When in early game play. The best way to conserve power without the battery charger is;

say you had a "welder" with 0% power and you are about to craft a "seaglide" with the necessary components, including a battery which is 100% power.

Simply replace the 0% battery in the welder with the 100% battery. Then you will still have the components to craft the seaglide.

Once the seaglide is built it will have a 100% battery in it. There for you have just fully recharged a battery to 100%. This workes on any tool with a battery (apart from the "heat knife")