Power for portable project

Question

I decided to rip apart a cheap super Nintendo and wire it to a small screen. I am using it to learn more about electronics, practice soldering, using an oscilloscope etc.

One of the challenges I have is how it would be battery powered. I have wired the screen and Nintendo into a breadboard and plugged in my bench supply. Nothing will turn on below 7v and with a game playing, I see about .565A. Both devices came with 12v power supplies. In order to run the two devices off of battery, what is the most effective way of going about it? When I look for videos and tutorials on this stuff I find a lot of RC hobbyist stuff.

Can I get a 3.7v LiPo battery and boost that, is that unwise? Or do I get a pair of 3.7v LiPo batteries and put them in series and boost that?

Answer 1

Taking a look the unofficial SNES schematics will be hugely helpful here.

Referring to this document reveals that most of the SNES runs off a 5V supply generated by a 7805 regulator (U12). Replacing this regulator with a more efficient switching regulator will significantly improve your battery life.

However, the SNES also expects a 9V supply to be available on VS. 7V is actually not sufficient for this rail; you will likely find that sound is distorted or muted when running on a 7V supply. You will need to either use at least 3 cells to supply enough voltage to this rail, or use a boost converter to provide this supply. (The draw on this rail should be minimal; it's just running a LM324 quad op-amp and a few analog components in the amplifier.)

Using a boost regulator, especially from 3.7V, is not advisable. The efficiency of boost regulators is often not very good, and would be compounded when running a second regulator for VCC. You're much better off driving two separate buck regulators from a higher voltage, or potentially generating 5V with a buck regulator and 9V with a boost.

Answer 2

You can get cheap RC lipo batteries which give 2S~6S (series) li-po that can handle current nicely (they do not, however, provide over-current protection). My suggestion would be get one that has more than 12~13V drained, use a buck converter with UVLO set at drained voltage + safety margin and use that to power your system. Anything that requires boost instead of step down will require higher current from batteries, and anything in between will require buck-boost which is a bit less readily available in breakouts.

So, e.g.:

4S 5500mAH Li-Po (you can get ones with less capacity cheaper)

4S = 4 batteries in series, which gives ~3.7V*4=14.8V safely discharged and 4*4.2V=16.8V charged.

You can then use a readily available LM2956 step down converter (although you can find more efficient or smaller options) to step your 14.8~16.8V to 12V.

This reduces a bit the current requirement (current, not power!) and should be an easier load on the battery.

There is one problem with RC batteries though. They tend to not provide over-current, over-draining, thermal, etc protections, so I'd recommend integrating an UVLO circuit to your setup and possibly a fuse.

Page 27 of LM 2956's datasheet shows a simple schematic and description of how to implement this (note that Z1 in your case would probably have to be ~14V):

So, how long would it last?

Say your setup consumes 12V * 0.6A = 7.2W

Quick google around says that at 3.7V a LiPo cell has 30% left, so we can use 70% of battery power.

LM2956 efficiency is about 85~90% for 15V to 12V:

So:

5500mAh*14.8V*0.7*0.8/7.2W = 6.3h

Which seems quite enough to me!

Then depending on your uses you could buy two smaller batteries instead of a huge one, etc etc