W111 Battery

I had recently noticed that the battery on the 250SE was not in good health.   The battery disconnect switch I built a few years ago has a battery health indicator included.   Even when connected to the trickle charger it was showing orange.   I also noticed that the trickle charger would sometimes go into error mode.    My battery tester showed that it was well below where it should be.     I had only replaced the W111 battery in the 250SE about four years ago, which is a pretty short life.

I was never happy with the battery I put in the car four years ago.   It was a DIN53MF battery and it was quite a bit shorter than the frame allowed.   This small size gave me only 500CCA; 54Ah and a reserve capacity of 90 minutes.   In theory nothing wrong with these specs, but my experience tells me that a small battery on these old cars just doesn’t last very long.   Even if a trickle charger is used.   The W111 battery is under different strains than in a modern car.   These old starters can draw quite a lot of current and older cars require more cranking.   Additionally, the alternator, at 35A, is quite small.   At idle, it probably barely provides enough power if multiple accessories are being used.

This time, I was keen to find a much better battery for the car.   I wanted at least 650CCA.   The battery also needed to fit properly in the factory frame.   This wasn’t as easy as you might think.    As far as I can tell, there is not a global standard for battery sizes.   From what I can tell, the USA uses ‘group sizes’, the are DIN sizes and then there are another set of battery sizes.   I couldn’t even find a nice simple publication of available battery sizes and what they meant.

That meant I had to go to to the manufacturer websites and examine their product catalogs as compared to the measurements I made.   Once I was close I had to take the frame into the various shops and try the frame.   I even contacted somebody selling a W111 sedan to see what their Bosch battery was!

I started to narrow it down to a two candidates.   A Century 67MF and a SuperCharge MF78.   In the end I went with neither!   The other frustration I found was that most battery stores carry at most two brands.   This made it hard to go to once place and try out their batteries.   I found a shop in Tempe that stocked a number of brands, including Century and SuperCharge.

The proprietor of the shop was a bit perplexed as to what I wanted to do but seemed happy enough for me to start trying my frame on his batteries.   He didn’t have the MF78 in stock, but I ended up finding something better.    I went with a SuperCharge MF66H.   This W111 battery gave me a big boost in capacity.   I now had 750CCA, 80Ah and a reserve capacity of 154 minutes.   I had previously ruled this size out as the frame wouldn’t fit.   The SuperCharge is a bit different to the other brands in that the top moulding allowed the frame to just fit over.   The major difference was the vent ports did not stick out to be flush with the side of the battery.

I did have to give the frame a few knocks with a rubber mallet to get it to go on, but overall it fit snugly.    To fit the battery I had to buy a battery terminal spreader as I had slightly damaged the terminal on the old battery installing it.

The last thing I did was to give the new battery a quick test, which showed up as 810 CCA, well above the rated capacity. This is pretty typical as most batteries are better than their rated capacity when new and gradually degrade over time.  As can be seen in the pictures above, the battery is a perfect fit in the frame and gives me the capacity I was looking for.

Due to the weather, I have not taken the car on a proper drive yet, but when moving it to get the DS out, it started up very quickly.