Is there any reason to upgrade my battery size in Deus Ex: Human Revolution?

• sometimes extended cloak will buy you enough time to hack some computers (e.g. funicular)
• eliminating wasted energy - if you have only 2 cells and you eat nutrition which restores 3 cells, you'll waste up to 50% of energy

Having a complete battery set is useful for when cloaking and when you need to perform multiple takedowns quickly.

Partially drained cells will recharge to full. For example if you have 4 cells and only use 1 and half cells when cloaked, the cell which was half used will recharge to full.

So unless you’re going to play the game in stealthily, spending most of the time the using the cloaking augmentation there is no real need to upgrade your battery size past 3 cells.