Need to replace that battery in your trunk? Sealed or not? Here is all the information you'll need from the very informative and talented Herman Chan. Thanks Herman!:
At 05:31 AM 10/24/2001 -0400, you wrote:
Date: Wed, 24 Oct 2001 01:26:32 -0700
From: Herman Chan 
Subject: [uuc] Re:  Replacement Battery Observations

>I read several postings to avoid sealed maintenance-free batteries 
>due to their higher charging requirements than their non-sealed 
>counterparts.

Aside from ensuring that there are proper venting provisions for 
trunk or interior installation, I don't recall seeing any convincing 
caveats regarding installing a particular type of battery in any BMW, 
outside of E36 convertibles which require a vibration-resistant 
battery.

>So, if BMW recommends a maintenance free battery, I wonder if the "charging
>voltage" information is true. Just a datapoint. But I'm curious what other
>digesters input on this would be.

You must be referring to the article commonly circulated among the 
club newsletters cautioning against maintenance-free/lead-calcium 
chemistry batteries.

Prior to my recent purchase of an Interstate MTP-91 as a replacement 
for my original BMW battery, I sent their tech dept. a copy of the 
article and asked them to comment.  I can forward a copy of their 
response if you're interested.

Interstate's response allayed any concerns I might have had, and my 
battery has not caused any problems in my car, which given its usage 
pattern (sparingly driven with mostly short-medium duration trips) 
would probably tax a battery more heavily.

The Interstate's clone, the Sears International Group 48 battery also 
seems to be quite popular among E36 owners.  Both are made by Johnson 
Controls (Hey, Carlos!), as is the Energizer equivalent now found in 
Pep Boys outlets.  This battery is the most powerful Group 48/91 
battery on the market, and fits most E36 models.

However, six-cylinder E36 owners might not want to give up the extra 
capacity that their original Group 92 batteries have, but outside of 
BMW/Douglas, I don't think anybody makes a 92 battery, so many people 
install 48/91 batteries that the four's and Z3's have as standard. 
Others have had success fitting some of more readily available 49/93 
batteries.

My only gripe with the Interstate is that unlike the OEM Douglas 
battery which has the vent port on the end adjacent to the Positive 
terminal post, the Interstate's vent port is located in the middle of 
the battery on the side between the posts, necessitating a vent 
extension kit which the dealer should provide.  With the kit's 
extension tube and butt connector to hook up to the original vent 
tube, the total length might only be adequate to barely reach the 
body grommet, depending upon how the hose is routed.  A quick run to 
the store for some longer tubing to replace the original took care of 
this.

>FYI, the battery for an E36 convertible uses a different battery designed
>for (per Bentley) higher vibration conditions. Interesting, why would the
>convertible have more vibrations in the battery area?

The battery in the convertible is used as a vibration damper and 
rides in a moving tray.  A "regular" battery that fails if used in 
this application would probably have its warranty voided by the 
manufacturer.

Herman