Take a 'mage' whose magic takes the form of manipulating (non-magical!) energy and matter in an approximately 10m radius around them. Examples of this include high speed air currents around the mage that serve as protection or forcing oxygen away from a person to cause suffocation although they can't affect anything below electrons and neutrons etc. They also can't turn matter into energy or vice-verse.
Knowledge of science isn't required for them to perform magic- their mind 'magically' (haha!) works out how to make their spells happen, but they do have to obey any laws of physics magic doesn't directly ignore by its existence (meaning they can't create energy/matter etc). Their greatest weakness is that they can't make use of chemical energy, from themselves or batteries.
What could a mage like this-call her Anna- use as 'mana' batteries to store energy to power her spells? Ideally, the object in question should be:
Light weight, so she can carry a few of them
Inconspicuous, so she doesn't stick out like a sore thumb
Not made of illegal substances- she should be able to acquire it through legal channels, although paperwork is fine.
'Rechargable' in her own home.
Magic in this universe is a legally accepted thing with compulsory magical education for mage-kids and various licenses for using magic in cosmetic surgery, construction etc.