Do I need three batteries?

I prefer to use 4 myself but you might get by with 2 high CCA batteries on an M11 if you're leaving it on charge when not in use. My advice is if its got 3, leave it that way. Never hurts to have extra juice.

 

When I bought my '99 W9, it only had two batteries. Never had trouble starting the N14, but it was warm weather. I added another battery before cold set in, just to make me feel better.

 

I had a battery explode when I hit the key after fueling a few years ago. Disconnected the bad battery, fired it up, and ran the rest of the day on the other 2.

...So yes, 2 will be plenty. Heck, you may even be able to get by with only 1 if it'll turn the motor. If you go THAT route, though, I wouldn't shut 'er down anywhere you don't have access to additional juice to get 'er fired back up again if that 1 battery isn't at 100%.

 

If I remember correctly the CCA rule of thumb minimum is 150 CCA per 100 CID on gas engines and 300 CCA per 100 CID for diesels. At 61 cubic inches per liter an M11 is around 670 cubic inches and should have around 2000 CCA battery capacity. I do not remember who's rule or thumb that was but it does not care how many batteries are used.
Rather than try to get by with less CCA I would consider automotive batteries. They are not designed for service like commercial batteries. Ten hour per day operation, constant vibration, shock, etc.. However 1000 CCA is still 1000 CCA. Three 750's equal 2250 CCA, may not cost more than two 1000 CCA's and have more reserve capacity.
Your starter and cables will thank you and you will thank them if you ever choke it down in an intersection.

 

You could put two in it, but if you've got my luck something's going to come up where it can't be plugged into the battery tender, and / or you will have to start running the truck when it's cold out.

Some of mine have 3, some have 4.

 

How about getting batteries that are dry with the acid in a separate container like the old days and fill and install the batteries when you need them . Unlimited shelf life and if you retire that truck you'll have them for another .

 

Get three batteries--your starter will thank you.

 

If you are using a battery tender new batteries should last just about forever.