As Apple introduced the new iPad in 2012, the 2048x1536 Retina display meant a 70 percent bigger battery appeared. With a four-inch iPhone 5 practically confirmed, and the Retina display continuing to be used, how will Apple approach the implementation of Li-ion batteries in the upcoming device?

Noam Kedam, vice president of marketing for Lyden Energy, a company making batteries, writing for Forbes, said Li-on batteries haven't progressed in 20 years. "Absent a change in battery chemistry, Li-ion is going to impose some limitations on where Apple can go with the iPhone 5's design and spec sheet," Kedam said.

There's probably going to be two keys changes to the iPhone 5 compared to the 4S and iPhone 4: a larger, 4-inch, screen and an upgrade of the A5 processor (the new iPad user the A5X processor). 4G LTE is also a possibility, and it would further affect battery life.

With the aforementioned 3 features, the new iPad introduced a 70 percent bigger battery. The iPhone is condensed in form factor, so even with a taller device, Apple is limited by how it can increase the battery.

"This is because Li-on battery technology is failing in two areas: energy density and thermal sensitivity," Kedam said. Kedam added battery manufacturers are finding it harder to gain increased performance out of Li-ion batteries. Therefore, Apple has to increase the battery to offer greater performance. And, if the iPhone 5 is probably going to increase to (or beyond) a four-inch screen, an efficient battery is needed.

Kedam added "Packaging efficiency" is where Apple may have an advantage over competitors - the way it builds its devices, basically. There are two different implementations for a battery: a dedicated area - "carved," in Kadem's words -, or using three layers of technology, the screen, display, and battery. The Motorola RAZR and RAZR MAXX have the latter implementation.

Apple has used the former implementation with previous devices, but what about the iPhone 5? Apple has made its iPhone and iPads thinner in successive generations; "This approach makes more room for a battery that nonetheless maintains the optimal X/Y ratio and thickness to maintain maximal energy density and thus run-time per charge," Kedam added.

However, as the size of the technology decreases - like the rumored micro-SIM European carriers are stockpiling - then more space will be available.

Kedam also thinks charging and discharging the device can generate heat, while generating heat through a powerful charger could reduce battery life. Using the current iPhone charger on a bigger battery will take a longer time to charge.

Ultimately, Apple is probably going to reduce the thickness of the new iPhone while increasing the weight - perhaps slightly oxymoronic - along with meeting the demands of a bigger screen, and improved performance.