^ This. Also, i have been exactly where you are and honestly it is just exhausting. I took my fp to a shoot in the cascades last winter thinking the cold wouldnt kill it that fast... boy was i wrong. Had the same wiggle issue with a cheap cable and lost about 20 mins of a really good interview because the screen just went black without warning. Its honestly not as good as i expected from a brand like Sigma. Unfortunately the fp is just a massive power hog and they really dropped the ball on the internal management for a camera this expensive. If you wanna stay under that 150 mark and not mess with heavy v-mounts, i found a solid combo that doesnt break the bank and actually stays put on a cage:

• Baseus 65W 20000mAh Power Bank - this thing is a tank and actually puts out enough juice for 4k without the camera complaining about low voltage.
• SmallRig Portable Power Bank Holder 2336B - it bolts right to the side of your cage and keeps the brick from sliding out when you are moving fast.
• ZITAY Sigma fp DMW-DCC11 Dummy Battery to USB-C - this cable is the secret sauce because the connector is tighter than most generic ones. Tbh i spent way too much on fancy rigs before realizing this simple setup worked better for handheld stuff. That Zitay cable is key because it has a built-in converter that actually talks to the camera properly. Just make sure you gaff tape the cable to the cage near the port... its the only way to be 100% sure it wont wiggle loose while you are running around. Good luck in Seattle, hope the rain doesnt make the power issues even worse.

Yeah, the internal battery is basically just a placeholder, its honestly pathetic for anything beyond a 5-minute clip. I had issues with the camera shutting down constantly because the voltage would dip on cheaper packs. Not as good as I expected from Sigma for a pro tool, but you can definitely fix it without going full V-mount. Heres what I use to keep it alive on my cage:

• Anker 737 Power Bank 24000mAh 140W PD for reliable power delivery that wont spike.
• SmallRig Universal Power Bank Holder 2790 to bolt it directly to your setup.
• Kondor Blue USB-C to USB-C 3.1 Gen 2 Cable because the fit is way tighter than generic ones. Quick tip: please buy a dedicated cable clamp for your cage if you havent yet. If that USB-C port wiggles even a millimeter, the recording stops instantly. Its a total lifesaver for handheld doc work.

🙌

Building on the earlier suggestion, an NP-F plate setup might actually be more reliable than a standard power bank for a long documentary shoot. It bolts directly to the cage for a cleaner rig.

• SmallRig NP-F Battery Adapter Plate Lite 3018
• SmallRig DC5521 to DMW-BLC12 Dummy Battery 3248 This combo provides consistent 7.4V output. Its a solid alternative that stays well under your 150 budget and wont wiggle around while you're moving.

Finally someone says it. Ive been thinking this for a while but wasnt sure.