Pass through power bank

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I want to use a power bank as battery back up for a Raspberry Pi. The
power bank has to have pass through charging so that I can leave the
power bank connected to the mains while the Pi is powered from the
output. I gather not all power banks can be used that way.

Does anybody have any general or specific advice about how to choose
the power bank?

Re: Pass through power bank
On 24/01/2015 14:07, Gordon Levi wrote:
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You haven't been reading the group messages, have you?

Someone recommended this to me:

   http://uk.rs-online.com/web/p/product/7757508

and there is a 10 Ah one as well.  Also a smaller 2Ah unit.
--  
Cheers,
David
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Re: Pass through power bank
On 25/01/2015 6:40 AM, David Taylor wrote:
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I just got a similar unit from dx.com

What I found was if there is power being put into the unit, then no power comes out, so it won't work as a UPS type of  
supply, which basically is what you want, correct?

Mine is a 20000mAh. Sure it has 2.1A and 1A outputs, but nothing comes out unless you either pull out the input plug, or  
switch off the source of input power.
http://www.dx.com/p/bp-l2k-1-0-lcd-20000mah-dual-usb-mobile-power-source-w-led-for-iphone-samsung-silver-white-328011#.VMQQGy6WXKA

May pay to check the unit before you purchase.
BTW
I got mine for phone backup, not as a micro UPS.

Cheers Don...






--  
Don McKenzie

http://www.dontronics-shop.com

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Re: Pass through power bank

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My post was intended to check the unit before I purchased! Thanks for
eliminating the DX one.

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Re: Pass through power bank
On 24/01/2015 21:36, Don McKenzie wrote:
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Yes, that's the problem.  I guess you could always ask before purchase.  
  Anyway, I can confirm that RS 10 Ah unit /does/ work as a UPS.  I have  
one of my Raspberry Pi PCs working on it right now.  Unplugged the main,  
and the RPi carried on working.  Replugged the mains and the device  
continues to supply power to the RPi.

--  
Cheers,
David
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Re: Pass through power bank
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I once got a pack somewhere (I think it was included with an old HD 808
camera I ordered via ebay) that exibits this behaviour. I was more or
less happy until I found out it did not recharge the battery when  
something is connected to the output.

Another one for the checklist: does it recharge when power comes back?
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[J|O|R] <- .signature.gz

Re: Pass through power bank
On 03/02/2015 16:51, Oscar wrote:
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Yes, it does.  You can unplug it from the mains, plug it back in, and  
the charging light resumes as you would expect.

--  
Cheers,
David
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Re: Pass through power bank
On 1/24/2015 2:40 PM, David Taylor wrote:
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The spec says, "Input power source rating: 500mA @ +5V"

I'm not sure if that limits the input current to 500 mA or if it is  
saying 500 mA is required from the power source.  If the unit won't draw  
more than 500 mA it won't power the rPi at max current draw without  
draining the battery, no?

--  

Rick

Re: Pass through power bank
On Sun, 25 Jan 2015 16:21:08 -0500, rickman wrote:

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Does that mean that this:
https://www.modmypi.com/pi-modules-upis-advanced-stacking

is currently the best bet at GBP 48.99?  This charges at 350 mA, but then  
again it is a UPS and can run off mains, batteries or solar cells and  
seems to accept supply voltages from 5v - 18v.

This looks a bit more reasonable when you know that Amazon flogs 2500 mAh  
'phone chargers, i.e. a pocketable 2500 mAh hard-cased Li-poly battery  
for anywhere between GBP 9 - GBP 25 and only the more expensive ones have  
chargers and a micro-USB output connector.



--  
martin@   | Martin Gregorie
gregorie. | Essex, UK
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Re: Pass through power bank
On 26/01/2015 01:50, Martin Gregorie wrote:
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It's only a quarter of the capacity, though.  In any case, as I am only  
experimenting, I preferred the more versatile in-the-power-line unit,  
although it's not a true UPS.

--  
Cheers,
David
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Re: Pass through power bank
On 25/01/2015 21:21, rickman wrote:
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The instructions are along the lines of: if the input source can supply  
1A or more the unit will draw 1A otherwise it draws 0.5A.  Not sure how  
it determines that.  Mine has been running an RPi B with Wi-Fi and a GPS  
attached for 20 hours now, and as soon as I started it the "charge  
greater than 80% LED" was flashing, and it still is.  When running, that  
RPi takes 0.4A.  My USB ammeter is only USB-A connectors, so I can't  
easily measure the current into the unit, or know how it determines  
whether to draw 0.5A or 1A.

--  
Cheers,
David
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Re: Pass through power bank
On Mon, 26 Jan 2015 11:01:57 +0000

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USB devices negotiate current draw with the host when connected.
Default is 100/150mA (USB2/3) maximum 500/900mA.  If the data pins are
shorted the device knows it's a dumb charger and can draw 500-1500mA.
ISTR there's a recent extension (USB3.1?) to this that provides 2A at 5V
but I don't recall the details, and of course Apple chargers/devices
have their own method.


Re: Pass through power bank
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There are (at least) 3 different schemes for the data lines to indicate
it's a dedicated charging port. Shorting (usually done with a 200 ohm
resistor) is one way. Another is 2.0V on one and 2.7V on the other data
line to indicate 10W output (2.1A), or 5W output if the voltages on the
two data lines are are swapped.  Another is 1.2V on the DP line.
There are USB PSU chips which detect which method the USB device is
trying to use, and adapt the effective resistance and voltages on the
data lines to match it (such as Texas Instruments TPS2511).

However, the mess with USB charging standards (the Chinese standard, EU
standard, Apple, and USB Consortium standard) mean many devices nowadays
work out the current available by steadily increasing the draw whilst
monitoring the voltage drop, and back off a bit when the voltage drops
below some device specific threshold (such as 4.75V), or if the supply
cuts out. This can mean that a low spec charging cable can severely limit
the ability to draw max current from a supply even when both the device
and the supply are capable of high current use.

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10W (2.1A) can be available from USB chargers to Chinese and EU standards.

The USB consortium standards allow for up to 1.5A on USB2 dedicated
charging port I think, but they also have standards for up to 90W or 100W
for laptops but using different connectors and higher voltages. I've never
seen these in use anywhere, possibly because you have to pay a license
fee to implement their standards, which I don't think is the case if you
use the Chinese or EU dedicated charging port standards.

--  
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

Re: Pass through power bank
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FWIW: I have an Anker 15,000 mAh unit (bought from amazon) and I powered
a Pi from it last week - noticed the following morning it was down to 1
dot, so plugged it in to charge - and it started charging. However it's
taken 3 days to charge fully - while powering in the Pi.

That, I suspect is the biggest issue with these units - the ability to
take 2x the output capacity to enable charging in a sensible time span...

I suspect it would go off an stay off when it ran out of juice though.

Gordon

Re: Pass through power bank
On Tue, 03 Feb 2015 17:10:12 +0000, Gordon Henderson wrote:

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Might this have something to do with limitations in the mains PSU you  
were using to charge it? I assume that's a USB charger, so what's its  
maximum current capability?

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Might be worth testing that.


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martin@   | Martin Gregorie
gregorie. | Essex, UK
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Re: Pass through power bank
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2 amps.

However who knows if the battery device actually senses the input voltage
or what. USB power is notoriously rubbish.

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One day maybe!

Gordon

Re: Pass through power bank
On Tue, 03 Feb 2015 18:32:26 +0000, Gordon Henderson wrote:

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As a 72 hour charge for that battery should (in theory) need just 208mA,  
a 2 amp PSU should have plenty of capacity for running both.

Charging a LiPO or Li-ion battery is fairly complex: a good charger  
provides constant current at a 1C rate (a 1000mA cell is charged at 1  
amp) until the full pack voltage is reached and then constant voltage  
until the current falls to an arbitrary small value, when the charger  
reports fully charged and turns off.

My guess is that the built-in charger was designed on the assumption that  
there is no load on the pack when its being charged, so the running RPi  
is simply eating some of the 1C current, meaning that the battery is  
being changed at a rather lower rate than expected. That shouldn't hurt  
the battery as the charging circuit will just dimly sit and wait until  
the battery gets to the end of the constant current phase. After a power  
outage I imagine that the charger restarts in constant current mode.


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martin@   | Martin Gregorie
gregorie. | Essex, UK
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Re: Pass through power bank
On 2/4/2015 7:45 AM, Martin Gregorie wrote:
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I think the issue is that once the battery is fully charged the switch  
to constant voltage charging may reach a point where the battery is  
being discharged.  I assume if the voltage starts to droop the charger  
will revert to constant current mode, etc. switching back and forth,  
charging and discharging the internal battery.  Such light charge cycles  
will eventually wear out the battery sooner than if it were managed more  
like a "proper" USP.  That would imply that there is a charging circuit  
for the battery and a separate supply for the output which does not draw  
from (and disrupt) the operation of the charging circuit.

The documents on the Anker web site support this.  The lighter 10,000  
mAHr unit says it will be damaged if used this way and the larger unit  
mentioned here says it "may" be damaged.

--  

Rick

Re: Pass through power bank
On Wed, 04 Feb 2015 13:05:21 -0500, rickman wrote:

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That seems quite likely. There's certainly some missing information.  
There's nothing specific on the Amazon site advertising the 15000 mAh  
battery. About all I could see is that both USB sockets have the same  
label alongside them and IIRC there's a comment about charging two  
devices at once.

Methinks this could be a case where an SLA may be the better solution  
simply because you can use a much simpler charger. Building one that  
switched between 'fast charge' and 'float' (or even ON and OFF) on  
voltage should be simple enough. I notice that COTS UPSes don't seem to  
be in a hurry to switch from SLA to to LiPO chemistry.


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martin@   | Martin Gregorie
gregorie. | Essex, UK
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Re: Pass through power bank
On 2/4/2015 1:41 PM, Martin Gregorie wrote:
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Funny, my need for a UPS for my pi is because when the Internet  
connection is flaky and needs to be reset, I cycle the UPS that powers  
my entire bench!  lol  I want the pi to not go down when I do that just  
like my laptop.  I might be able to do this with a super cap on the 5  
volt line...

--  

Rick

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