To regulate the voltage you can use an UBEC. I have 3A (5A available too) UBEC U3 Pro. There are many to choose from but for the Pi it is important to get one with the filter capacitor on it otherwise you may have issues with Wireless devices. You can find them on hobbyking or ebay. Input voltages up to 22Volts or as specified on the UBEC.
I then bought very 2x small NimH in size but pretty big capacity 1500mah battery packs. I had to use two packs (2 x 6volt) in series (12volt) because the input voltage should be more than 7.2v into the UBEC for it to work properly at max rating. So try and find batteries for your solutions, size and capacity that will give roughly +-10v (but more voltage will last longer) charged and fall to ~6v when discharged to have a smooth power experience.
I made all the splitting cables my self, and attached a micro USB
cable too. Everything with connectors, because you need to disconnect
the batteries and charge them but also you may want to use it for
another project too. You can buy all the splitters and ready made
cables too if you want.
- You can find LiPro Balance charger on ebay, like the one in the picture that can charge and charge variety of batteries. Read the manual carefully on how to charge batteries! Overarching can cause fire or make the battery useless.
- Using LiPo (Lithium Polymer) is very good for fast discharge, and longer battery life. But if not recharged properly they can blow up. So if you unsure stay away!
- I would recommend using NiMh (Nickel Metal Hydrite) because they can be charged easily and wont blow up if overcharged by mistake. For the Pi they will last long because you are not discharging them rapidly like in use for hobby RC, where LiPo is the clear winner any way over these standard NiMh.
- If you have a bit of extra cash and want even better NiMh batteries, look for nano tech NiMh. These can be charged at 2-4 times the normal rate, have double to quad capacities and last very long, due to the nature of nano positioning of NiMh (the charging chemicals are stacked precisely in parallel using nano tech, enabling more capacity, faster discharge and charge) These are becoming more popular than LiPo because they can supply incredible amounts of power in short time without affecting life time as in traditional NiMh.Again, hobbyking sell them.
- Pairing batteries in parallel increased mah and voltage stays the same. Pairing batteries in series increases voltage but mah stays the same. In RC this is an important decision for max power (more mah) output vs longer lasting (voltage) So its safe to say that more voltage is better for the Pi for longer running setups.
- I do not represent hobbyking in any way, they just sell everything (quality, not el cheapo knock offs) you would possibly need for these kinds of projects and have allot of good tutorials and tips on batteries! Everything can be found on EBay but be careful of quality vs price.
My Pi can run with WiFi and HD Stream for several hours off this configurations. If you get Nano tech it could possibly run for a few days without interruption.