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 .

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.

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.

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.

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.

On Wednesday, June 20, 2018 at 6:27:33 AM UTC-7, 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 drew up a zero-loss battery switching circuit in a few minutes. It is based on 2 power p-channel mosfets. A dual opamp compares two battery inputs and selects the higher voltage by turning on its mosfet. Transient suppressors protect the circuit from voltage spikes. The total component count is 12 (16 if you want LED indicators). Send me a private email if you want a copy of the circuit.

Tom

On Wednesday, June 20, 2018 at 6:27:33 AM UTC-7, 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.


John (OHM),

Yup, I have one of Shaun's (on your recommendation) and some of the anti-revere-irrigation ones. I haven't installed either.

My current install uses the Schleicher rotary switch to select: Battery 1, Battery 2, Battery 1+2 (through diodes), Battery 3 (tail). I use a bank of capacitors connected through a power resistor to energize the circuit when the switch is between batteries.

If I had to do it again I would use ideal diodes (though as others have mentioned this is really not nearly a necessary with LiFePO4 batteries since they go through voltages below 11v quite quickly on my battery tester). I would also separate the batteries to individual switches to eliminate the switch as a single point of failure.

Andy Blackburn

9B

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

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?

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