Hello Bimmerphiles! I recently got an unpleasant surprise when installing a new battery in Joanne’s 1995 325is (E36). Might a similar surprise await you?

There exists controversy among bimmerphiles as to which 3 Series is the best: E30, E36, or E46. You will note that I did not mention the E21, E90, or F30. I guess your opinion will depend upon which model’s attributes are most important to you.

Anyway, my aforementioned surprise was due to a construction difference among the E30, E36, and E46 cars. On the E30 (those with trunk-mount batteries) and E46, the battery is contained in a topless metal compartment on the right side of the luggage compartment. Indeed, on the E30 the battery sits in a “plastic” tray within this metal compartment. This is one of the nicest designs I have ever seen. So, on the E30 and E46, any damage due to decamped battery electrolyte will be readily apparent when the battery is removed and said damage would hopefully be contained in the battery compartment. Not so on the E36!

Believe it or not, Joanne’s 200,000-mile 1995 E36 was on its second battery until recently. The BMW OE battery lasted fifteen years and the Interstate replacement lasted until August of 2017. This might be a testament to my use of a battery maintainer whenever the car sits idle for more than a day.

In replacing the Interstate and cleaning up a bit of corrosion in the battery compartment, I noticed that the inboard side of the battery compartment is open to the spare wheel compartment! I noticed this when I dropped a tool and it disappeared, only to be retrieved after the spare wheel was removed.

And that is when I got The Surprise.

Photos 1 and 2 depict the corrosion I found when I removed the spare wheel, which had been in place for years and having received only periodic air pressure checks. Luckily, after removing the corrosion by liberally scrubbing with a Scotch Brite pad and Simple Green Industrial Cleaner and Degreaser (pH of about 9; you can also use a mixture of baking soda and water), I found the underlying metal to be OK. After flushing with water and drying overnight, the first-coat primed area is as shown in Photo #3.

After a second coat of primer and two coats of Rust-Oleum gloss black, things looked as shown in Photo #4: almost good as new. Incidentally, in the upper-right corner of the photo, you can see the bottom of the new battery, an Odyssey Extreme PC1200MJ AGM (Absorbent Glass Mat).

In a traditional flooded-cell lead-acid automotive battery, the cell plates are suspended in liquid electrolyte, a mixture of sulfuric acid and water. In an AGM battery, the electrolyte is absorbed into fiberglass mats fitted between the cell plates. So an AGM battery is less likely to spill electrolyte. Indeed, the Odyssey instructions indicate that the battery can be installed in any orientation except inverted. Other claimed advantages of AGM batteries are longer service life, higher deep-discharge capability, slower self-discharge rate, higher vibration resistance, and higher cranking amperage for a given case size. A known disadvantage is higher cost, which may be offset by longer life (the OE battery in Joanne’s E36 is a hard act to follow). Possible disadvantages of AGMs are that they have a lower maximum operating temperature (some may not be suitable for an underhood environment), and (according to the Battery University Website) if your non-smart voltage regulator is set to charge at more than 13.8 volts, this could overcharge the battery on a long drive. The Odyssey Battery Technical Manual suggests a “float-charge” voltage of 13.6 volts (roughly equivalent to the required voltage-regulator setting) and a charging voltage of 14.7 volts, but does not directly address voltage-regulator setting.

I found this a bit confusing, so I contacted Odyssey battery. These folks have always responded promptly to my queries. Regarding temperature limitations, Odyssey says that the max operating temperature for the PC1200 battery is 113 °F, clearly below underhood operating temperatures. For the metal-jacketed PC1200MJ however, the maximum temperature is 176 °F. For a trunk-mounted battery, I think the non-metal jacket PC1200 would be OK, but for a few more dollars, I went with the PC1200MJ.

Regarding charging voltage, the Odyssey Product Guide’s warranty section specifically states that the alternator should be set to provide between 14.0 and 14.7 volts, and if the charging system is not set within these limits, shortened battery life can be expected, as well as a voided warranty. This can be a problem on BMWs, some of which use a charging voltage lower than 14 volts (forget the newer “smart” charging systems, where battery voltage is not held constant). The charging system on Joanne’s E36 runs at about 14.2 – 14.3 volts after things warm up, while my M3, which also has an Odyssey, runs at about 13.6 volts. o we shall see how things work out. Perhaps I should have read the fine print more thoroughly…

I chose the PC1200MJ because, while it has a lower cold-cranking amperage rating than BMW specifies (540 vs. 700), the PC1200MJ is short enough not to interfere with the battery compartment’s “plastic” cover. Since the car is rarely if ever started in cold temperatures, I think that 540 cold-cranking amps should suffice. Again, we shall see. Odyssey does not offer a direct-replacement battery for the E36.

Evidently BMW believes that AGMs are superior batteries, as BMW has been installing them for some years now, especially in Bimmers with “smart” charging systems. I am interested in knowing what battery life your Bimmer experiences, regardless of whether it has an AGM battery. From reports I read on iATN (the International Automobile Technicians Forum), BMW battery life ain’t what it used to be.

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 tech tips, repair/maintenance questions, repair horror stories, emissions-inspection sagas, product evaluations, etc.

© 2018; V.M. Lucariello, P.E.

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