mud2005
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Simple battery circuit feasible?

Sun Jun 04, 2017 1:26 am

I'm new to the pi world and electronics in general, I bought a 7" display and a raspberry pi 3.
They both run great off AC adapters, but I'm looking to run off battery power.
my display can handle 5V-12V I'm wondering if this simple circuit would work to run the pi and display and also be able to charge the batteries using an external 2S battery charger.
I realize I may be in over my head and this circuit is way too simple, if so feel free to say so, I'm here to learn :)
I plan on only charging the battery when the pi is off
diagram.png
diagram.png (12.61 KiB) Viewed 3112 times

W. H. Heydt
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Re: Simple battery circuit feasible?

Sun Jun 04, 2017 2:39 am

Any design work I ever did pre-dates Li Ion batteries (which is what you appear to be planning to use) by decades. That said, I don't want to be anywhere near by if you try that circuit. Li Ion batteries finicky. In addition, a Pi runs on 5v+/-5% (that is, 4.75v to 5.25v). If I'm reading what you're doing correctly, it looks like you're going to hit the Pi with 7.4v. If you're *lucky* all you'll do is trip the polyfuse. If you're not lucky, you'll need a new Pi.

Some recommendations... Don't use Li Ion (or LiPo) until you have some experience. Use a good voltage regulator (with an adequate current capacity--2.5A) to control the power going into the Pi. Learn standard symbols for drawing circuits. Specify your circuit elements precisely so that others can tell exactly what you're using.

levelcrow
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Re: Simple battery circuit feasible?

Sun Jun 04, 2017 3:27 am

I'm guessing you're going to use Lithium ion as well due to the 3.7V value; please don't mess around with those types of batteries without doing your research and using the necessary specialized hardware such as a charge controller.

Li-ion may be common but if you handle it incorrectly in terms of charge current, discharge currents, or charge voltage you could end up starting a very violent fire; one that may burn your house down if you aren't around to catch it.
http://www.mpoweruk.com/lithium_failures.htm

You can't rely on Lithium ion to give a stable voltage either. The voltage is going to vary depending on how charged it is, using two of these batteries in series is only going to make the problem worse.

Honestly, it may be safer to just use a commercial battery like one of those cell phone power banks as they've already got the charging and voltage regulating circuitry in there. You can step up the 5V out of the power bank to the 7.4 for the display and also feed just the 5V to the Pi. You'd still have to figure out the proper circuit logic for some sort of charger such that it only charges the battery when the Pi is off. This is a much easier and safer project to start out with, and I won't spoil the fun but will say that if it were me then I'd probably use some sort of MOSFET and some sort of 7400 series inverter or buffer chip.

Please let me warn you that there are some reports that cheaper cell phone power banks don't have circuitry designed to handle discharge and charge at the same time, please make sure to account this before experimenting ESPECIALLY with applications you'll leave plugged in unattended. Lithium ion battery fires are a serious matter and will absolutely burn your house down if not caught in time; they are absolutely in the realms of possibility when tinkering with them and the risk of such an accident must be kept in mind at all times.

MarkTF
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Re: Simple battery circuit feasible?

Sun Jun 04, 2017 4:19 am

As previous posters have noted, 3.7 V implies you're using LiPo batteries. The usual charge algorithm for LiPo batteries is constant current to a particular voltage and then constant voltage until the current goes down to a specified level. The implication is that the voltage and current into the battery have to be carefully monitored and controlled. In the circuit you've drawn this can't be done because the charger can not distinguish between current flowing to the battery and current flowing to the display and the Pi. Errors in charging can destroy LiPo batteries, sometimes in spectacular (i.e. they catch fire) fashion.

Some battery technologies, like the lead-acid batteries commonly used in automotive applications, are charged using a constant voltage process and might work in a circuit similar to what you've drawn. You'd probably need a regulator to both the Pi to get a fixed 5 V and to the display to get a fixed 7.2 V.

mud2005
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Re: Simple battery circuit feasible?

Sun Jun 04, 2017 4:34 am

thanks for the replys, just to clarify, I was planning to use a charger such as: https://hobbyking.com/en_us/turnigy-e3- ... -plug.html
and as for the current to the PI it wouldn't be 7.4V as I had planned on a UBEC such as: https://www.adafruit.com/product/1385
I have some experience w/ balancing/charging lipo cells w/ rc planes and electric bikes, but am by no means an expert

edit: would it be possible to add a switch to the display and PI so when charging the current only goes to the batteries?

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Gavinmc42
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Re: Simple battery circuit feasible?

Sun Jun 04, 2017 6:44 am

Hobby King have plenty of 2S packs.
Just make sure it has the charger/balancer connector for the charger.
Get two packs, then you can charge one while using the other.
Then the circuit is even simpler

Get a flame proof bag at the same time ;)
ubec should be fine if it can handle the max current likely.

I have not yet but I would go for 2-3S LiFePO4.
Use AA cells and external charger. these are getting cheaper now.
Check other posts for more info on LiFePO4
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achrn
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Re: Simple battery circuit feasible?

Sun Jun 04, 2017 7:47 am

mud2005 wrote: I plan on only charging the battery when the pi is off
diagram.png
As drawn, though, you can't control that, unless what you mean is that you only plan on connecting mains power to the charger when the Pi is off. In that case, it might work, but I wouldn't trust a cheap charger conencted across the battery without powering the charger. There should be something to stop the battery discharging back through the charger, but lithium batteries being as picky as they are, I wouldn't do it. You could wire in a changeover switch so that the battery is connected to either the charger or the Pi & screen, but I'm not sure why - it would be just as easy, and more certain, to wire up your batteries to quick-release connectors and just unplug and replug.

UBECs run Pis very happily (providing they have the current capacity, which most do). I have a couple doing that .

Given your apparent familiarity with RC gear, beware that RC lipos generally have less in the way of built-in protection circuitry than the lipos in laptops and other consumer electronics - you are at risk of damaging cells if you over-drain them, and damaged cells can reputedly do bad things when you next charge them.

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Gavinmc42
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Re: Simple battery circuit feasible?

Sun Jun 04, 2017 9:26 am

Given your apparent familiarity with RC gear, beware that RC lipos generally have less in the way of built-in protection circuitry than the lipos in laptops and other consumer electronics - you are at risk of damaging cells if you over-drain them, and damaged cells can reputedly do bad things when you next charge them.
That might be a good clue, check RC UBEC's, some might have a low voltage shutoff.
Trying to charge and run at the same time is much less simple.
It is one of the reasons I am thinking 2 x LiFePO4 standard size cells.
External charger will charge the cells separately and a linear low drop reg is simpler.

It really depends on how long you want to run the Pi off batteries.
26650 LiFePO4 are 3300mahr, AA are 600mahr
I'm dancing on Rainbows.
Raspberries are not Apples or Oranges

mud2005
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Re: Simple battery circuit feasible?

Sun Jun 04, 2017 5:23 pm

thanks all, this gives me a lot to think about.
there is also this option: https://www.adafruit.com/product/1566
I could plug the PI into the 2A and the screen into the 1A and it has a charging circuit built in all for $40 and has a very high capacity, almost seems to good to be true.

W. H. Heydt
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Re: Simple battery circuit feasible?

Sun Jun 04, 2017 8:07 pm

mud2005 wrote:thanks all, this gives me a lot to think about.
there is also this option: https://www.adafruit.com/product/1566
I could plug the PI into the 2A and the screen into the 1A and it has a charging circuit built in all for $40 and has a very high capacity, almost seems to good to be true.
They note that it is NOT a good "UPS" due to the way the power behaves when charging during use.

The price of "power banks" has come way down. Consider this one: https://www.monoprice.com/Product?p_id=15122 with a smidge more capacity (10.4Ah) than the Adafruit unit, but at less than half the price, which can provide 2.4A and can charge at 2A. For slightly less than the price of the 10Ah Adafruit unit there is: https://www.monoprice.com/Product?p_id=15122 with a capacity of 27.2Ah, though the max output is only 2A.

I use the Monprice 10.4Ah pack to power a Pi2B and an RPF 7" touch screen. It runs for about 5 to 6 hours on a charge.

mud2005
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Re: Simple battery circuit feasible?

Sun Jun 04, 2017 9:24 pm

awesome! thank you

mud2005
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Re: Simple battery circuit feasible?

Sat Sep 29, 2018 6:45 pm

I have been running the pi from a 12000mah usb battery for a while now
https://www.bestbuy.com/site/tzumi-pock ... lsrc=aw.ds

It works great BUT lately I have been seeing the low voltage icon often. I have a hefty load on the battery w/ the pi, 7" screen, and amplifier
It's only an issue when running n64 emulator so far.
I am using a 20awg short cord from the USB supply to the pi

I'm wondering if there is a small board I can use to "regulate" or "step up" the power to 5v w/o wasting too much power?

disclaimer: I am an electronics noob so I need a "simple" solution :)

sparkyhall
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Re: Simple battery circuit feasible?

Sat Sep 29, 2018 7:11 pm

I've been using the 'Wide input shim' from Pimoroni on a Pi3 and Pi2, works really well 6xAA Nimh batteries:
https://shop.pimoroni.com/products/wide ... 8104321034

The shim doesn't get hot so it seems efficient, not tested on a Pi3+ though.

wolf.z
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Re: Simple battery circuit feasible?

Sat Sep 29, 2018 9:47 pm

mud2005 wrote:
Sat Sep 29, 2018 6:45 pm
...

I'm wondering if there is a small board I can use to "regulate" or "step up" the power to 5v w/o wasting too much power?

disclaimer: I am an electronics noob so I need a "simple" solution :)
Your initial idea lacks an under voltage protection. So your Li cells may have a short life. In addition, depending on the charger's regulating strategy, charging won't work at all, having a second power sink (Pi, display) connected. The power bank approach, using a power bank capable of being charged and provide power in parallel is the most "simple" solution from a user's perspective.
As long as the undervoltage still lets run the Pi and the display, why engage any additional step up convertor?

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davidcoton
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Re: Simple battery circuit feasible?

Sun Sep 30, 2018 12:10 am

mud2005 wrote:
Sat Sep 29, 2018 6:45 pm
I'm wondering if there is a small board I can use to "regulate" or "step up" the power to 5v w/o wasting too much power?
The USB battery will already have a 5v regulator. If you put in another voltage convertor, you will still need better power connections from your convertor to the Pi. Just get a better USB lead between your battery and Pi (eg 18awg, very short cord).

Of course you may just be drawing the maximum current your battery can supply (I don't see that rating in the spec). In which case a second convertor won't help.

Ideally any regulator needs to control the voltage at the Pi end of the cable. I don't know of any commercial products (whether battery or mains) that do so: they all regulate at the supply end and may (or may not) make an allowance for cabble losses.
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GAP
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Location: New South Wales Australia

Re: Simple battery circuit feasible?

Sun Sep 30, 2018 3:59 am

mud2005 wrote:
Sun Jun 04, 2017 1:26 am
I'm new to the pi world and electronics in general, I bought a 7" display and a raspberry pi 3.
They both run great off AC adapters, but I'm looking to run off battery power.
my display can handle 5V-12V I'm wondering if this simple circuit would work to run the pi and display and also be able to charge the batteries using an external 2S battery charger.
I realize I may be in over my head and this circuit is way too simple, if so feel free to say so, I'm here to learn :)
I plan on only charging the battery when the pi is off
diagram.png
18650 Lithium Li-ion Batteries coupled with below should protect your cells from over charging/discharging and allow you to achieve what you have drawn.
https://www.ebay.com/itm/2S-20A-7-4V-w- ... 100623.m-1

As for connecting a charger to the circuit I use a Double Pole Double Throw (DPDT) Centre Off switch on my model train locos. Switch set to centre it Pi OFF, set to left Pi is On, set to right charger is connected and Pi is disconnected.

I have a schematic but it is in .ppt format and I cannot figure out how to attach it.

For a charger look for a balanced charger like this one it also has overcharge protection built in.
https://www.ebay.com/itm/iMAX-B6-AC-Lip ... 3546656310

For a holder try this.
https://www.ebay.com/itm/Simple-Plastic ... 0008.m2219

18650 Lithium Li-ion Batteries are popular with the G Scale model train crowd.

mud2005
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Joined: Tue May 23, 2017 3:40 am

Re: Simple battery circuit feasible?

Sun Sep 30, 2018 6:02 am

Thanks for the replies! this gives me a lot to think about, that wide input shim looks interesting :)

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