what battery do i need?

Question

So i want to buld a raspberry pi portable. I am new to electronics and have a question.

It has to have a built in display and i will probably just connect a keyboard to it to configure it.

And i would like to give it a tablet like shape and just connect a usb mouse (I would also like to make it this way so i do not need a keyboard when using it, because it is portable and the mini keyboards cost a lot)

I found a backup display on amazon wich is 7 inches and needs 12v and 5 Wats

The raspberry pi needs 5 volts at 2.5 amps

So my question is: what battery do i need?

I watched a video about the types and i am going with a NiMH battery because the video said it was safe.

But what kind of ratings do i need?

Do i need a 17v battery and put the screen and pi in series, or do i fry something then?

And how di charge it? do i have to take out the battery when charging or can i connect the pi and screen to the charging circuit or something?

please help and sorry for this stupid question.

the-dubster · Accepted Answer

I'm not going to directly tell you which battery to use - but I AM going to say DO NOT put the screen and Pi in series - you'll kill the Pi or possibly them both! The same battery CAN supply both power requirements - just use regulators to give the 5V supply (The 12V can come directly from a 12V battery). If you use a higher voltage battery (14.4V for example) then a 12V Reg and a 5V reg will do the trick nicely. For regulators I often fall back on the 78XX series of 3 leg regulators (7805 for 5V and 7812 for 12V), depending on the power requirements will determine which style you need. Here are some links for the regs. Higher current devices are available and a heatsink will be required for full load. TAIWAN SEMICONDUCTOR - TS7805CZ - IC, V REG +5.0V, 7805, TO-220-3 | CPC UK http://cpc.farnell.com/taiwan-semiconductor/ts7805cz/ic-v-reg-5-0v-7805-to-220-3/dp/SC10582 STMICROELECTRONICS - L7812CV - V REG +12V, 7812, TO-220-3 | CPC UK 2A variant of the 5V STMICROELECTRONICS - L78S05CV - 5V, 2A Positive Voltage Regulator in TO-220-3 Package | CPC UK Also this is an option - the 7" touchscreen for the Pi - no need for 12V then as the included board takes care of the supply for you! Raspberry Pi 7” Touchscreen Display

shabaz · Answer

Hi Vincent,

It would be easier to use a display which also requires the same voltage (5V), so you can run both the Pi and the display from a single 5V source. There are USB power sources (they have Li-Ion cells inside) with a USB output of 5V (there is an in-built DC-DC converter) which are intended for charging mobile phones. They can be used to power the Pi and display (you don't need 10V, the Pi and display are wired in parallel).

Also, the car reversing/backup displays are low-res, it may be hard to make out any text if it was used as a tablet. Some state a high res but actually output a standard def image which is unusable for text. About the minimum would something like the Pi 7" capacitive touch display, or some HDMI display perhaps.

Also, it would be good to check out some existing projects like this, for ideas, e.g. this URL: https://learn.adafruit.com/7-portable-raspberry-pi-multitouch-tablet/overview

You're right to be concerned about safety (that URL unfortunately does not mention battery safety, and I'm not sure I'd use a zip-tie around a Li-Po cell. But the rest of the info may be useful there), the off-the-shelf 5V USB power packs should be safe if some known reputable one is used (check the reviews on Amazon etc) from a reputable source. Make sure it can supply perhaps 2A or more (the Pi and display may consume this much).