Basic idea is that these convert solar energy into mana. Now that Urthona's sun is dying, they have lost some power and take much longer to reach a full charge. You can still use them every turn but not always to their full potential.

Nice card. Sensible version of the 2-mana mana artifact. Seems a bit better than Sphere of the Suns, but that did seem nerfed a bit harder than one might expect.

I don't know if you've encountered the rule that a mana artifact for is equivalent to a nonbasic land with the same rules text. You can see that on things like Adarkar Wastes vs Talisman of Progress, Shimmering Grotto vs Prismatic Lens, etc. I mention it because if you converted Tendo Ice Bridge to a mana artifact following that rule, you'd end up with precisely this except that drawing colourless mana wouldn't recharge it. Accordingly, this may turn out to be rather strong.

Which isn't to say you should immediately nerf it. The flavour is good, and good mana stones are a good thing to have. Just letting you know that that rule of thumb suggests this may be on the strong side.

I decided to 'nerf it' to make it an artifact version of Tendo Ice Bridge because there are a lot of ways to get charge counters onto artifacts nowadays.