Why don't you put the batteries in the legs and just run some wire up to the upper body?
The two halves of the Titan are not going to be permanently attached, but magnetised so it can be disassembled for easier transport. The legs and the upper body will each make a relatively "flat" piece to transport, without the big voids the overall shape would have. (I had considered making it fold up at its hip joints, but that wouldn't pack very efficiently - even if the hip joints didn't have pistons that would fall apart under those circumstances.)
Running a connection up through that kind of joint is not likely to be reliable. Running wire outside with a breakable electrical connection wouldn't be a problem though.
However, it would mean deviating from the design, and I'm not actually sure exactly where in the legs I'd put a battery box. The "Pelvis" is going to be mostly structural engineering to make the hip joints work, so there's not really much room there. The upper thighs are large enough - but I'd have to re-engineer my plans for the structural skeleton there.
The tl;dr answer is that it's the only practicable place to keep the power supply.
Are you going to be detailing an interior replete with princeps etc?
Yes, there will be an interior, complete with a Princeps, Techpriest, 2x Moderati and 2x weapons targeting servitors. Some sources list an extra servitor and Techpriest, but space is fairly tight in there anyway! (Even though I'm likely going to omit one of the internal bulkheads/doors)
I'll probably throw in some maintenance servo skulls though. (I'd imagine they're MIU-Linked to the Techpriest, letting him work through them and access areas he otherwise couldn't).
All except the servitors should be removable. Partly so the crew can have "on foot" models to allow them to appear not in the Titan (although the Techpriest is going to have a standing model anyway), partly to offer up another possibility.
The plan is to leave some parts swappable to leave open the possibility of a later project to make parts to reconfigure the chassis into a Chaos Titan - likely Khorne dedicated based on my planned colour scheme and the number of parts that are practicable to swap. (Armour-wise, the shin, loin, head and back plates).
The former case is simply that the colour scheme is going to be similar to the Legio Ignatum - a Maroon, Yellow, Black, Brass combination taken from (as I've said before) this steam locomotive
. That colour scheme works for Khorne, so that's good.
The other part is that I won't have the opportunity to defile every last armour plate - with Khorne, that works fine. Things like the Brass Scorpion and Blood Slaughterer are actually very "clean". They're not covered in rust or dribbling daemonic pus all over the place, which means that having some "plain" armour plates won't be out of place.
Leaving the crew interchangeable will mean an opportunity for either rotting crew (as a daemon engine) or some traitorous fanatics driving it.
But that is (at very best) a long way off. Still, planning ahead never hurts.
As an aside, I have no idea how the Vulcan Mega bolter is meant to hold the 14,000 rounds claimed in Imperial Armour. With conservative ammunition dimensions, I can about see a way (although not fully practical) to get about 2,000-2,500 rounds per hopper.
With my own interpretation of the mega-bolter, with much longer barrels (you may notice the ejection port is much further back on my version) and the larger cartridge to make use of that, I'm getting about 1,200 rounds per hopper.
(Basically, my interpretation starts with the idea that each half of the Mega Bolter is roughly a 50mm version of the GAU-8 Avenger*).*Although the Avenger doesn't eject cases, it feeds them back into the magazine. It stops them damaging the plane as they're ejected.
However, rather than the normal interpretation of bolter rounds, I'm playing it as something a bit different, with the fuse instead being more like that on the Bofors 3P ammunition.
The full description on that is here
, but the short version is that it comes with proximity fuses (to allow it to detonate near hard to hit aerial targets), time fuses (to allow it to detonate at a given distance) or impact fuses (to let it explode on impact, or delayed to go off after penetrating the target).
50mm calibre (although I'm not sure on a canonical source for that, it's what I'm going with) is not something to use against infantry in the way bolter ammo is normally fused (to go off when it hits its target).
I see the Mega-Bolter as being able to air-burst munitions, blasting apart entire squads with well timed and aimed bursts of just a fraction of a second (perhaps even delaying detonations until after the rounds have penetrated intervening terrain). Against harder targets like vehicles, it might use direct impact.
Of course, using that kind of ammunition in a Gatling gun is crazy... but so is a Titan. Stupidly awesome over-engineering is routine in the GrimDark.