Artificial Storage Capacity for Magic

Utility

The use of storage capacity is necessary for all but the most competent mages. Without it a mages capabilities are limited, most may even struggle to conjure a fireball. To use it the user must draw on the magical energy held in the storage material, which can then be used for whatever purpose the mage wishes. This itself may pose a challenge to inexperienced magic users, especially when experiencing a high-stress environment like a battle, where magic may be instrumental to victory. In this case, the battery can be bound to a certain spell, which will give the energy for immediately, should the mage cast it. Enchanting a battery this way does not require extraordinary magical acumen, but an intimate knowledge of rune crafting.
For lesser mages, it is practical to carry around an on-demand supply of energy which can be expended at one's leisure. Refilling them is easy, as the magical energy inherent to any being replenishes slowly, meaning that a mage can fill up any number of batteries by himself, given enough time.
The attempt to give lesser mages a way to cast magic for prolonged periods of time and make it their livelihood was the primary reason for research in the area. The inventor, a gnome named Sergios Zaratos, gave the template away for free, as he wanted to see a world filled with magic, that no longer was a privilege of the extremely gifted.

Manufacturing

Almost any material can be used to store excess magic. As such it possible to construct makeshift batteries from lesser material with little effort. The only required parts are the magical runes, that any mediocre wizard or runemaster can apply with little effort. The problem with these batteries of questionable quality is both their charging power and stability. At best they can bind enough power for three of four fireballs, at worst they are just fancy trinkets that are not useful. It is also possible that they start to leak the energy they hold, making them useless.
Making a battery is not the problem, building it to last and be effective is.
Several factors must be combined to allow the construction of high-quality batteries. Materials that are able to store large amounts of energy must make up the core of the battery. Obsidian, gold, and platinum are the best available, but generally, any refined ore will do. Even pure steel can conserve a substantial amount of magic. To activate its preserving abilities the runes must be engraved directly on the storage material. A high-end rune forge is required to ensure their excellent quality and it is advisable to enlist a master runesmith to engrave them.
The shell of the material is irrelevant, however, it should be able to withstand physical pressure and be easily transportable. Steel is the material used most often, as it is both relatively cheap and durable enough. The shell should leave a hole through which the magical energy can be funneled, both to load the battery and access its contents. It is also advised to enchant the shell to enhance its durability. If enchanted with sufficient skill it is possible for the enchantment to take the energy it requires from the battery itself, rather than being limited by its own power. If this is the case, the enchantment will stop working when the battery runs out but will work as soon as the battery is charged again. This is not an easy feat and requires a competent mage to do the enchantment.
As such, building a high-quality battery requires competent personnel, good equipment, and expensive materials.

Access & Availability

Artificial storage of magic is essential for any mage worth their salt. As the capacity of the mage's body is limited, most manage three or four fireballs without extensive training, and the ability to draw from the energy found in the surrounding world is fragile without decent practice, using batteries provides a good way for most to enhance their capabilities. It is also useful for people who bought magical items but lack the ability to refill them.
To accommodate the needs of these magic users or regular people, any runeforge or larger shop will try to have some of them in supply. Most traveling merchants, especially those who cater to far of hamlets will try to provide a small selection, in case they come across a wandering mage. Prices may vary, the cheapest are given away practically for free, while those produced by reputable sellers may cost as much as a carriage.