Philes’ Forum – Winter 2019


By Vic Lucariello, Sr.

Hello Bimmerphiles! This time out I have a follow-up to my last column on AGM [Absorbent Glass Mat] batteries and an AGM-conversion story on my E30 M3. But first, a correction.

In the last Philes Forum, I reported that Odyssey Battery tells us the maximum operating temperature for their PC1200 AGM battery is 113F. I wrote that 113F is clearly above underhood operating temperatures. I meant to say that 113F is below underhood operating temperatures, so a PC1200 might not be suitable for an engine-compartment-mounted battery. I was inundated with a veritable Colorado Blizzard of emails taking me to task for this error. Hey, in 32 years of writing Philes’, this was my first error. NOT!

As part of the preparation for my last column, I also contacted another AGM-battery manufacturer, Optima. These batteries, with their distinctive cylindrical-shaped cells, are quite popular with racers. Optima says that their red-top automotive AGM batteries have a maximum operating temperature of 125F. Same ballpark as for the Odysseys.

After my successful AGM-battery installations in my hotrod and in Joanne’s 1995 [E36] 325is, I decided to put one in my E30 M3. [A disadvantage of having multiple vehicles is that one of them seemingly always needs a battery or tires. This time, three of them needed a battery.]

Given that the M3 is a 4-cylinder, I reasoned that it should not need as large and powerful a battery as does the six-cylinder 325is, so I selected an Odyssey Extreme PC925MJT. The Interstate MTP-91 battery I removed is rated at 700 CCA [Cold Cranking Amps] while the PC925MJT is rated at only 330 CCA. The MJ stands for metal jacket, not really necessary in this trunk -mount application. The T stands for traditional SAE top battery posts. Without the “T”, you don’t get battery posts, so keep this in mind. A PC925 is somewhat less expensive than a “MJ” or “T”, and I believe some Internet vendors might be taking advantage of this. Other reasons for selecting PC925MJT are that it precisely fits the width of the “plastic” battery tray in the M3 [same tray for all the trunk-mount-battery E30s] and it is about 13 pounds lighter than the Interstate. My M3 is a street-only ride, but for a track-driven car 13 pounds is a LOT of weight reduction.

Since the Odyssey battery does not have hold- down “feet” at the bottom of its case like the BCA [Battery Council of America] Group 91 batteries BMW specifies for the trunk-mount E30s, I had to make something to hold the battery down and prevent it from sliding fore and aft in the battery tray. I chose to use a piece of aluminum-alloy channel commonly available from industrial- supply houses such as McMaster-Carr. The problem was that I only needed a piece of channel less than a foot long and McMaster-Carr only sells the stuff in longer lengths. Oh well, so now I have some nice aluminum-alloy channel in stock.

I really hate to drill holes in automobile bodies unless it is absolutely necessary. This is because said holes need to be rust- proofed, and there ain’t no rustproofing like factory rustproofing. [You 2002 and 320i owners probably scoff at that statement.] On the M3 I was able to secure the battery-hold- down bolts [I used 8-millimeter threaded rod also available from McMaster-Carr] to the “plastic” battery tray by fashioning a couple of reinforcements affixed to the bottom of the tray, mimicking the factory reinforcements, and then mounting 8-mm “nut serts” through the reinforcements and tray bottom. See Photo #1, which shows the underside of the removed battery tray.

Photo #1 – Underside of reinforced battery tray.

One other problem to be solved was that the existing OE battery negative cable was too short to reach the new battery. I was able to source a seemingly very nice quality negative cable from Battery Cables USA. The cable is #2 AWG [American Wire Gauge], closely matching the metric wire gauge of the OE cable, and 12 inches long. Twelve inches was a little more than necessary, but a little longer is preferable to a little too short. BCA will make you a custom cable to your specs, but I chose the 12-inch, which was in stock.

The completed project, with the battery cover removed, is depicted in Photo #2. BMW’s “plastic” battery cover fits nicely over the new battery, yet another reason I chose the PC925MJT. The Group 91 battery specified for the E30 has provision for external venting, but this is not needed for the AGM battery. I put all the removed parts together and have them squirreled away in a safe place in case this smaller-than-stock battery does not work out. The M3 can easily be returned to stock configuration if necessary.

Photo #2 – Completed installation.

We typically have sub-zero-F overnight temperatures here in SW Colorado in January and February, and my M3 hibernates in my enclosed trailer during the nasty months. I am happy to report that when I started the M3 last week, the S14 engine cranked over vigorously and started right up. So time will tell if my choice of a lesser-CCA battery was wise.

That’s all for now, bimmerphiles. See you next time.

Anyone wishing to contribute to Philes’ Forum can contact me at I’m interested in comments, tech tips, repair /maintenance questions, repair horror stories, emissions inspection sagas, product evaluations, etc.

© 2019; V.M. Lucariello, P.E.

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