8 ms·

Here's a fun bit: in the article they say that lithium-sulfur is hard to measure charge level for due to the voltage properties of charging and discharging. "T

Here's a fun bit: in the article they say that lithium-sulfur is hard to measure charge level for due to the voltage properties of charging and discharging. "The upshot is that voltage is not a good proxy for the state of charge and, to make things even more complicated, the voltage curve is asymmetrical for charge and for discharge." Since it would be bad if your battery suddenly died and you dropped out of the sky, they had to develop complex statistical and neural network algorithms to accurately determine state of charge to within a few percent. One black box for staying in the sky and another in case you end up on the ground!

mrfusion 6y ago
Can you just measure the current coming out of the battery and keep track of it?
- hn_acc_2 6y ago
  You could use this technique to estimate the remaining capacity starting from a full charge / known charge, but not to arbitrarily measure the remaining capacity of the battery
  - dougmwne 6y ago
    It sounds like the voltage curve is all over the place as the battery phases through its chain of different chemical reactions, unlike a normal battery. I take that to mean there's no way to just measure the voltage at a given point of time to estimate capacity, hence why the statistical method was required.
    - rbanffy 6y ago
      It's a good thing to measure the instant capacity, but the voltage and current (and, hopefully, thermal output) are being measured during the whole flight.
- hwillis 6y ago
  AKA joule counting- for normal li-ion it gets you to ~5% most of the time, up to 20% off at the start and end. That's assuming the voltage stays constant the entire time- in reality the first bit is at 4.2+ volts, and the last bit is down to ~3 volts. That's a 40% energy difference per electron that leaves the battery. It's also one of the reasons you get electronics that die suddenly at 5%- current gas gauges usually account for it, but older stuff wasn't always good at knowing when the voltage would drop off. Nowadays (and always, for the most part) the sudden shutoff is because electronics often pull very brief power spikes that drop the battery voltage below the minimum voltage temporarily. The chemistry takes a moment to recover after that. The problem with Li-S batteries isn't just that they have a goofy curve- that can be charted and saved, even as the battery degrades (Note- I'm mostly up on conventional chemistry. Don't know much about Li-S). It's more have a couple phases they go through during discharge. Impedance and other properties of the battery change, which changes the discharge characteristics of the battery, which changes the voltage. Proportionally, the swing in voltage is also larger (although this kind of thing is always changing, so I may be out of date). There's also a small amount of self-discharge and parasitic reactions that will consume electrons, but that number is necessarily fairly small and predictable. The main thing is that 50% of the energy variance is in the voltage, and you need to know a lot about the current chemistry inside the battery (as well as the future load profile) to be able to predict the voltage that all the remaining electrons will have as they leave the battery.
- ChuckMcM 6y ago
  Many battery monitoring circuits do this, sampling both voltage and current to compute power over time. With a suitable inductor to limit the rate of current change to be within the nyquist sampling interval of the monitor, you can pretty accurately measure charge going in or coming out of a battery. Combined with a model for the battery and you've got a modern battery monitor circuit.
- SigmundA 6y ago
  I added a shunt based AH meter to my RV that does this, seems pretty standard tech, not sure what the big deal is: https://www.magnum-dimensions.com/battery-monitor-kit https://www.magnum-dimensions.com/battery-monitor-kit
mulmen 6y ago
This makes me wonder how accurate float bulb based fuel level sensors really are. This sounds like a major problem and the solutions are interesting but potentially even better than what we are used to. My 1990 Toyota never read full. The buffer on the fuel sender was so extreme that by the time the needle made it to the "F" I had already burned 1/8 of a tank! My current car warns me when I have about 50 miles of fuel left, I wonder how much historical data it uses in that calculation. None of my motorcycles even have fuel gauges. I just keep an eye on the odometer and when I stop to stretch my legs I give the tank a shake or a peek.
- TheRealSteel 6y ago
  > float based fuel level sensors I initially read this comment thinking you meant floating-point based...
  - mulmen 6y ago
    Hah, didn't even think of that! I made a small edit to clarify.
  - cogman10 6y ago
    Dang floats ruin all calculations!
- nippoo 6y ago
  They're pretty inaccurate in light aircraft where you're often flying at an angle / slightly asymmetrically; in commercial airliners, under most flight conditions, they're generally within a couple percent. Aircraft will generally use fuel flow sensors and use those to calculate remaining fuel (by integrating the fuel flow over time) and float/capacitance sensors in tanks are used as verification / a sanity check (which can sometimes only end up being noticed inflight, eg https://www.flightglobal.com/safety/boeing-modifying-777-fuel-indicator-after-in-flight-discrepancy-incidents/138706.article https://www.flightglobal.com/safety/boeing-modifying-777-fue...) For more than you'd ever want to know about the fuel systems in a modern airliner, see http://www.b737.org.uk/fuel.htm http://www.b737.org.uk/fuel.htm
- 6y ago
cameldrv 6y ago
Or you could do what they did on Apollo (forget if it was the CM or LM). They had the problem of measuring how much was in a tank, but the tank was in 0G, so a float is no good. The proposed solution was some sophisticated radiation based thing where they measured the attenuation of some radioactive source through the tank. This wound up being highly complex, and the solution was simply to have a reserve tank. When the main tank ran out, you knew you had exactly the amount in the reserve tank.
- rbanffy 6y ago
  Another possible solution is to measure how much fuel (or energy) got in and how much is getting out. You know the nominal capacity and you know the flow rate.
  - cogman10 6y ago
    You'd have to account for charge losses (Some energy ends up just being heat while charging) but that could probably be simply guestimated on battery temp while charging and some constant factor. For example, assume a 90% charge efficiency for a battery at 20C... or whatever makes sense.
    - deleted 6y ago
      [deleted]
  - Cerium 6y ago
    That technique is called Coulomb counting and is the standard way of giving a precise battery charge percentage for lithium based batteries. The battery capacity estimate is improved over time based on the previous observed amounts of energy to enter or leave the battery.
- pokoleo 6y ago
  They could've put a variable-sized bladder in the tank, and measured the internal pressure. Right?
- brianpan 6y ago
manfredo 6y ago
State of charge tracking is tricky even for lithium ion batteries. I worked on it for a bit as part of a solar car competition. Usually it involves a mix of coulomb counting and voltage measurements. IIRC, a handful of laptops and consumer electronics have trained neural networks to assist in estimating SOC. As far as aircraft goes, it's sufficient to put a lower bound on the remaining charge. Realistically, the poor (abysmal, really) energy density of batteries pretty much precludes their usage in any serious aircraft. For sustainable air flight I'm more optimistic about syngas or hydrogen.
- m463 6y ago
  I've always wondered about those one-use aluminum-air batteries one guy was developing. (search for tevor jackson aluminum air battery) They might be more palatable for aircraft.
  - manfredo 6y ago
    Batteries are fundamentally unsuitable for aircraft. The energy density is easily an order of magnitude lower than thermochemical fuels: https://en.m.wikipedia.org/wiki/Energy_density#/media/File%3AEnergy_density.svg https://en.m.wikipedia.org/wiki/Energy_density#/media/File%3... Furthermore batteries don't get lighter as their charge is expended, as opposed to fuel that gets burned off. This means a plane powered by batteries has to land with the same mass of fuel as it does when it takes off. Many aircraft are worse at decelerating themselves than accelerating. This means they can actually take off with more mass than they can land, thus extending their range. This can't happen for battery powered planes. Hydrogen powered planes is less far fetched than it seems. The Soviets built a prototype: https://en.m.wikipedia.org/wiki/Tupolev_Tu-155 https://en.m.wikipedia.org/wiki/Tupolev_Tu-155 Cryogenic storage of hydrogen is less difficult than it seems. Because the engines are constantly drawing fuel from the fuel tank, the tank's pressure is constantly dropping which cools it down. Most of the difficulty around hydrogen storage is in long term storage where hydrogen permitting through the vessel is a concern. The main challenge for applying it to aircraft is being able to make a vessel light enough and in a form factor suitable for planes, namely fitting it in the wing.
BurningFrog 6y ago
No clue how technically feasible this is, but conceptually I like it: Have 10 small batteries, that you go through one by one, instead of the single big one. You only need to start worrying on the 10th one.
- taneq 6y ago
  It could work, and it'd certainly give you a good 'ground truth', but there's a couple of reasons currently why it's probably not the best option. Batteries tend to wear out faster when they're deeply discharged, and when they're discharged rapidly. This approach would rapidly, deeply discharge some of your batteries instead of slightly discharging all of them. You could work around it with a sort of 'wear leveling' type battery management system but you'll still be reducing the life of the batteries compared with using the cells all at once.
  - hvidgaard 6y ago
    If you have the ability to tell when you get somewhere in the region of 10-30% charge, you could always switch there and not deep discharge unless you really have to.
    - edaemon 6y ago
      The discharge rate could still damage the batteries.
  - _Understated_ 6y ago
    So it's kinda like RAID 4 then? In RAID 4 you had a dedicated parity disk. What if you allocated one of your 10 batteries as the "parity". It would not be used until the others had ran out of charge (as originally expressed above) And to prevent uneven wear levels, each time you plug in to charge, the parity battery changes to one of the other 10 randomly (or on a pattern but the end result is the same). So over time, assuming your prng was decent, you would have an even wear level. You'd probably want maybe two batteries as your parity in case of failure but it would still work. I'm on my first coffee and it hasn't properly kicked in but it sounds plausible in my head :)
monadic2 6y ago
Just pointing out that the engine dying does not imply you'll free fall from the sky.
- dehrmann 6y ago
  I went on a ride in a jet trainer. The instructions I got were "We only bail if there's an engine fire. If the engine goes out an I can't restart it, we glide to one of these airports; this plane is an excellent glider." On bailing out "it came with ejection seats, but the government doesn't like us having explosives, so we use parachutes."
  - imtringued 6y ago
    Ejection seats are bad for your body.
deleted 6y ago
[deleted]

mrfusion 6y ago

Can you just measure the current coming out of the battery and keep track of it?

hn_acc_2 6y ago
You could use this technique to estimate the remaining capacity starting from a full charge / known charge, but not to arbitrarily measure the remaining capacity of the battery
- dougmwne 6y ago
  It sounds like the voltage curve is all over the place as the battery phases through its chain of different chemical reactions, unlike a normal battery. I take that to mean there's no way to just measure the voltage at a given point of time to estimate capacity, hence why the statistical method was required.
  - rbanffy 6y ago
    It's a good thing to measure the instant capacity, but the voltage and current (and, hopefully, thermal output) are being measured during the whole flight.
hwillis 6y ago
AKA joule counting- for normal li-ion it gets you to ~5% most of the time, up to 20% off at the start and end. That's assuming the voltage stays constant the entire time- in reality the first bit is at 4.2+ volts, and the last bit is down to ~3 volts. That's a 40% energy difference per electron that leaves the battery. It's also one of the reasons you get electronics that die suddenly at 5%- current gas gauges usually account for it, but older stuff wasn't always good at knowing when the voltage would drop off. Nowadays (and always, for the most part) the sudden shutoff is because electronics often pull very brief power spikes that drop the battery voltage below the minimum voltage temporarily. The chemistry takes a moment to recover after that. The problem with Li-S batteries isn't just that they have a goofy curve- that can be charted and saved, even as the battery degrades (Note- I'm mostly up on conventional chemistry. Don't know much about Li-S). It's more have a couple phases they go through during discharge. Impedance and other properties of the battery change, which changes the discharge characteristics of the battery, which changes the voltage. Proportionally, the swing in voltage is also larger (although this kind of thing is always changing, so I may be out of date). There's also a small amount of self-discharge and parasitic reactions that will consume electrons, but that number is necessarily fairly small and predictable. The main thing is that 50% of the energy variance is in the voltage, and you need to know a lot about the current chemistry inside the battery (as well as the future load profile) to be able to predict the voltage that all the remaining electrons will have as they leave the battery.
ChuckMcM 6y ago
Many battery monitoring circuits do this, sampling both voltage and current to compute power over time. With a suitable inductor to limit the rate of current change to be within the nyquist sampling interval of the monitor, you can pretty accurately measure charge going in or coming out of a battery. Combined with a model for the battery and you've got a modern battery monitor circuit.
SigmundA 6y ago
I added a shunt based AH meter to my RV that does this, seems pretty standard tech, not sure what the big deal is: https://www.magnum-dimensions.com/battery-monitor-kit https://www.magnum-dimensions.com/battery-monitor-kit

mulmen 6y ago

This makes me wonder how accurate float bulb based fuel level sensors really are. This sounds like a major problem and the solutions are interesting but potentially even better than what we are used to. My 1990 Toyota never read full. The buffer on the fuel sender was so extreme that by the time the needle made it to the "F" I had already burned 1/8 of a tank! My current car warns me when I have about 50 miles of fuel left, I wonder how much historical data it uses in that calculation. None of my motorcycles even have fuel gauges. I just keep an eye on the odometer and when I stop to stretch my legs I give the tank a shake or a peek.

TheRealSteel 6y ago
> float based fuel level sensors I initially read this comment thinking you meant floating-point based...
- mulmen 6y ago
  Hah, didn't even think of that! I made a small edit to clarify.
- cogman10 6y ago
  Dang floats ruin all calculations!
nippoo 6y ago
They're pretty inaccurate in light aircraft where you're often flying at an angle / slightly asymmetrically; in commercial airliners, under most flight conditions, they're generally within a couple percent. Aircraft will generally use fuel flow sensors and use those to calculate remaining fuel (by integrating the fuel flow over time) and float/capacitance sensors in tanks are used as verification / a sanity check (which can sometimes only end up being noticed inflight, eg https://www.flightglobal.com/safety/boeing-modifying-777-fuel-indicator-after-in-flight-discrepancy-incidents/138706.article https://www.flightglobal.com/safety/boeing-modifying-777-fue...) For more than you'd ever want to know about the fuel systems in a modern airliner, see http://www.b737.org.uk/fuel.htm http://www.b737.org.uk/fuel.htm
6y ago

cameldrv 6y ago

Or you could do what they did on Apollo (forget if it was the CM or LM). They had the problem of measuring how much was in a tank, but the tank was in 0G, so a float is no good. The proposed solution was some sophisticated radiation based thing where they measured the attenuation of some radioactive source through the tank. This wound up being highly complex, and the solution was simply to have a reserve tank. When the main tank ran out, you knew you had exactly the amount in the reserve tank.

rbanffy 6y ago
Another possible solution is to measure how much fuel (or energy) got in and how much is getting out. You know the nominal capacity and you know the flow rate.
- cogman10 6y ago
  You'd have to account for charge losses (Some energy ends up just being heat while charging) but that could probably be simply guestimated on battery temp while charging and some constant factor. For example, assume a 90% charge efficiency for a battery at 20C... or whatever makes sense.
  - deleted 6y ago
    [deleted]
- Cerium 6y ago
  That technique is called Coulomb counting and is the standard way of giving a precise battery charge percentage for lithium based batteries. The battery capacity estimate is improved over time based on the previous observed amounts of energy to enter or leave the battery.
pokoleo 6y ago
They could've put a variable-sized bladder in the tank, and measured the internal pressure. Right?
brianpan 6y ago

manfredo 6y ago

State of charge tracking is tricky even for lithium ion batteries. I worked on it for a bit as part of a solar car competition. Usually it involves a mix of coulomb counting and voltage measurements. IIRC, a handful of laptops and consumer electronics have trained neural networks to assist in estimating SOC. As far as aircraft goes, it's sufficient to put a lower bound on the remaining charge. Realistically, the poor (abysmal, really) energy density of batteries pretty much precludes their usage in any serious aircraft. For sustainable air flight I'm more optimistic about syngas or hydrogen.

m463 6y ago
I've always wondered about those one-use aluminum-air batteries one guy was developing. (search for tevor jackson aluminum air battery) They might be more palatable for aircraft.
- manfredo 6y ago
  Batteries are fundamentally unsuitable for aircraft. The energy density is easily an order of magnitude lower than thermochemical fuels: https://en.m.wikipedia.org/wiki/Energy_density#/media/File%3AEnergy_density.svg https://en.m.wikipedia.org/wiki/Energy_density#/media/File%3... Furthermore batteries don't get lighter as their charge is expended, as opposed to fuel that gets burned off. This means a plane powered by batteries has to land with the same mass of fuel as it does when it takes off. Many aircraft are worse at decelerating themselves than accelerating. This means they can actually take off with more mass than they can land, thus extending their range. This can't happen for battery powered planes. Hydrogen powered planes is less far fetched than it seems. The Soviets built a prototype: https://en.m.wikipedia.org/wiki/Tupolev_Tu-155 https://en.m.wikipedia.org/wiki/Tupolev_Tu-155 Cryogenic storage of hydrogen is less difficult than it seems. Because the engines are constantly drawing fuel from the fuel tank, the tank's pressure is constantly dropping which cools it down. Most of the difficulty around hydrogen storage is in long term storage where hydrogen permitting through the vessel is a concern. The main challenge for applying it to aircraft is being able to make a vessel light enough and in a form factor suitable for planes, namely fitting it in the wing.

BurningFrog 6y ago

No clue how technically feasible this is, but conceptually I like it: Have 10 small batteries, that you go through one by one, instead of the single big one. You only need to start worrying on the 10th one.

taneq 6y ago
It could work, and it'd certainly give you a good 'ground truth', but there's a couple of reasons currently why it's probably not the best option. Batteries tend to wear out faster when they're deeply discharged, and when they're discharged rapidly. This approach would rapidly, deeply discharge some of your batteries instead of slightly discharging all of them. You could work around it with a sort of 'wear leveling' type battery management system but you'll still be reducing the life of the batteries compared with using the cells all at once.
- hvidgaard 6y ago
  If you have the ability to tell when you get somewhere in the region of 10-30% charge, you could always switch there and not deep discharge unless you really have to.
  - edaemon 6y ago
    The discharge rate could still damage the batteries.
- _Understated_ 6y ago
  So it's kinda like RAID 4 then? In RAID 4 you had a dedicated parity disk. What if you allocated one of your 10 batteries as the "parity". It would not be used until the others had ran out of charge (as originally expressed above) And to prevent uneven wear levels, each time you plug in to charge, the parity battery changes to one of the other 10 randomly (or on a pattern but the end result is the same). So over time, assuming your prng was decent, you would have an even wear level. You'd probably want maybe two batteries as your parity in case of failure but it would still work. I'm on my first coffee and it hasn't properly kicked in but it sounds plausible in my head :)

monadic2 6y ago

Just pointing out that the engine dying does not imply you'll free fall from the sky.

dehrmann 6y ago
I went on a ride in a jet trainer. The instructions I got were "We only bail if there's an engine fire. If the engine goes out an I can't restart it, we glide to one of these airports; this plane is an excellent glider." On bailing out "it came with ejection seats, but the government doesn't like us having explosives, so we use parachutes."
- imtringued 6y ago
  Ejection seats are bad for your body.

deleted 6y ago

[deleted]