According to the FAQ, the Raspberry Pi was not created to run on batteries. It needs to be supplied with a 5v stabilized power supply. Anything below 4,8v will make it reboot or behave strangely. Much more than 5v and you will damage it or destroy it.
This is particularly complicated with batteries, because when they are fully charged their voltage is greater then their nominal voltage, and while they get discharged their voltage drop to lower levels than the nominal. So a battery of 3,7v when fully charged gets to 4,2v, and when fully discharged drops to 2,7v. So how we regulate this madness to stable 5v?
One way is to use an UBEC (Universal Battery Eliminator Circuit). This component is responsible to stabilize varying input voltages to a single output voltage.
The UBEC I’m using is this one, easily found on Ebay or anywhere. He can be supplied with voltages between 5,5v and 26v, regulating the output to 5v or 6v, changing between then with a jumper. I’m using two batteries 18650, with nominal voltage of 3,7v, like these ones.
Both in series sum 7,4v, but look when they get fully charged they get more than 8,2v!
But that’s OK, our UBEC will regulate it to stable 5v. When we wire the batteries to the UBEC we get a regulated 5,2v power supply
Ok, now you are thinking: “Shouldn’t it be 5v?”
The answer is “yes”! But is kind of normal this deviation, and 5,2v has not fried my Raspberry YET, and I don’t think it will do (but I cannot guarantee what will happen to yours…). With the output power supply regulated to 5v, we just need to wire it to the corresponding pins on the Raspi (VCC and GND)…
Yes, we are already running Pokemon Yellow and we hacked a Playstation controller for testing purposes. But these are topics for another posts =).