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A complicated question about LiFePO4 batteries, - and switching.



 
 
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  #11  
Old June 19th 18, 10:28 PM posted to rec.aviation.soaring
jfitch
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Posts: 908
Default A complicated question about LiFePO4 batteries, - and switching.

On Tuesday, June 19, 2018 at 9:30:32 AM UTC-7, Andy Blackburn wrote:
John,

Have you tried any of these?

https://www.ebay.com/itm/15A-Solar-P...sAAOSw-0RbApaz

They claim almost zero voltage drop compared to Schottkey diodes. Of course with the rapid voltage fallow of LiFePO4 batteries, it may only make a tiny difference in effective capacity.

Andy Blackburn
9B


On Tuesday, June 19, 2018 at 6:23:07 AM UTC-7, OHM Ω http://aviation.derosaweb.net wrote:

- But I do have Schottkey diodes in each bus. Why, you might ask, do I have diodes if I have two separate electrical buses? Because I can bridge the two buses with another switch for that "just in case situation" (belts and braces engineering over design). I have never have to use this switch but then again you never know ...

Thanks, John OHM Ω


Being pedantic for a moment, the diode drop is proportional to the endurance reduction, and has little to do with the battery voltage characteristic. If you drop 0.7V on a 13V battery, you're going to be drawing 5% more power and have 5% less endurance. 0.3 across a Schottky is starting to look pretty small and across those virtual diodes not much at all.

The explanation is that almost all modern electronics are using switching power supplies to produce the required amount of power at 5 or 3.3V. Less input voltage to the power supply just means it draws more current to get the same output. On that basis alone a 13.5V LFP will last 10% longer than a 12.5V SLA, at the same AH.

If it weren't windy, I'd be flying instead of pedantic .
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  #12  
Old June 20th 18, 01:47 AM posted to rec.aviation.soaring
[email protected]
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Posts: 60
Default A complicated question about LiFePO4 batteries, - and switching.


If you drop 0.7V on a 13V battery, you're going to be drawing 5% more power and have 5% less endurance.


I have slightly different numbers. I use low drop (0.4 v) diodes and per battery switches with K2's. Operating rules are when bus gets below 11 volts, it's time to turn on another battery. (about 4-5 hours with glide computers and transponder, and not too talky on the vhf.)

Most of the electronics have switching power supplies, so as the volts go down they adapt and the amps go up. The K2's have a fairly flat discharge curve up until they fall off the cliff. All the diodes do is slightly adjust where on the cliff edge we stop and go to another battery.

So at the worst case switchover point the batteries are at 11.4 and I'm loosing 3.5% in the diodes. Costing perhaps a few minutes out of 4 hours. Seems simple and cheap to prevent bus glitches and prevent circulating currents between the batteries.

The old plan was pairs of SLA's as friends. They got bought each year at the same time, charged in parallel, and discharged in parallel. This also worked, but the K2's seem to last for years and provide more run time.
  #13  
Old June 20th 18, 01:55 AM posted to rec.aviation.soaring
jfitch
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Posts: 908
Default A complicated question about LiFePO4 batteries, - and switching.

On Tuesday, June 19, 2018 at 5:47:17 PM UTC-7, wrote:
If you drop 0.7V on a 13V battery, you're going to be drawing 5% more power and have 5% less endurance.


I have slightly different numbers. I use low drop (0.4 v) diodes and per battery switches with K2's. Operating rules are when bus gets below 11 volts, it's time to turn on another battery. (about 4-5 hours with glide computers and transponder, and not too talky on the vhf.)

Most of the electronics have switching power supplies, so as the volts go down they adapt and the amps go up. The K2's have a fairly flat discharge curve up until they fall off the cliff. All the diodes do is slightly adjust where on the cliff edge we stop and go to another battery.

So at the worst case switchover point the batteries are at 11.4 and I'm loosing 3.5% in the diodes. Costing perhaps a few minutes out of 4 hours. Seems simple and cheap to prevent bus glitches and prevent circulating currents between the batteries.

The old plan was pairs of SLA's as friends. They got bought each year at the same time, charged in parallel, and discharged in parallel. This also worked, but the K2's seem to last for years and provide more run time.


But you are also losing and additional 3% in the higher current draw, 6.5% total, 15 minutes out of the 4 hours. Those of you running two batteries should consider simply running them in parallel. Just make sure you charge them in parallel too. This is actually better for battery life (particularly true for SLAs) because the average depth of discharge is less. Might want to check with the battery manufacturer and see of the BMS will like that - some do, some don't.
  #14  
Old June 20th 18, 02:15 AM posted to rec.aviation.soaring
[email protected]
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Default A complicated question about LiFePO4 batteries, - and switching.

https://www.digikey.com/product-deta...5PBF-ND/812018

Even with a 0.4V drop forward, you're still ahead of SLA on voltage to the electronics and no switching required in flight.
  #15  
Old June 20th 18, 03:49 AM posted to rec.aviation.soaring
[email protected]
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Posts: 3
Default A complicated question about LiFePO4 batteries, - and switching.

On Tuesday, June 19, 2018 at 5:55:05 PM UTC-7, jfitch wrote:
On Tuesday, June 19, 2018 at 5:47:17 PM UTC-7, wrote:
If you drop 0.7V on a 13V battery, you're going to be drawing 5% more power and have 5% less endurance.


I have slightly different numbers. I use low drop (0.4 v) diodes and per battery switches with K2's. Operating rules are when bus gets below 11 volts, it's time to turn on another battery. (about 4-5 hours with glide computers and transponder, and not too talky on the vhf.)

Most of the electronics have switching power supplies, so as the volts go down they adapt and the amps go up. The K2's have a fairly flat discharge curve up until they fall off the cliff. All the diodes do is slightly adjust where on the cliff edge we stop and go to another battery.

So at the worst case switchover point the batteries are at 11.4 and I'm loosing 3.5% in the diodes. Costing perhaps a few minutes out of 4 hours. Seems simple and cheap to prevent bus glitches and prevent circulating currents between the batteries.

The old plan was pairs of SLA's as friends. They got bought each year at the same time, charged in parallel, and discharged in parallel. This also worked, but the K2's seem to last for years and provide more run time.


But you are also losing and additional 3% in the higher current draw, 6.5% total, 15 minutes out of the 4 hours. Those of you running two batteries should consider simply running them in parallel. Just make sure you charge them in parallel too. This is actually better for battery life (particularly true for SLAs) because the average depth of discharge is less. Might want to check with the battery manufacturer and see of the BMS will like that - some do, some don't.


I've used two K2 LifePo and one K2 charger. They are used and charged in parallel as per K2 advice. No problems in several years of use.
  #16  
Old June 20th 18, 12:21 PM posted to rec.aviation.soaring
Shaun McLaughlin[_2_]
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Posts: 8
Default A complicated question about LiFePO4 batteries, - and switching.

I have re-wired 2 glider panels in the last year or so and used John's
presentation as the starting point.

Fuse on each battery - a battery master switch for each battery (ON/OFF)
- IDEAL DIODES - +12V power bus - TYCO breaker for each device - DEVICE
- GND - DIODE - BATTERIES

I can recommend the Ideal Diodes and Tyco breakers (although they are not
cheap). I liked the idea of reducing the number of connections (separate
fuse and switch) by having a combined fuse/switch. Since first installing
the Ideal Diodes I know they have been used on a number of other gliders at
my club and everyone seems happy so far.

http://re-voltage.eu/

I installed the Diodes behind the panel rather than on each battery as I
didn't want to damage them while handling the batteries. However they can
be used to connect both batteries to the same charger safely.

This setup means I only need to switch a battery off if there is an issue.
It allows me to combine battery types (SLA/LiFePO4) if the need arises.

I went down this path of having both batteries always on and sharing the
load after having issues switching batteries in flight. If I wasn't quick
enough I would find my FLARM rebooting, then having to wait to download the
last 'trace' onto the SD card. I found this quite frustrating while running
ridges in the Alps! My stop gap fix to this was to add a capacitor and a
rotary make before break switch to select the batteries, but this was still
less than ideal and lacked redundancy.

Shaun

At 13:23 19 June 2018, T0hNIOKEpiBodHRwOi8vYXZpYXRpb24uZGVyb3Nhd2ViLm5ldA ==
wrote:
A beginning comment: With modern avionics drawing less power than their
pre=
decessors and LiFePo4 with greatly increased energy capacity, maybe only
on=
e battery is needed these days for most glider pilots. Just sayin'.=20

I agree that our batteries should not be run in parallel (without

diodes).
=
If one is of a significantly different voltage than the other, then the
hi=
gher one will charge the lower one possible at a high rate (current) that
y=
ou don't want. This can potentially blow one or both battery fuses (you
do=
have a fuse on each battery, right???), or might cause one battery to
fail=
in a bad way, either of which can leave you in a poor situation
especially=
during flight.

I have a presentation on my thoughts on the subject of two battery

systems
=
at http://aviation.derosaweb.net/presentations#wiring. =20

Here is my setup on my current glider.

- Two batteries (lithium)
- Each battery has a fuse.
- Two separate power buses with one battery powering each.
- One bus is for "communications" (radio, transponder, FLARM) and the
other=
bus is for "navigation" (GPS, Vario, etc.). =20
- Each bus has a separate breaker (5A) which is also a master switch (a
tru=
e breaker/switch by Tyco
http://www.aircraftspruce.com/catalog/elpages/pbci=
rcuitbrkr2.php?clickkey=3D5950).=20
- But I do have Schottkey diodes in each bus. Why, you might ask, do I
hav=
e diodes if I have two separate electrical buses? Because I can bridge
the=
two buses with another switch for that "just in case situation" (belts
and=
braces engineering over design). I have never have to use this switch

but
=
then again you never know ...

Thanks, John OHM =E2=84=A6


  #17  
Old June 20th 18, 02:27 PM posted to rec.aviation.soaring
OHM Ω http://aviation.derosaweb.net
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Posts: 131
Default A complicated question about LiFePO4 batteries, - and switching.

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.
  #18  
Old June 20th 18, 02:37 PM posted to rec.aviation.soaring
John Godfrey (QT)[_2_]
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Posts: 265
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 like the ideal diodes, dislike the 0.3v drop across the Tyco breakers, run my batteries to one bus through low loss Schottky diodes.

I also believe that the number of angels on the head of the pin should be represented in Roman, not Arabic numbers in base 8.
  #19  
Old June 20th 18, 03:56 PM posted to rec.aviation.soaring
OHM Ω http://aviation.derosaweb.net
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Posts: 131
Default A complicated question about LiFePO4 batteries, - and switching.

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
  #20  
Old June 20th 18, 04:06 PM posted to rec.aviation.soaring
Charlie M. (UH & 002 owner/pilot)
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Posts: 1,034
Default A complicated question about LiFePO4 batteries, - and switching.

To add to a good post, part of the reason to NOT use most breakers as a switch is you derate (drop the ampacity) of the breaker over time due to wear. Add in a hot ambient (low save, sitting on the ramp on a sunny day, battery voltage drop, etc.) and the breaker can trip early/lower amperage.

Digression, but related.
Industrial environment, the company added a building with lots of lights. I suggested breaking up the light rows and putting them on lighting contactors but that added quite a bit of money.
The old plant manager stated, "We'll just use the breakers!".
A few years later (breakers switched twice a day, 5-6 days a week) we lost lights in the middle of summer as the building got hot.
Great money saver.......:-(
 




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