That's the idea of the mod - these arn't "batteries" that you just recharge. They are energy packs you place in until they drain.
They are an imperfect solution to the problem of powering your suits, alowing for higher-drain work while still not being "click and forget".
I balanced the size around the amount of batteries that go into crafting them, with a little loss (they are still smaller in-gride than the batteries involved).
If you want to be using them, expect to take a few extra packs along with you.
If you just want larger batteries, there are other mods for that.