Reddit reviews POWERADD Pilot Pro2 23000mAh Power Bank 4.5A DC (5V/9V/12V/16V/19V/20V, 3 Ports) Output External Battery with LCD Display for MacBook, Laptops, Smartphones and Tablets

Something like this if you want lightweight and 1-1.5 recharges:
http://www.amazon.com/Poweradd-23000mAh-Multi-Voltage-Portable-Notebooks/dp/B015OAJFOC/ref=sr_1_42?ie=UTF8&qid=1457760566&sr=8-42&keywords=External+battery+pack+laptop

Or this if you're looking to get closer to 24-30 hours (3-4 recharges) and can carry a little extra:

http://www.amazon.com/MAXOAK-50000mAh-Portable-External-Notebook-Most/dp/B00YP823NA/ref=sr_1_18?ie=UTF8&qid=1457760443&sr=8-18&keywords=External+battery+pack+laptop

Excellent question -- although the T470 allows for swapping its external battery while the internal keeps the lights on. You might find yourself happiest by stocking one or more 72Wh 61++ batteries. From personal experience, using an external battery with extra cords while travelling is a nuisance. A big nuisance. You will need more desk space and so on. Not cool. When I was still using a 12" machine ten years ago, I found a slim battery matching its size and so it was under it and that was good. I am unaware of any slim batteries to fit right under the T470. It would need to be hell of a slim TBH, for example there's an Anker which is 184 x 124 x 15mm which is about one half in both directions of the T470 area. I am not quite sure the laptop would be very stable with it -- but if you do have a 61++ in the laptop, who knows, it might just level it out. Or you might be able to jerry-rig some feet if it doesn't. But we are really going off the chart here. (As an aside, note how PackedPixel avoids taking any desk space at all. They are on it.)

With that aside.

I am sure there are many products on the market but I am only aware of the Razer Power Bank which can deliver 2.25A at 20V over USB C. 150 bucks only buys you 12,800mAh. That's not cool.

However, if you consider the T470 also has the good ole' rectangular tip, then practically all laptop external batteries are on the table. For example https://smile.amazon.com/Poweradd-23000mAh-Multi-Voltage-Portable-Notebooks/dp/B015OAJFOC/ will do, even if it doesn't ship with the rectangular tip, a multitude of converter plugs from the old to the new are readily available on eBay and Amazon for about 1-2 bucks. This or any other similar will be below 100 bucks for the roughly maximum capacity FAA allows (gosh, people say it's the TSA that limited the battery capacity, they might enforce it but it's not them who limited it). There are 50 000 mAh monstrosities for those who do not fly.

And of course the Portable AC Outlet Battery Pack by ChargeTech will charge everything happily between green earth and blue sky. It has a price to match the capabilities. And converting the voltage to AC just to be converted back to DC is ... not efficient. This is a last resort and for that it's horrendously expensive.

https://www.amazon.com/dp/B015OAJFOC/ref=cm_sw_r_cp_api_RLJjzbZ6BKXJX

I've been using the following at 19V w/o issue:

https://www.amazon.com/gp/product/B015OAJFOC/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

This is what I use to power a uZeus in my portable case, works great:

POWERADD Pilot Pro2 23000mAh... https://www.amazon.com/dp/B015OAJFOC?ref=ppx_pop_mob_ap_share

get something like this, www.amazon.com/dp/B0083EQS3I/_encoding=UTF8?coliid=I2QAQFXKWFOQY2

then a battery pack with dc output like this https://www.amazon.com/dp/B00HFMUBYG/_encoding=UTF8?coliid=I2DH2C7QW3L3G8 or this https://www.amazon.com/Poweradd-Pilot-Pro2-23000mAh-Smartphones/dp/B015OAJFOC/

you will probably need a wire adapter like this also since i'm not sure what comes with the battery pack. https://www.amazon.com/Valley-Enterprises-2-1mm-5-5mm-Adapter/dp/B00GPVWRRC

My T460s and phone are currently running off one of these:

https://www.amazon.com/Poweradd-Pilot-23000mAh-External-Smartphones/dp/B015OAJFOC

...it'll happily charge my X230 & X201s too, came with all the relevant adaptors, but I got it to make up for the battery life downgrade. No fires yet.

Some points you may want to consider first:

• You lose the battery time remaining indicator. It's not a huge deal mind, since you'll probably have a full internal battery when the power pack gives up
  
• These power pack get very close to the limit of what's allowed on flights.
  
• The adaptors stick out of the laptop much more than a normal plug, which can get annoying if it sticks out into the aisle on trains, e.g.
  
  But I find it useful enough it comes everywhere. I can also charge it with the thinkpad AC adapter, so I don't need to bring one just for the battery.
  

That's what I mean. Regular battery packs only output 5v @ 2A, what you need is a special Li-ion battery pack that's capable of outputting ~12V @ 3-4A. Like this.

no, TB2 doesn't have this ability.

Your best best is getting a power bank with 20V output and using a MagSafe cable to barrel connector.

I used to do this when I had a MagSafe MacBook.

something like this

https://www.amazon.com/Poweradd-Pilot-Pro2-23000mAh-Smartphones/dp/B015OAJFOC/ref=sr_1_sc_3?ie=UTF8&qid=1502556284&sr=8-3-spell&keywords=laptop+powerbank

with this

https://www.amazon.com/Connector-DC-MagSafe-Cable-5-5x2-5mm/dp/B00JQ4I4D6/ref=pd_sim_107_3?_encoding=UTF8&pd_rd_i=B00JQ4I4D6&pd_rd_r=DF0KN0Y4X430XVH5JHBQ&pd_rd_w=dPVbl&pd_rd_wg=NNvgA&psc=1&refRID=DF0KN0Y4X430XVH5JHBQ

it looks like you could get 12v and 3A out of something like this.

https://www.amazon.com/Poweradd-Pilot-Pro2-23000mAh-Smartphones/dp/B015OAJFOC/ref=sr_1_3?ie=UTF8&qid=1502918241&sr=8-3&keywords=12v+computer+battery

Naw dog.

Been powering my my laptop on the go with this thing for about 4 years now. Laptop cord stayed at home.