Hello all -

I am nearing completion of my 1972 AT2 restoration and will soon be reinstalling the electrical.

Some guidance/recommendations regarding the battery would be helpful.  I'm looking at a MotoBatt MB9U which according to their website is the proper battery for '72 AT2.  They list the dimensions as being 5.35" lgth x 3" width x 5.25" high.  I don't have the measurement I took of my battery box here with me but I believe the depth (which will accommodate the length of the battery) is nearly an inch shorter. 
So my question is:
Who's using a MotoBatt battery in their AT2 and what model # is it?  Or, who's using some other sealed cell battery that they're satisfied with and what brand & model # is it?  I'm not dead set on the MotoBatt.  I just want a quality & properly fitting sealed cell battery.... meaning I'm not going to have to rewire my leads b/c the terminals are in the wrong place.

Secondly, regarding the lighting operation.  I had no familiarity with this bike (or any AT2) prior to the restoration. In fact, I acquired it as an abandoned project in boxes and loose parts.
My question is:
Once all of the electrical is hooked up, should I expect the lighting to operate without the engine running?

I welcome your wisdom

Being 12 volt & electric start yes everything should operate without motor running. I'm no help with battery dimensions but someone will be along soon that has done it [used motobat] as Motobat batteries seem popular nowdays.

I’ve used the motobatt on the AT2 and dt125. At first I thought it was going to be too tall but it actually has a plastic spacer attached to the bottom that you take off then it’s short enough to fit. I think I had to shim the side up with some closed cell foam pad to take up the extra space there. 

Thank you for your thoughts fellas.  My specific issue is trusting the MotoBatt application of their MB9U battery which according to their website is the proper battery for '72 AT2.  They list the dimensions as being 5.35" lgth.  The depth of the battery box on the AT2 is just about an inch shorter than that, so the battery would protrude out by that amount.  Not being familiar with the AT2 (I obtained it mostly disassembled) I don't know if that is workable.  It looks as if the backside of the oil tank is integral in holding the battery in place, so there may be a distance allowance there.  But if this were correct you'd think there would be some sort of padding/cushioning designed to be on the backside of the oil tank.  The schematics (covering both AT2 & CT2) are ambiguous.  On the Electrical drawing they show a battery strap but that's got to be specific to the CT2 whereas there's no place that I can see for a battery strap to attach at the top of the battery box on the AT2.  The Frame drawing shows the more accurate AT2 battery box but without any reference to any sort of securing strap.  Thus my speculation of the oil tank holding the battery in place.  But it'd be hard to believe that Yamaha engrs. would've used the backside of the the oil tank to secure a battery without the use of any cushioning/protection.  And that brings me back full circle to my concern of the length of the MottBatt recommended MB9U battery.
So I guess I've now got another question... on the AT2, what is the factory designed method of securing the battery?
Regarding the battery dimensions (my initial question) I can mount the oil tank and take some rudimentary measurements and simply go find a battery that'll fit.... but I was hoping someone that's already crossed that bridge would've recalled exactly what they used (without modifications to the leads) and shared it here.

  
 

DaveinSC shared his experience with the Motobatt.

Shared parts lists can be confusing.  Poor job was done transitioning to digital.. AT and CT are different animals when it come to electrical/battery...  prefix 261 for AT battery box, 248 for CT. 

Pretty sure same battery box was used on AT from 69 all the way through the 74-76 DT125 12v generator/starter models.  It's not a rare part or battery.  Just get a "regular" battery for the AT if you don't trust Motobatt.

Lots of manuals and parts lists in Tech Library as well as pictures in threads here....  here's a picture from a manual showing strap over oil tank end.