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A complicated question about LiFePO4 batteries, - and switching.



 
 
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  #21  
Old June 20th 18, 04:50 PM posted to rec.aviation.soaring
[email protected]
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Default A complicated question about LiFePO4 batteries, - and switching.

On Wednesday, June 20, 2018 at 9:27:33 AM UTC-4, OHM Ω http://aviation.derosaweb.net wrote:
Andy,

I have used the ideal diodes mentioned by Shaun McLaughlin (http://re-voltage.eu/ US$27 each) as I was given a pair to test with. They do what they say with minuscule voltage drops across them. Full disclosure, I am currently using two 1N5821 Schottky diodes in my panel right now (page 74 of my presentation) - small, simple, cheap, plentiful, robust, and easy to mount..

That being said - the availability of inexpensive ideal diodes has exploded in recent years mainly for use in systems like solar panel arrays. See https://www.ebay.com/sch/i.html?_fro...es%29&_sacat=0. I also like the increasing range of mounting styles available allowing for placement options behind our increasingly cramped panels.

One other comment: If you use diodes* (of any type) you can run two batteries in parallel safely. The danger was always been that without diodes, and with two batteries at different voltage output levels, that one battery could cross-charge the other. This could potentially happen without any current limit (!) until they equalize voltages. On a single bus with two batteries and diodes, the battery with the greatest voltage level would power the bus by itself until the separate battery's voltages equalized.

- John OHM Ω

* Teaching Moment - A diode only allows current to flow in one direction. In our case current will be limited to flowing from the battery to our avionics. As current flows through a diode there is a small voltage drop across the input to the output. We want to avoid, or limit, this voltage drop in our glider based power systems as we have no generating power source as found in airplanes.


I wonder about those "ideal diodes". They are presumably similar to what's in the batteries' BMS, I would guess power FETs that are switched on or off - and when "on" they conduct well with minimal (but not quite zero?) voltage drop. The "ideal diodes" must also have a sensing circuit that responds to the small voltage drop, amplify it and feed it to the gates of the FETs. (Seems like a Schottkey diode would be a good enough substitute.) The ones in the BMS can conduct in both directions (for charge and discharge), perhaps there are separate FETs for each direction.
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  #22  
Old June 20th 18, 05:23 PM posted to rec.aviation.soaring
jfitch
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Default A complicated question about LiFePO4 batteries, - and switching.

On Wednesday, June 20, 2018 at 7:56:31 AM UTC-7, OHM Ω http://aviation.derosaweb.net wrote:
QT,

All breakers suffer a voltage drop of some amount (see more below). Therefore some pilots like fuses as they have no voltage drop at all. I went away from fuses as they don't give me a positive indication of a trip, are difficult to fix in (soaring) flight, and are fragile.

Breaker voltage drop - As you probably know current is passed inside of breakers through a tiny "heater" (for want of a better phrase). The more current, the hotter the "heater" gets, until finally things get too hot and the heat trips a bi-metallic strip, and the current flow is interrupted. This "heater" can be thought of as a resistor. Small value breakers have a high value resistance and thus they drop a high voltage across them. For a 1A Tyco W31 breaker this is 0.6v! Ouch! A 5A breaker only drops 0.15v, 10A = 0.1v, 15A = 0.09v, etc. That is why I only recommend 5A breakers and above to be used. These are maximum voltage drop values so I am still holding with choosing breakers over fuses. See the specs at See https://www.alliedelec.com/m/d/6526f...72ec951f9f.pdf.

PS - I had stated before that ONLY switch/breakers like the Tyco W31 may be used as a switch. I said that pull-type breakers should NEVER be used as a switch. I just found out that the pull-type Tyco W23 breaker is rated to be used as a switch! Huzzah! But I digress.

- John OHM Ω


Glass fuses also suffer a voltage drop - how do you think they work? They are simply a resistive element that heats beyond the melting point above the label current. In fact a Bussman 1A AGX fuse has a resistance as much as 0..15 ohms, or a 1.5V drop. See http://www.cooperindustries.com/cont...AGX_Series.pdf

In addition, fuses are mounted in holders, and the ones used by many German glider manufacturers have at least 5 extra sliding connections in them: Faston to holder, holder to fuse, fuse to holder cap, holder cap to holder, Faston back to circuit - 5 more opportunities for voltage drop or failure. I've found them to age poorly and become quite unreliable. If you are going to breakers, I'd pick the Klixon 7277/7274 series, these are more compact and made for aircraft use.
  #23  
Old June 20th 18, 06:03 PM posted to rec.aviation.soaring
OHM Ω http://aviation.derosaweb.net
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Default A complicated question about LiFePO4 batteries, - and switching.

Joe - Damn. There goes my electrical merit badge. Mea Culpa. Revamping my presentation as we speak. The Bussman AGC spec sheet tells the tale and seems to ping to a 1A fuse having a typical drop of 0.31Vdc. http://www.cooperindustries.com/cont...AGC_Series.pdf

About Klixon 7277/7274 breakers. I read the specs and don't see it mentioned that it can be used as a switch like the Tyco. Only a mention of an option for an "auxiliary switch". Also, the maximum voltage drop for the 5A Klixon model is 0.25Vdc versus that of the Tyco which is 0.15Vdc. https://www.sensata.com/sites/defaul...ata sheet.pdf and https://www.alliedelec.com/m/d/6526f...72ec951f9f.pdf.

My $0.01 (de-rated).

- John
  #24  
Old June 20th 18, 06:12 PM posted to rec.aviation.soaring
Benedict Smith
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Default A complicated question about LiFePO4 batteries, - and switching.

At 14:56 20 June 2018,
T0hNIOKEpiBodHRwOi8vYXZpYXRpb24uZGVyb3Nhd2ViLm5ldA ==
wrote:
QT,

All breakers suffer a voltage drop of some amount (see more

below).
Theref=
ore some pilots like fuses as they have no voltage drop at all. I

went
awa=
y from fuses as they don't give me a positive indication of a trip,

are
dif=
ficult to fix in (soaring) flight, and are fragile.=20


- John OHM =E2=84=A6

John,
You can connect a 12v led indicator in parallel with the fuse, it will
light up if the fuse blows, regarding fixing them in flight, I would
want to know why it had blown before replacing it, not easy to
diagnose electrical faults and fly safely at the same time!
Ben.

  #25  
Old June 20th 18, 10:43 PM posted to rec.aviation.soaring
jfitch
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Posts: 907
Default A complicated question about LiFePO4 batteries, - and switching.

On Wednesday, June 20, 2018 at 10:03:22 AM UTC-7, OHM Ω http://aviation.derosaweb.net wrote:
Joe - Damn. There goes my electrical merit badge. Mea Culpa. Revamping my presentation as we speak. The Bussman AGC spec sheet tells the tale and seems to ping to a 1A fuse having a typical drop of 0.31Vdc. http://www.cooperindustries.com/cont...AGC_Series.pdf

About Klixon 7277/7274 breakers. I read the specs and don't see it mentioned that it can be used as a switch like the Tyco. Only a mention of an option for an "auxiliary switch". Also, the maximum voltage drop for the 5A Klixon model is 0.25Vdc versus that of the Tyco which is 0.15Vdc. https://www.sensata.com/sites/defaul...ata sheet.pdf and https://www.alliedelec.com/m/d/6526f...72ec951f9f.pdf.

My $0.01 (de-rated).

- John


Similar to Tyco, rated at 5000 cycles loaded, 10,000 unloaded. Also, the Tyco voltage drop is actually 0.195V, so they're pretty close. Tyco specs resistance as 0.03 ohms +/- 30% so max is 0.195, Klixon specs 0.25V drop max at max current. That Bussmann spec is typ, they don't spec max.
  #26  
Old June 21st 18, 02:05 AM posted to rec.aviation.soaring
John Godfrey (QT)[_2_]
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Default A complicated question about LiFePO4 batteries, - and switching.

On Monday, June 18, 2018 at 9:22:24 PM UTC-4, wrote:
Another question regarding the above battery type! There are variety of techniques used when switching between batteries - on/off/on switches, good switches with a cut out between the movement, two batteries with a diode to prevent flow between both batteries, etc.
The question I have concerns an older glider with limited battery space (Libelle), previously using SLA batteries and and a simple switch between the two. I will be upgrading with a new panel, likely LiFe batteries and want a back-up as well. While I understand the function of the diodes in the switching circuit, is there any reason to be concerned about their use with the LiPo battery and the internal Battery Management System? Imagine two LiFePO4 batteries that have equal charges, only one is discharging at a time until some threshold is reached and they switch over as determined by diodes outside the internal battery system. Is there a reason to question this type of circuit with the use of this new battery technology and the Battery Management System within?
While I love these new batteries, I don't want to bring on any new problem in the air, being quite aware that the wiring is only to contain smoke.

Thanks ahead, Mark


Are the angels on the pin all the same size?
  #27  
Old June 21st 18, 03:23 AM posted to rec.aviation.soaring
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Posts: 7
Default A complicated question about LiFePO4 batteries, - and switching.

Are the angels on the pin all the same size?

Scholars have spent considerable time debating this question, but likely not as much time as some of the arguments and discussions on the R.A.S. !

But seriously, Angels are pure intelligence and don't occupy space. And for a more thoughtful answer one would really need to know if it is the head of the pin you are referring, or the sharp point, it could make a difference.

John - my Libelle still has the original contest letters on it - QT

Mark Guay
  #28  
Old June 21st 18, 09:16 AM posted to rec.aviation.soaring
krasw
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Posts: 433
Default A complicated question about LiFePO4 batteries, - and switching.

On Wednesday, 20 June 2018 20:15:06 UTC+3, Benedict Smith wrote:
At 14:56 20 June 2018,
T0hNIOKEpiBodHRwOi8vYXZpYXRpb24uZGVyb3Nhd2ViLm5ldA ==
wrote:
QT,

All breakers suffer a voltage drop of some amount (see more

below).
Theref=
ore some pilots like fuses as they have no voltage drop at all. I

went
awa=
y from fuses as they don't give me a positive indication of a trip,

are
dif=
ficult to fix in (soaring) flight, and are fragile.=20


- John OHM =E2=84=A6

John,
You can connect a 12v led indicator in parallel with the fuse, it will
light up if the fuse blows, regarding fixing them in flight, I would
want to know why it had blown before replacing it, not easy to
diagnose electrical faults and fly safely at the same time!
Ben.


Fuse blows it there is a short in wiring. You would like to continue supplying current via LED to shorted wires, so you can see that fuse was blown?
  #29  
Old June 21st 18, 01:35 PM posted to rec.aviation.soaring
Charlie Quebec
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Posts: 88
Default A complicated question about LiFePO4 batteries, - and switching.

A 1K resistor in series will fix that.
  #30  
Old June 21st 18, 04:01 PM posted to rec.aviation.soaring
John Godfrey (QT)[_2_]
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Posts: 263
Default A complicated question about LiFePO4 batteries, - and switching.

On Wednesday, June 20, 2018 at 10:23:41 PM UTC-4, wrote:
Are the angels on the pin all the same size?

Scholars have spent considerable time debating this question, but likely not as much time as some of the arguments and discussions on the R.A.S. !

But seriously, Angels are pure intelligence and don't occupy space. And for a more thoughtful answer one would really need to know if it is the head of the pin you are referring, or the sharp point, it could make a difference.

John - my Libelle still has the original contest letters on it - QT

Mark Guay


I still think the core issue is whether the count is represented in Arabic or Roman, and in what base. The "head" of the pin is generally agreed to be the "not pointy" end, but I will defer to St. Thomas and the Talmudic scholars for final guidance. Angels can change state as proven by the Large Haydron Collider experiments.

I loved that Libelle, if you ever want to sell it please let me know.

QT
 




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