i'm not 100% sure as it has been a while since i had 7970, but i'm fairly sure the battery itself is compatible between 7970 & the external battery versions of 6770/6870/9070. however, i think the external battery is sold with the e-tube battery mount. the wire exiting from that mount is different for e-tube vs 7970.

2 options are:
1) buy only a new battery mount (i suspect this may be possible--either through an official source or ebay)
2) use one end of an e-tube wire and splice it to your existing battery mount. it's not hard work to do the splicing--it's just that the wires are very narrow gauge. i've coached 1st timer solderers (is that a word?) through it.

shimano's grommets are, well, just grommets. depending on where/how you drill, you may find it easier to use other grommets. sometimes i have found the shimano grommets don't work all that great, even in a frame designed for it. one spot on my tarmac comes to mind.

if you're careful, drilling the frame is NBD. if you are willing to keep some wires exposed, it would be fairly trivial, but if you want to sink all of them it is harder as you will need a larger opening to fit the b-junction.

One easy example would be to drill at the top of the DT and a single hole near the bottom, then run the RD connector externally along the underside of the CS. The FD & battery leads will be short and external as well. 2 small holes, very easy.

Another thought that comes to mind is whether you can drop wires down the ST and get them to run up to the front of the DT and back to the RD (you'll have to drill rather at the back or along the CS). If your Tarmac is "open" at the bottom bracket, this would be trivial. Many (most I've seen) have a metal shell, though. If you can get those wires routed, you can then drop a B junction down the ST without having to drill a larger hole in the BB.

You'd then need 2 small holes for the FD & battery wires.

Some frames I've drilled (a Cannondale Scalpel) had internal plugs in the down tube used during the molding process. I have yet to see a Specialized bike with that, but it could happen. I was able to drill through the plugs but it gets a little awkward due to how you have to reach the plugs with a drill (through the BB shell and the HT opening). Very unlikely, though.

I'd give it a shot. You like Di2. It's proven. You can get deals on it. It's available now. And, while I've drilled a brand-new frame, your attitude toward THIS frame means a bit lower risk.

Also, give Shimano a quick call (the 949 area code #). They have some very knowledgeable people there, and they can definitively answer whether the old battery itself will work in a new mount. They have been remarkably helpful.

(Hm...last thing that comes to mind is I know the new internal batteries have the firmware built into the battery itself; there is no mount. I wonder if this could be a problem for the external batteries--if they can accept new firmware or have some limiter I don't know about. It is possible the FW is built into the external mount, not the battery.)

One more way to test: Order/borrow/find a new charger from your shop/a friend/backcountry, and see if your battery charges. You then know that at least all the leads/connectors match up.



(As I thought about it, there may be a way to