Any electricians in the audience? Need some help on a project.

[MoFo]

I'm trying to sort out a power circuit, so I can have something run off battery, recharge the battery, or run off a wall socket.  Hoping there's an electrician who can check my wiring diagram and make sure it'll work - I don't want to backfeed any of the circuits, or fry the components.  :D

[KursadA]

Can you provide more details? What is the power consumption of your circuit? What voltage level and battery chemistry are you using?

[MoFo]

Well I was hoping to do this via PM since it's *wildly* off topic, but okay...

 

I'm building a Raspberry Pi handheld device.  I want to run it off a cell phone power bank to get a decent battery life (I need 5000 - 10000mAh flat pouch li-po cells, can't source them locally at a decent enough price and don't want to roll the dice on cheap cells from China, since they've got a sketchy reputation for short lifespans and false mAh ratings).  The power banks I'm looking at don't support pass-through charging; most automatically shut off battery out once power in is detected, so I can't just plug the device in and use it while it charges.  The ones that *do* have pass-through don't input enough current to charge the battery AND run the device at the same time anyway, and since the input is limited, with the device on I'd just be drawing down the battery more slowly.

 

So my plan is to have a 3-way switch, so I can select between battery power, battery charge, and mains power.  So, switch to the left, it runs off battery; switch in the middle, I can plug it in and re-charge; switch on the right, I can plug it in and run it off A/C  (through a USB cell charger).

 

The Pi is 5V/1A.  The screen will run off 5V/1A, so I figure 5V/2.5A should work, factoring in power connections and a few ancillary components (audio amp, etc.)  So, not *major* currents in terms of 'somebody's going to die if this goes wrong', but the Pi and screen probably can't handle much over-voltage, and I don't want to wreck them.  And backfeeding the lipo cell could be a bad thing, I'd imagine.

 

In terms of a circuit, I'm thinking something along the lines of this:  

 

 

 

(the lipo cell is a stand-in for the power bank since there aren't any banks in the Fritzing library; it's got a USB out and a USB in, so just imagine the power bank's charge circuit is between the battery and USB connectors)

 

So basically, three questions: a.) will this work; b.) should I be looking at SPTT or DPTT slide switches (assuming it will work) and c.) am I over-thinking this and I should just go with an eBay lipo pouch after all...

[dnl42]

You might check out Sparkfun. ISTR they had a board that might manage this...

[KursadA]

A typical electrician will not be able to help you with this. As I suspected, this is an electronics question. While it would work, you may be overthinking it with the Rube Goldberg switch setup. We engineers have figured out reverse current protection and load sharing for battery chargers a long time ago.

 

What you exactly need is a circuit built around a DC-DC converter IC with battery charging , pass through and load sharing functions. These ICs are commonly used in many consumer devices; and there are hobbyist boards built around them as well. One example is the Adafruit PowerBoost 1000C.  Based on a quick look at its specs, it looks like it would work for you assuming you chose a lipo battery that can provide 2A. Note that with this circuit you don't need a power bank, just a battery.

[MoFo]

LOL.  This is actually an effort to work around a 1000C.  :D

 

That was actually my original plan (using a powerboost).  The problem is that, like I said, I want 5000 - 10,000mAh of battery, and places like Sparkfun and Adafruit max out at 2500mAh.  So the only option is lipo cells from China, bought off eBay, but they have a pretty poor reputation - sellers vastly overstate their capacity, or the cells are low quality and degrade quickly, etc.  Much like 3rd party cell phone batteries.  I don't want to buy a "10,000mAh battery" only to find out the actual capacity is closer to 2000mAh, then have it crap out on me a month from now.  Also, I do need a pouch form factor, so 18650's and 14500's are out.

 

I can buy an Anker or Xiaomi power bank and know I'm getting reliable batteries.  And by using the power bank's charge circuit, I eliminate the cost of the powerboost, which is a nice bonus.  It would be nice if I could just rip the batteries out of a power bank, since they're the perfect form factor and capacity, but they cells themselves won't have a charge protection circuit (it's integrated on the PCB) and I do not want to solder directly to the cell itself.

 

Sigh.  The more I think about it, the more I think I should just bite the bullet and buy a battery off eBay, and hope for the best.  :(

 

 

And FWIW, when I asked for input on a different forum, I got an even more Rube Goldberg solution - use the power bank to re-charge the 2000mAh lipo running through a powerboost 1000C.  So, mains -> power bank -> power boost -> lipo -> power boost -> device, and going from 5V -> 3.7V -> 5V -> 3.7V -> 5V.  :)

[KursadA]

So it looks like the Powerboost 1000C was indeed the right solution, but you can't trust the batteries in the market. You may have a point, but as long as you stick to well-known brands in the market you should be fine. At least the Powerboost 1000C is a well-documented, known quantity while we don't know anything about the particular characteristics of the power bank charging circuit.

 

As for the available Li-Po cells being from China - where do you think all the other Li-Po batteries are made in? I will give you 4 options and let you guess the right one:

 

a) Canada: where they are lovingly built in a building converted from a Victorian orphanage in Toronto, by the happy workers of a socially responsible progressive company which pledged to donate half of its profits to transgender Inuit children from disadvantaged backgrounds.

 

b) Germany: where they are made in a 17th century Benedictine monastery about 20km. west of Geislingen by a group of monks who have been making high-performance batteries since 1872.

 

c) Sweden: where they are made in a beautiful, modernist architecture factory by a super happy and well-educated Swedish work force, each of whom can speak 5 languages with near-native fluency and enjoy unparalleled social rights such as 6 years paid paternity leave (8 years for mothers). 

 

d) China: where they are built in a depressing factory in a suburb of Shenzhen, by a migrant work force most of whom were introduced to running water and indoor plumbing about 4 years ago.

 

Once you figure out the correct answer, I would just suggest buying the right batteries and put together a circuit using the Powerboost. ;-)

 

 

Edited April 6, 2017 by KursadA

[MoFo]

43 minutes ago, KursadA said:

c) Sweden: where they are made in a beautiful, modernist architecture factory by super happy and well educated Swedish work force, each of whom can speak 5 languages with near-native fluency and enjoy unparalleled social rights such as 6 years paid paternity leave (8 years for mothers). 

 

So you're saying I should look at Ikea, then?  :)

 

I don't care that the batteries are made in China.  My concern is that a significant proportion of the lipos being sold on eBay are low-quality cells.  Whether they're re-selling factory rejects or mislabelling lower capacity cells, I don't know, but they do have a reputation for under-delivering.  If there *was* a well known brand I could stick to, I'd be happy.  It's the roll-the-dice aspect I'm concerned about.  And having to wait two months for replacements if the ones I order *do* suck.

 

But, point taken.  I'll go the Powerboost route.  Thanks for the advice.