Pass through power bank

[Martin Gregorie]

On Wed, 04 Feb 2015 13:05:21 -0500, rickman wrote:

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.

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.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

 

[Martin Gregorie]

On Wed, 04 Feb 2015 19:53:17 -0500, rickman wrote:

Yeah, I know you prolly think I'm a heather, but I've used this sort of
rig for long-term NiCd and NiMH maintenance over the last couple of
decades and never had a problem with any of them.

What's a heather?

For 'heather' read 'heathen'.

--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

 

[Martin Gregorie]

On Thu, 05 Feb 2015 09:25:31 +0000, Tony van der Hoff wrote:

On 05/02/15 02:33, Mel Wilson wrote:

Wild-assed guess, for laughs, follower of Heath-Robinson, the British
Rube Goldberg?


Heathen?

Yep. Uncaught typo. Though maybe I should have said 'electronic heathen'.

--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

 

[Andrew Gabriel]

In article <mau2aj$ljj$1@dont-email.me>,
Martin Gregorie <martin@address-in-sig.invalid> writes:

For that sort of job, I'd probably use 4 AA size NiMH or hybrid cells on
the end of a LM358 plus transistor constant current source set to feed
them 1% of capacity + RPi current draw and, if I was feeling keen, a 7805
or equivalent to drive the RPi. The point being that NiMH last forever if
trickle charged at 1% of the rated capacity, i.e. 10mA into a 1000mAh
NiMH.

Yeah, I know you prolly think I'm a heather, but I've used this sort of
rig for long-term NiCd and NiMH maintenance over the last couple of
decades and never had a problem with any of them.

Agree, or even a sealed lead acid battery with a DC-DC converter to 5v
(easily available in the form of cigarette light socket mobile chargers).

Lithium batteries have special properties making them ideal for portable
use, but that doesn't apply here. They are much more complex to charge,
and absolutely mustn't be continuously trickle or float charged. That
makes them unsuitable for use in a UPS of the type discussed here,
unless there was more complex circuitry designed in to the UPS to take
them out of the circuit once charged. It seems the products people have
identified are not designed this way, which is probably why the
manufacturers don't support simulteneous charge and use.

The easiest way to build one would be a 12V SLA battery and charger
designed to float at 13.8V, and a DC-DC converter to supply 5V for the
Pi. (These are available at 2.1A nowadays, and even 2 x 2.1A.)
You could add some circuitry to turn off the load if voltage drops to
10V (to protect the SLA battery), and if you expect the battery to be
run down to this level, use a deep discharge battery, which is capable
of deep draining without seriously shortening its life. Battery capacity
will typically decline linearly to nothing over about 5 years in float
charging at 13.8V - use that to decide how often to replace the battery.
Sizing the battery and charge current capacity depends on the load and
the min hold-up time you need after a power outage. Another factor is
how long you are prepared to wait for the battery to fully recharge
after power restoration before you have enough energy stored to
satisfy your min hold-up requirement again - you have an at-risk time
when the UPS will not yet be able to meet your hold-up requirement.

With batteries which need constant current charging (or controlled
current charging) such as NiCd/NiHM/LiPo, you need a different
approach.

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

 

[Martin Gregorie]

On Thu, 05 Feb 2015 13:57:11 +0100, A. Dumas wrote:

On 05/02/2015 01:53, rickman wrote:
On 2/4/2015 4:18 PM, Martin Gregorie wrote:
Yeah, I know you prolly think I'm a heather, but I've used this sort
of rig for long-term NiCd and NiMH maintenance over the last couple of
decades and never had a problem with any of them.

What's a heather?

http://www.urbandictionary.com/define.php?term=heather , obviously.

Nope, never seen *that* definition before.

--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

 

[Tony van der Hoff]

On 05/02/15 02:33, Mel Wilson wrote:

On Wed, 04 Feb 2015 19:53:17 -0500, rickman wrote:

On 2/4/2015 4:18 PM, Martin Gregorie wrote:

Yeah, I know you prolly think I'm a heather, but I've used this sort of
rig for long-term NiCd and NiMH maintenance over the last couple of
decades and never had a problem with any of them.

What's a heather?

Wild-assed guess, for laughs, follower of Heath-Robinson, the British
Rube Goldberg?

Heathen?

 

[Martin Gregorie]

On Thu, 05 Feb 2015 13:54:26 +0000, Andrew Gabriel wrote:

Agree, or even a sealed lead acid battery with a DC-DC converter to 5v
(easily available in the form of cigarette light socket mobile
chargers).

I thought about suggesting that, but its fairly hard to find 6 or 12v
SLAs of under, say, 2.5AH and anything bigger is likely to be overkill
for running an RPi, Also, looking through the Sunpower catalogue, I
didn't find any multistage chargers for 6v SLAs. However, if you use,
say, a 7.2Ah 12v SLA I agree its dead easy: connect a 2-stage charger to
one side and use a cigarette socket 2v->5v converter to provide the 5v.

Again, I've done this for some years. I use a pair of 12v 7.2AH SLAs to
run the instruments in a glider and have a pair of Sunpower 2-stage
chargers to keep them topped up at home. Most glider instruments want
12v, but the PNA I use for navigation wants 5v via a mini-USB socket, so
I use a cannibalised cigarette socket 12v->5v converter to power it. Yes,
the PNA has an internal battery, but that's only good for 2-2.5 hours and
a good cross country flight is 4 hours plus. 'Cannibalised' because you
get more reliable connections by removing the converter's plastic case,
installing the PCB in a small Maplins metal box and soldering the 12v
inputs to it. Stuffing the unmodified converter into a Maplins/Farnell/RS
car lighter socket is a recipe for bad connections: all those lighter
sockets are just rubbish for anything except heating a lighter. For home
use, mounting the converter in a plastic box would probably be OK, but
these (tiny, dirt cheap) converters are switch-mode devices that put out
a lot of RF hash: not good to have anywhere near an airband radio set.

The easiest way to build one would be a 12V SLA battery and charger
designed to float at 13.8V, and a DC-DC converter to supply 5V for the
Pi. (These are available at 2.1A nowadays, and even 2 x 2.1A.)

Mine is a Compaq iPAQ in-car charger that I bought off eBay in 2004 and
is rated at 1.4 amps.

You could add some circuitry to turn off the load if voltage drops to
10V (to protect the SLA battery).

Nice touch - a job for one of the 8 pin PICAXEs?

Battery capacity
will typically decline linearly to nothing over about 5 years in float
charging at 13.8V - use that to decide how often to replace the battery.

They deteriorate anyway. My 7.2AH glider batteries are always put on
charge after a day's flying and never left on charge for more than 12
hours. My panel draws less than 1A at worst case (measured with
everything on and the radio on receive at 400mA). I cycle the batteries
once a year to check capacity and notice that even good quality Yuasa
batteries only hold full capacity for the first three years and then
decline quite steeply (>20% a year) after that. I've seen 500 cycles
quoted but don't think its that much in real life.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

 

[A. Dumas]

On 05/02/2015 01:53, rickman wrote:

On 2/4/2015 4:18 PM, Martin Gregorie wrote:
Yeah, I know you prolly think I'm a heather, but I've used this sort of
rig for long-term NiCd and NiMH maintenance over the last couple of
decades and never had a problem with any of them.

What's a heather?

http://www.urbandictionary.com/define.php?term=heather , obviously.

 

[Johny B Good]

On Thu, 05 Feb 2015 09:25:31 +0000, Tony van der Hoff
<tony@vanderhoff.org> wrote:

On 05/02/15 02:33, Mel Wilson wrote:
On Wed, 04 Feb 2015 19:53:17 -0500, rickman wrote:

On 2/4/2015 4:18 PM, Martin Gregorie wrote:

Yeah, I know you prolly think I'm a heather, but I've used this sort of
rig for long-term NiCd and NiMH maintenance over the last couple of
decades and never had a problem with any of them.

What's a heather?

Wild-assed guess, for laughs, follower of Heath-Robinson, the British
Rube Goldberg?


Heathen?

That was my thought too but it's one hell of a typo.
--
J B Good

 

[Johny B Good]

On Thu, 05 Feb 2015 13:57:11 +0100, "A. Dumas"
<alexandre@dumas.fr.invalid> wrote:

On 05/02/2015 01:53, rickman wrote:
On 2/4/2015 4:18 PM, Martin Gregorie wrote:
Yeah, I know you prolly think I'm a heather, but I've used this sort of
rig for long-term NiCd and NiMH maintenance over the last couple of
decades and never had a problem with any of them.

What's a heather?

http://www.urbandictionary.com/define.php?term=heather , obviously...

....confirming that he meant "heathen".
--
J B Good

 

[rickman]

On 2/5/2015 10:09 AM, Martin Gregorie wrote:

On Thu, 05 Feb 2015 13:54:26 +0000, Andrew Gabriel wrote:

Battery capacity
will typically decline linearly to nothing over about 5 years in float
charging at 13.8V - use that to decide how often to replace the battery.

They deteriorate anyway. My 7.2AH glider batteries are always put on
charge after a day's flying and never left on charge for more than 12
hours. My panel draws less than 1A at worst case (measured with
everything on and the radio on receive at 400mA). I cycle the batteries
once a year to check capacity and notice that even good quality Yuasa
batteries only hold full capacity for the first three years and then
decline quite steeply (>20% a year) after that. I've seen 500 cycles
quoted but don't think its that much in real life.

I'm surprised that SLA batteries die so quickly. I guess they are
rather different from the lead acid battery used in automotive apps
where they can last much longer. I worked for the railroad once and saw
batteries that were 20 years old still working fine. They were backup
for crossing signals and were on trickle charge 99.9% of the time.

--

Rick

 

[Martin Gregorie]

On Thu, 05 Feb 2015 22:36:50 -0500, rickman wrote:

I don't know what the reason for using SLA batteries in your glider is,
but wouldn't Li-ion be a lot lighter for the same capacity?

The weight difference isn't significant: a factor of two at the most. My
glider is a Standard Libelle. It is a single seat type, 15m span, weighs
206kg ready to fly less pilot. More details and pics here in case you're
interested:
http://www.gregorie.org/gliding/libelle/index.html

I was picturing a drone type aircraft.

Understood. We, the gliding community, have always been wary of Li-ion
batteries since a lot of us are or have been model flyers and are well
aware of their reputation for catching fire while being charged and in
crashes. The Boeing 787 fun and games hasn't helped their rep. either.
They were using SAFT Li-ion batteries.

I don't need the extra capacity these chemistries offer since even one of
my pair of 7.2Ah SLAs can run my panel for 14 hours plus. Besides, a new
7.2AH SLA currently sells for GBP 16.80 as against GBP 86.56 for a
Lithium iron phosphate drop-in replacement. This price does include the
charger but its still only a 7AH battery. The alternative US sourced K2
12 LFP 7, another 12v 7AH SLA drop-in replacement, is $US 140 without the
charger, so that looks like a lot more money for very little benefit.

Admittedly the LiFePO4 chemistry is *much* safer than the SAFT cells that
Boeing was using but I still don't need something behind my seat that can
spew toxic smoke after a bad accident: in a glider the pilot is the nose
weight that puts the balance point, known as the CG (centre of gravity)
where it needs to be for safe flight: in this case about 30-35 cm behind
the seat back. Heavy, removable items such as batteries tend to be fitted
as close to the CG as possible. Yes, we do need to know what we weigh and
the acceptable weight is always written on a placard in the cockpit - and
checked by weighing and recalculating it every 7 years.

-----
The only glider type that does get serious benefit from lithium battery
chemistry is something like the Antares 20E, an electric self-launching
motor glider. It is a single seat 20m span beast with wings full of
batteries. It carries enough power to take off and climb 10,000 ft or to
motor 150-200km if the weather turns bad. Its an excellent, very fast and
efficient glider too. One has flown 1500km in Scottish wave. The pilot
had declared a 750 km triangle for his day's flight, but got round it so
fast he decided it would be rude to stop so early and went round a second
time before heading home.

http://www.lange-flugzeugbau.de/htm/english/products/antares_20e/
antares_20E.html


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

 

[Gordon Levi]

Gordon Levi <gordon@address.invalid> wrote:

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?

Here's the one I chose <http://www.mi.com/en/mipowerbank10400/>. They
don't actually specify pass through charging but some of the rave
reviews do
<http://gadgetadda.com/2014/11/24/xiaomi-10400mah-mi-power-bank-review-bang-for-the-buck/>.
I'm in Australia and chose this seller on eBay
<http://www.ebay.com.au/itm/XIAOMI-10400mAh-Portable-Power-Bank-Li-ion-Battery-Charger-5V-2-1A-iPad-iPhone-/261478644375#ret>.
I was a little worried before it arrived that it might be a fake
<http://en.miui.com/thread-19104-1-1.html>. I'm fairly sure it is not
but I haven't taken it apart to check. Looking on the bright side, if
they are worth faking they must be good!

The instructions are only in Chinese but the LEDS are informative and
easy to interpret. The "Power button" is more mysterious. It seems to
be a momentary switch that will reset the power bank circuitry and
turn off the output power only while it is pressed. Looking on the
bright side again, that makes it an ideal reset switch for the Pi.

I have followed with interest the discussion on the possible
variations of pass through charging but have not yet observed how mine
behaves. I'll post again if I find out.

 

[rickman]

On 2/6/2015 9:37 AM, Martin Gregorie wrote:

On Thu, 05 Feb 2015 22:36:50 -0500, rickman wrote:

I don't know what the reason for using SLA batteries in your glider is,
but wouldn't Li-ion be a lot lighter for the same capacity?

The weight difference isn't significant: a factor of two at the most. My
glider is a Standard Libelle. It is a single seat type, 15m span, weighs
206kg ready to fly less pilot. More details and pics here in case you're
interested:
http://www.gregorie.org/gliding/libelle/index.html

I guess the batteries are a small part of the weight of the aircraft so
not so important. I worked on a radio design for a drone (powered, not
a glider) which was probably about the same size as your glider but with
a smaller wingspan. The radio unit (an aluminum box) had a stud for a
ground cable. In the design review they spend some 20 minutes
discussing (arguing) over whether the stud should be shortened a half
inch to save weight. lol

So I'm thinking weight is important in any aircraft and especially so in
a glider.

I was picturing a drone type aircraft.

Understood. We, the gliding community, have always been wary of Li-ion
batteries since a lot of us are or have been model flyers and are well
aware of their reputation for catching fire while being charged and in
crashes. The Boeing 787 fun and games hasn't helped their rep. either.
They were using SAFT Li-ion batteries.

But I understand the concern for safety. I believe that many Li
batteries are not allowed on commercial passenger flights as cargo.
Sort of like the oxygen generators on that flight that crashed some
years back because of the fire.


I have thought of getting into flying, but I have many, many other
things in the way at the moment. I am very into kayaking, but need a
hip replacement before I make any plans.

--

Rick

 

[Martin Gregorie]

On Sat, 07 Feb 2015 14:06:17 -0500, Dennis Lee Bieber wrote:

It also has the minor problem that, at cruise altitude, the
difference
between stalling from going too slow, and ripping the wings off from
going supersonic, was around 20 knots. The pilot had to concentrate on
airspeed control.

Ah, the so-called coffin corner, where stalling speed and the IAS at that
altitude (actually the TAS (True Air Speed) coincide. Get fractionally
fast and the you're beyond airframe limits: fractionally slow and stall/
spin is the likely result. I believe U2 pilots know it well.

That is what happens when the required cruising altitude is "as high as
she'll go".


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

 

[Martin Gregorie]

On Sat, 07 Feb 2015 22:09:10 -0500, rickman wrote:

I'm not so sure of that. I bought my 17 foot boat from a 70+ guy who
had cancer. He was still kayaking until he got sick. Plus it is such
great exercise. You may *need* to be fit to fly, but you will *become*
fit by kayaking. It fixed my lower back trouble by strengthening my
torso (core muscles). In fact, kayaking is a great social activity with
most participants in their later years. Flat water kayaking is a bit
tame for the 20 year old crowd I guess.

Yeah, I assumed you meant white water or ocean kayacking. I'd forgotten
how popular flat-water kayacking is in the US and Canada and know very
little about it.

Good point about the social aspect. I think those who don't take part in
what are inherently group sports don't understand that side of the game.

Its the main difference between gliding and GA flying: since you need at
least three people to launch a glider (tug pilot/winch driver, wing
runner and glider pilot) almost by definition its a group game, unlike
GA, where you can wander onto the field, do the walk round, climb in,
start up and fly.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

 

[Christian Brunschen]

In article <mb7jn0$8lb$3@dont-email.me>,
Martin Gregorie <martin@address-in-sig.invalid> wrote:

On Sat, 07 Feb 2015 22:09:10 -0500, rickman wrote:

I'm not so sure of that. I bought my 17 foot boat from a 70+ guy who
had cancer. He was still kayaking until he got sick. Plus it is such
great exercise. You may *need* to be fit to fly, but you will *become*
fit by kayaking. It fixed my lower back trouble by strengthening my
torso (core muscles). In fact, kayaking is a great social activity with
most participants in their later years. Flat water kayaking is a bit
tame for the 20 year old crowd I guess.

Yeah, I assumed you meant white water or ocean kayacking. I'd forgotten
how popular flat-water kayacking is in the US and Canada and know very
little about it.

Good point about the social aspect. I think those who don't take part in
what are inherently group sports don't understand that side of the game.

Its the main difference between gliding and GA flying: since you need at
least three people to launch a glider (tug pilot/winch driver, wing
runner and glider pilot) almost by definition its a group game, unlike
GA, where you can wander onto the field, do the walk round, climb in,
start up and fly.

Being at the launch point and helping is part of the fun! (At least it
was for me when I was actively gliding.)

And there's also a lot of waiting around the launchpoint for suitable
weather - perhaps depending on where you are in the world!

// Christian

 

[Martin Gregorie]

On Sun, 08 Feb 2015 15:57:21 -0500, rickman wrote:

On 2/8/2015 7:10 AM, Martin Gregorie wrote:
On Sat, 07 Feb 2015 22:09:10 -0500, rickman wrote:

I'm not so sure of that. I bought my 17 foot boat from a 70+ guy who
had cancer. He was still kayaking until he got sick. Plus it is such
great exercise. You may *need* to be fit to fly, but you will
*become* fit by kayaking. It fixed my lower back trouble by
strengthening my torso (core muscles). In fact, kayaking is a great
social activity with most participants in their later years. Flat
water kayaking is a bit tame for the 20 year old crowd I guess.

Yeah, I assumed you meant white water or ocean kayacking. I'd forgotten
how popular flat-water kayacking is in the US and Canada and know very
little about it.

Good point about the social aspect. I think those who don't take part
in what are inherently group sports don't understand that side of the
game.

Its the main difference between gliding and GA flying: since you need
at least three people to launch a glider (tug pilot/winch driver, wing
runner and glider pilot) almost by definition its a group game, unlike
GA, where you can wander onto the field, do the walk round, climb in,
start up and fly.

GA?

General Aviation: see https://en.wikipedia.org/wiki/General_aviation
for a pretty good definition.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

 

[Martin Gregorie]

On Sun, 08 Feb 2015 12:26:39 +0000, Christian Brunschen wrote:

In article <mb7jn0$8lb$3@dont-email.me>,
Martin Gregorie <martin@address-in-sig.invalid> wrote:
On Sat, 07 Feb 2015 22:09:10 -0500, rickman wrote:

I'm not so sure of that. I bought my 17 foot boat from a 70+ guy who
had cancer. He was still kayaking until he got sick. Plus it is such
great exercise. You may *need* to be fit to fly, but you will
*become* fit by kayaking. It fixed my lower back trouble by
strengthening my torso (core muscles). In fact, kayaking is a great
social activity with most participants in their later years. Flat
water kayaking is a bit tame for the 20 year old crowd I guess.

Yeah, I assumed you meant white water or ocean kayacking. I'd forgotten
how popular flat-water kayacking is in the US and Canada and know very
little about it.

Good point about the social aspect. I think those who don't take part in
what are inherently group sports don't understand that side of the game.

Its the main difference between gliding and GA flying: since you need at
least three people to launch a glider (tug pilot/winch driver, wing
runner and glider pilot) almost by definition its a group game, unlike
GA, where you can wander onto the field, do the walk round, climb in,
start up and fly.

Being at the launch point and helping is part of the fun! (At least it
was for me when I was actively gliding.)

Quite. Most clubs have a pretty comfortable club-house. The social aspect
includes the point that it will tend to collect a group of like-minded
individuals which means that its a nice place to hang out. Many (all?)
pilots tend to have a fairly black sense of humour, so its nice to know
that the associated jokes and viewpoints aren't likely to be appreciated.

For instance, I like Sir Charles Kingston-Smiths comment that "The only
time you can have too much petrol in board is when the plane is on fire".

Look him up if you don't recognise the name.

And there's also a lot of waiting around the launchpoint for suitable
weather - perhaps depending on where you are in the world!

Also true, but forecasting is getting better: take a look at Blipspot /
RASP (respectively US/American terms for the same software or XCWeather.
There's also a growing network of ground-based FLARM receivers in the UK
and Europe, so you can see if your friends are flying and where they're
going. All this stuff is available on the 'Net and many of the tracking
sites the use radar feeds are now taking FLARM feeds as well.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

 

[Graham.]

On Wed, 04 Feb 2015 19:53:17 -0500, rickman <gnuarm@gmail.com> wrote:

On 2/4/2015 4:18 PM, Martin Gregorie wrote:
On Wed, 04 Feb 2015 13:49:30 -0500, rickman wrote:

On 2/4/2015 1:41 PM, Martin Gregorie wrote:
On Wed, 04 Feb 2015 13:05:21 -0500, rickman wrote:

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.

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.

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...

For that sort of job, I'd probably use 4 AA size NiMH or hybrid cells on
the end of a LM358 plus transistor constant current source set to feed
them 1% of capacity + RPi current draw and, if I was feeling keen, a 7805
or equivalent to drive the RPi. The point being that NiMH last forever if
trickle charged at 1% of the rated capacity, i.e. 10mA into a 1000mAh
NiMH.

Yeah, I know you prolly think I'm a heather, but I've used this sort of
rig for long-term NiCd and NiMH maintenance over the last couple of
decades and never had a problem with any of them.

What's a heather?

https://www.youtube.com/watch?v=JnUOPsjC2qk#t=84

--

Graham.

%Profound_observation%