As esferas recarregam quando são deixadas de fora em um Highstorm. Elas pegam e armazenam o Stormlight da tempestade. Como Highstorms ocorrem a cada dois dias, não é difícil recarregar as esferas, mas Kaladin é um sujeito bastante ativo, com não muitas esferas, então não é de surpreender que ele precise procurar Stormlight onde puder.
Além disso, no final de Words of Radiance ...
...they are entering the period of the season called the Weeping, where Highstorms slow then stop altogether. This is what Dalinar's military plan is relying on, that the army can reach the Parshendi and end the war before the Highstorms resume, and not have his people exposed on the Shattered Plains with no shelter from the storms. So if you're in that portion of the plot, Kaladin is looking for more spheres because there's been nothing to recharge them lately, and he's not the only one doing so.