Imperative form of "saber":
(tú) sabe
(vos) sabé
(usted) sepa
(vosotros) sabed
(ustedes) sepan
We do use the "saber" in imperative, but seldom.
Your example for "ustedes" (as we in Mexico would address a group):
- Sepan quién fue el primer presidente, para el examen.
In daily speech though, we would rather say "sépanse"; the "se" in this case only adds some sort of emphasis (maybe as if your teacher said "
do know").
More colloquially though, we might say something like: "Hay que saber quién fue el primer presidente para el examen". "Hay que", an impersonal form to state
what is needed, substitutes the imperative.
*edit* By the way, a Mexican teacher would most probably say "apréndanse quién fue el primer presidente para el examen".
This is closer to "know by heart", and also "se" adds an emphasis on the action.
Your second example can very well be expressed with the imperative of "saber": "Sabe (tú) siempre dónde puedes recargar la batería", but it might sound slightly artificial. You can use "está seguro [be sure] de saber siempre dónde..." or, way better, "asegúrate [make sure] de saber siempre dónde..."