Playing on Minecraft 1.11.2 and while troubleshooting mods in my private modpack I found the following lines in the latest.log (Made sure I'm using all the latest versions of the libraries and dependencies)

[00:39:28] [Client thread/ERROR]: Using missing texture, unable to load rftools:textures/blocks/powercellw.png, java.io.FileNotFoundException
[00:39:28] [Client thread/ERROR]: Using missing texture, unable to load rftools:textures/blocks/powercellu.png, java.io.FileNotFoundException
[00:39:28] [Client thread/ERROR]: Using missing texture, unable to load rftools:textures/blocks/powercells.png, java.io.FileNotFoundException
[00:39:41] [Client thread/ERROR]: Using missing texture, unable to load rftools:textures/blocks/powercelle.png, java.io.FileNotFoundException
[00:39:41] [Client thread/ERROR]: Using missing texture, unable to load rftools:textures/blocks/powercelld.png, java.io.FileNotFoundException
[00:39:41] [Client thread/ERROR]: Using missing texture, unable to load rftools:textures/blocks/powercelln.png, java.io.FileNotFoundException

This still happens today, although the error text looks a bit different now:

[22:30:26] [main/ERROR] [TEXTURE ERRORS]: +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=
[22:30:26] [main/ERROR] [TEXTURE ERRORS]: The following texture errors were found.
[22:30:26] [main/ERROR] [TEXTURE ERRORS]: ==================================================
[22:30:26] [main/ERROR] [TEXTURE ERRORS]:   DOMAIN rftools
[22:30:26] [main/ERROR] [TEXTURE ERRORS]: --------------------------------------------------
[22:30:26] [main/ERROR] [TEXTURE ERRORS]:   domain rftools is missing 6 textures
[22:30:26] [main/ERROR] [TEXTURE ERRORS]:     domain rftools has 1 location:
[22:30:26] [main/ERROR] [TEXTURE ERRORS]:       mod rftools resources at /home/joe/MinecraftMods/RFTools/bin
[22:30:26] [main/ERROR] [TEXTURE ERRORS]: -------------------------
[22:30:26] [main/ERROR] [TEXTURE ERRORS]:     The missing resources for domain rftools are:
[22:30:26] [main/ERROR] [TEXTURE ERRORS]:       textures/blocks/powercelln.png
[22:30:26] [main/ERROR] [TEXTURE ERRORS]:       textures/blocks/powercells.png
[22:30:26] [main/ERROR] [TEXTURE ERRORS]:       textures/blocks/powercellw.png
[22:30:26] [main/ERROR] [TEXTURE ERRORS]:       textures/blocks/powercelle.png
[22:30:26] [main/ERROR] [TEXTURE ERRORS]:       textures/blocks/powercelld.png
[22:30:26] [main/ERROR] [TEXTURE ERRORS]:       textures/blocks/powercellu.png
[22:30:26] [main/ERROR] [TEXTURE ERRORS]: -------------------------
[22:30:26] [main/ERROR] [TEXTURE ERRORS]:     No other errors exist for domain rftools
[22:30:26] [main/ERROR] [TEXTURE ERRORS]: ==================================================
[22:30:26] [main/ERROR] [TEXTURE ERRORS]: +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=

I'm pretty sure that this is actually either a vanilla or Forge bug in their .mtl handing. Long-term, I'd like to get off of .obj and .mtl files and onto .json's though.

I'm not in favor of that. I make my models in Blender and obj/mtl is the only thing that Blender can output that MC can read.

Do you plan to ever edit the powercell in Blender again? If not, could I convert just it to JSON?

I would rather keep it as a blender model

Fair. I'll turn to Forge and see if something with them can get this to stop happening.

Has there been any updates to this issue?

Thank you for taking the time to create this issue, as we're no longer supporting versions below 1.12, I'm going to go ahead and close it but if this bug seems to continue over to newer versions, please do report it again so we can investigate the issue further.