Re: [bmw] Replacing a battery on 99 E46 323

[mkohlbrenner@xxxxxxxxxxx]

Aleksandar Mitreski <mitreski@domain.elided> wrote:

> ... The only difference between stock BMW batery and Interstate MTP 91
> is that the BMW has an exaust pipe that releases certain nitrogen gases.

Actually, it is hydrogen that is released from automotive batteries.

> The Interstate Batery does not relase any gases so there is no need
> for an exaust pipe...

This isn't really true.  Modern automotive batteries tend to be of the
"Sealed Lead Acid" type, but this is really a misnomer -- they all
have a pressure relief valve in them in case too much pressure builds
up inside.  During the normal course of battery charging, some of the
water can be broken down into its components, hydrogen and oxygen.  The
intent of "sealing" is that these components are retained and should
recombine so that you don't have to keep adding distilled water like in
the good ole' days.  However, if you either overcharge or charge too
quickly, you can build up enough pressure that it needs to be vented.

I have worked with truly "sealed" lead acid batteries in my job and
have seen them get surprisingly pregnant when overcharged.

The real difference between the OEM battery and your new battery is
the presence of the hose, not the vent itself -- both have a vent and
both WILL vent hydrogen under similar circumstances.

You should ask yourself why BMW chose to put this hose on your battery.

The answer is simply that since your OEM battery is in an enclosed space,
rather than in the relatively well vented engine compartment, they had
to ensure that any hydrogen released from the relief valve is vented
to the exterior of the car.

If your new battery decides to release hydrogen, you will be venting
this potentially explosive gas into your enclosed trunk.

To confirm my understanding on this subject, I did a quick search on
"lead acid battery hydrogen vent bmw" which yielded, among others:

http://www.njbmwcca.org/Bulletin/bmw11-2002.pdf

This topic is discussed in the middle of that document.  It also
mentions that you can get some sort of exhaust kit to retrofit to your
new battery.

Regards,
Mike Kohlbrenner
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