging as if the vehicle
is being driven. It is also delivering charge into the recipient
battery during the brief time of the donation.
Could a battery explosion take place then, and is it more likely with
a vanagon with its battery in an enclosed box inside the vehicle than
with a vehicle with its battery in an open engine compartment? Of
course the battery box is open and the van is open during the
operation, but the gas has less opportunity to dissipate than it does
with an open engine compartment. And the precautions that are
recommended in jump starting instructions are given for vehicles with
engine compartment battery placement.
BTW, just due to no need, I have neither given nor gotten a jump with
my vanagon. But this discussion has prompted me to consider the
proper location for grounding. I suppose the best place would be on
a frame member under the vehicle, since the usual grounding locations
in the engine compartment are all too far away. Where would you
ground the jumper cable on a vanagon?
Do you know how many vehicle battery explosions happen during a
year? I have no idea.
David
---- Alistair Bell <albell@SHAW.CA> wrote:
> I would bet that any venting to the outside is not for hydrogen gas,
> but rather to remove any sulfuric acid fumes from immediate area, to
> reduce corrosion of nearby parts.
>
> you are not going to get an explosion under the seat from the small
> amount of hydrogen gas produced by the battery. A battery can explode
> from within if hydrogen and oxygen are not allowed to escape, but
> that is different than the fear of vented hydrogen combining rapidly
> with the co-produced or atmospheric oxygen, in the space under the
> seat of a vanagon. Hydrogen and oxygen are the products of
> *overcharging* lead acid batteries.
>
> its a different order of magnitude if you have a vast bank of
> charging batteries, ie a diesel electric submarine.
>
> alistair
>
>
>
>
> On 1-Oct-10, at 10:43 AM, Loren Busch wrote:
>
> RE: Stock battery venting
> I'll try to get a picture and post a link to it.
--
David McNeely
