not paying nearly £500 for a couple of camera batteries!, I found a cheaper option, if i needed to buy the batteries and charger it would have been about £70 for after market (knock off) makita batteries and charger. (plus rigging)
This could be used to power all kinds of stuff as well, not just a camera.

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Tools used-
Yorkshire tea- (UK) https://amzn.to/2WUYBlv (USA) https://amzn.to/3heqfRQ
Battery adapter- (UK) https://amzn.to/3zzlZD5 (USA) https://amzn.to/38XdTZH
Voltage regulator- (UK) https://amzn.to/3hoxQgB (USA) https://amzn.to/2YFXd73
3M VHB sheet- (UK) https://amzn.to/3E49dQ5 (USA) https://amzn.to/3hkQB4y
Makita batteries- (UK) https://amzn.to/3nmB73P (USA) https://amzn.to/2YFY1sB
Cheeseplate- (UK) https://amzn.to/2V5Dkof (USA) https://amzn.to/3nkfcu3
Baseplate- (UK) https://amzn.to/3kPcD29 (USA) https://amzn.to/3A67Gqc
Rods- (UK) https://amzn.to/3eJu14g (USA) https://amzn.to/3lcw4R0

Update-
Ive been making another of these for my 700d which I film at home with and I have modified that one and the one on my C100 with one of these-
(UK) https://amzn.to/3D2v7CW (USA) https://amzn.to/3lfJfQV (wire in parralel between battery and regulator board) and fitted both circuit boards into one of these-
(UK) https://amzn.to/3yNAx1Y (USA) https://amzn.to/3njAYON

it disconnects the battery when it falls below a set voltage to protect from over discharge. turns out im terrible for leaving the camera running and have ruined one of the batteries by discharging it too much a few times!

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