About batteries

[Sewdhull]

17 hours ago, Dan Robinson said:

Is there more context to the chart?  I'm minded to think that with that kind of voltage  drop temperature is playing a part due to pulling too much from the battery. 

 

Happy to learn though.

The voltage drop you are seeing is a function of the internal resistance of the cell. Free flow of the charge is restricted by chemistry and essentially the surface area of and distance between the internal electrodes. A cell is a rolled up sandwich of the two electrodes and the electrolyte between them.

It is similar but not identical (because batteries are chemical) to the voltage dropping across wires and contacts and MOSFETs in the circuit under load.

 

Edited November 16, 2023 by Sewdhull
More thoughts

[Pseudotectonic]

He is right, if you discharge below ~3.0 V per cell you are just going to kill your LiPo, and since you drop down to ~3.0 V sooner with a higher C, you get less usable mAh out of the battery.

 

Extrapolating to 0 V is irrelevant, even if you can do that, which you can't, because if we are drawing our own lines, I could just extrapolate the higher C to drop to 0 sooner, which is probably more likely to be the case.

[Lozart]

On 17/11/2023 at 18:40, Pseudotectonic said:

He is right, if you discharge below ~3.0 V per cell you are just going to kill your LiPo, and since you drop down to ~3.0 V sooner with a higher C, you get less usable mAh out of the battery.

 

Extrapolating to 0 V is irrelevant, even if you can do that, which you can't, because if we are drawing our own lines, I could just extrapolate the higher C to drop to 0 sooner, which is probably more likely to be the case.

 

You're right, but it's what the battery manufacturers do when they say that a battery is a "nominal" capacity. For example the one in that graph is a nominal 2500mAh, which is at the point where charge would reach zero.

[Pseudotectonic]

The nominal capacity is usually measured with 0.2 C down to a lowest acceptable voltage like 2.75 V, it is never rated to discharge to 0 V because below the 2.75 V (or whatever they decide to be the cut-off voltage) it is irrelevant for all intent and purposes

 

In that graph for example, Samsung decides the end voltage to be 2.5 V and actually has two capacity rating for 0.2 C and 10 A (which is about 0.4 C), and they are rated on paper to be 2500 mAh and 2450 mAh respectively (and the actual measurements confirm roughly with these numbers)

 

https://www.powerstream.com/p/INR18650-25R-datasheet.pdf

 

But every manufacturer measures it differently, some measures with higher C, some has different cut-offs, but the paper capacity is always rated to the cut-off voltage and not to zero

[Sewdhull]

There is no charge below a certain voltage because of the chemistry.

 

You can see the voltage drops at the end because of this at all discharge rates. 

 

Lithium and most other chemistries exhibit this behaviour and even when some charge is left damage occurs to the cell even if it is left unused. 

 

It's not like a water bucket emptying to nothing that you can refill, more like a fire which burns to embers that you can restore. Let the fire burn too low and it goes out, even tho there's stuff in the fire still.

[Sewdhull]

Here is one of the best cells around.

 

It has a high discharge ability and low IR and therefore low losses. Probs 3% 

 

https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.molicel.com/wp-content/uploads/INR21700P42A-V4-80092.pdf&ved=2ahUKEwiiz-jrztKCAxVFT0EAHU7-C9IQFnoECBQQAQ&usg=AOvVaw0LQw5T1Ox5wcEGxkfy-Cls

 

 

Sadly too large for most Airsoft use where the width makes it hard to fit anywhere.

 

This cell with a higher IR has more loss, but again these are really good cells, same size as the Titans physically. Probs 4% loss here.

 

https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.molicel.com/wp-content/uploads/DM_INR18650A-V4-80078.pdf&ved=2ahUKEwjx7aSH0NKCAxVYgP0HHbKzBTwQFnoECBMQAQ&usg=AOvVaw17W4w_UcgiPWscdq5s9FWU

 

Once you get to mediocre cells the losses ramp up and the capacity difference becomes more pronounced.

 

The other thing to bear in mind is that the cell temperatures are high at the end of some of the discharge curves which will extend the curve, which you will not see in normal use.

 

Here's a mediocre cell, for us any way. 20% loss

 

https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.overlander.co.uk/pub/media/datasheets/2600mAh_Li-Ion_Cell_Specification.pdf&ved=2ahUKEwjygaLj1NKCAxVQQEEAHfs8ARQQFnoECDQQAQ&usg=AOvVaw3807Y43t7mbC3QqqU1wsCT

Edited November 20, 2023 by Sewdhull
Extra info

[Leo Greer]

It's why I typically use packs in the range of 5K Mah with plenty of discharge. I run a spare mag pouch on my stock, switch the stock around, and just use the same battery for everything. Obviously that's overkill, but it's nice to not care about your battery for multiple game days on end.

[Dan Robinson]

1 hour ago, Leo Greer said:

It's why I typically use packs in the range of 5K Mah with plenty of discharge. 

 

 

Fuck me, are you jump starting your truck from your pew as well? 😄

Edited November 21, 2023 by Dan Robinson

[Lozart]

8 hours ago, Dan Robinson said:

 

 

Fuck me, are you jump starting your truck from your pew as well? 😄

 

I've got a couple of Titans that are 7000mAh.

[Sewdhull]

Titan have very misleading information about thier batteries and are expensive.

Thier C ratings are all over the place.

 

They claim 48A or 16C from 2 cell packs and also parallel cells to make high capacity packs and still only claim 16C. 

The very best 18650s will give you 35A.

 

Nice to have that sort of capacity to draw on tho.

[Lozart]

2 hours ago, Sewdhull said:

Titan have very misleading information about thier batteries and are expensive.

Thier C ratings are all over the place.

 

They claim 48A or 16C from 2 cell packs and also parallel cells to make high capacity packs and still only claim 16C. 

The very best 18650s will give you 35A.

 

Nice to have that sort of capacity to draw on tho.

 

I've gone back to standard LiPo. Any performance benefit they claim is slim at best, the chemistry of the cells doesn't lend itself to repeated short bursts either. The robustness is nice and the lack of self discharge is good but not worth the money they charge.

[Sewdhull]

I think if they used better cells you'd see better results.

High capacity doesn't automatically transfer to good current capabilities 

[Rogerborg]

10 hours ago, Lozart said:

I've got a couple of Titans that are 7000mAh.

 

What's that in fractions of an M67 hand grenade?  🤔

 

7.4V = 18%

11.1V = 27%

if my numbers are correct

[Lozart]

14 hours ago, Sewdhull said:

I think if they used better cells you'd see better results.

High capacity doesn't automatically transfer to good current capabilities 

 

Undoubtedly, though I do question their marketing spiel that a low C, high capacity battery is equivalent to a smaller high C battery in this application.

[Sewdhull]

Motors don't work in C, it's dimensionless, they work in Amps and that would be a much better way.

Like quoting TPA for example instead of RPM. And quote motor torque not high low etc. All this sort of thing muddies the water if you want to compare things.

Most of Titans blurb is meaningless, some is just wrong. Even the bits that are in fact true are also true comparing lipos. 

[Lozart]

52 minutes ago, Sewdhull said:

Motors don't work in C, it's dimensionless, they work in Amps and that would be a much better way.

Like quoting TPA for example instead of RPM. And quote motor torque not high low etc. All this sort of thing muddies the water if you want to compare things.

Most of Titans blurb is meaningless, some is just wrong. Even the bits that are in fact true are also true comparing lipos. 

 

Unfortunately most airsofters wouldn't know a Newton-metre if it stole all their apples.

 

While motors don't work in C they do respond differently when the supply current is governing their performance; if a battery can't supply enough current quickly enough then performance will suffer in an AEG because that initial spin up is where the motor will spend a lot of its time. It's less relevant in things like drones or RC planes because once you get the rotor spinning and you've overcome that moment of inertia then the motor can settle into a fairly consistent running current. Once you reach that "steady" state then the capacity calculations etc make more sense because your load behaves more like the graphs that battery manufacturers like to throw around. Even when you run an AEG in constant full auto, the load is never stable. Every cycle the gears have to compress the spring and then it releases so the load comes off the motor. The current draw is never smooth and the battery's ability to deliver a lot of current quickly becomes more important than the mAh rating as long as it is sufficiently high. 

[Sewdhull]

Drone currents vary by a magnitude compared to airsoft.

 

We either have semi or auto. Semi draws more power for a shot, requiring more current as the motor starts from 0 RPM.

Auto is a varying load for sure but cells don't mind that if it falls within their specs.

 

For sure mAh rating is less useful than the current the battery can supply, and you'd need to do power calculations to estimate battery life. Way easier to get a pack and see how long it lives ( For me it only needs to last half a day, I can change it at lunch.

 

For me, I use the largest batteries (physically) that will fit in my gun, current being more important than capacity. I have a couple of watt meters so i can see what my  load is and therefore what type of cells I can get away with.

My preference is Lithium iron Phosphate (Life), then Cylindrical cells, then Lipos. When I flew drones weight was an issue so they are all Lipo. I have some enormous packs, now in ammo boxes.

 

I'm leaning towards 12.8v Life packs if they will fit, because they are so forgiving in use, last a really long time and supply good current, the voltage not being as high as the 12.8v 4 cells would give you because as we have seen in tis thread, voltage drops quickly and stays at a level for a good while before the battery is exhausted. However, often there isn't enough room for such luxuries.

 

[Lozart]

13 hours ago, Sewdhull said:

 

 

I'm leaning towards 12.8v Life packs if they will fit, because they are so forgiving in use, last a really long time and supply good current, the voltage not being as high as the 12.8v 4 cells would give you because as we have seen in tis thread, voltage drops quickly and stays at a level for a good while before the battery is exhausted. However, often there isn't enough room for such luxuries.

 

 

 

Where are you finding decent LiFe stick packs? They were all the rage for airsoft a few years back but most places only seem to have one or two choices these days (or are you repurposing RC packs?)

[Dan Robinson]

On 22/11/2023 at 11:56, Lozart said:

. Every cycle the gears have to compress the spring and then it releases so the load comes off the motor

 

Would be fun to see that on a sillyscope

 

RE - Moment of inertia and steady state - one of my drones has nearly 12" blades and the thing weighs a few KGs.  I'd guess there is a fair impact with a rapid speed and direction change.  You should see the pitch change when it tries an emergency stop from 40ish miles an hour.  I had a very squeeky bum when it was avoiding a massive oak tree once after chasing a dirt bike across a field LOL.

[Sewdhull]

Even those little drones draw some mighty amps at fully throttle as they whizz about.

You'll see 70C or even 100C packs being used. There's some tests online you can Google.

3 hours ago, Lozart said:

 

 

Where are you finding decent LiFe stick packs? They were all the rage for airsoft a few years back but most places only seem to have one or two choices these days (or are you repurposing RC packs?)

I use xt connectors, because they are better so I don't need to repurpose anything, Nuprol do some life packs, I have one and it's fine.

 

https://www.tactical-clothing.co.uk/product/nuprol-np-power-1100mah-99v-20c-li-fe-slim-stick-type-battery-216629?gclid=CjwKCAiAjfyqBhAsEiwA-UdzJDECrsOk5S3L-TZEarDw3YC1QjQkIYJuQ9zcAi3Kpuk0A0wBlpJ8LBoCUkEQAvD_BwE

[Sewdhull]

Vapex and Nuprol packs look the same, because the voltage is lower motors won't pull as much current from a 3S life Vs a 3s lipo or lion.

However I haven't used the life's much, as I just have the one for testing.

I'll measure some of my batteries and let you all know what they are like.

[Sewdhull]

18 hours ago, Dan Robinson said:

 

Would be fun to see that on a sillyscope

 

RE - Moment of inertia and steady state - one of my drones has nearly 12" blades and the thing weighs a few KGs.  I'd guess there is a fair impact with a rapid speed and direction change.  You should see the pitch change when it tries an emergency stop from 40ish miles an hour.  I had a very squeeky bum when it was avoiding a massive oak tree once after chasing a dirt bike across a field LOL.

Your wish etc

 

http://www.airsoftlab.eu/docs/experiments/motor_current/

[Lozart]

19 hours ago, Sewdhull said:

Even those little drones draw some mighty amps at fully throttle as they whizz about.

You'll see 70C or even 100C packs being used. There's some tests online you can Google.

I use xt connectors, because they are better so I don't need to repurpose anything, Nuprol do some life packs, I have one and it's fine.

 

https://www.tactical-clothing.co.uk/product/nuprol-np-power-1100mah-99v-20c-li-fe-slim-stick-type-battery-216629?gclid=CjwKCAiAjfyqBhAsEiwA-UdzJDECrsOk5S3L-TZEarDw3YC1QjQkIYJuQ9zcAi3Kpuk0A0wBlpJ8LBoCUkEQAvD_BwE

 

 

I have no doubt that they draw high currents, I was more suggesting that the load is less peaky.

[Sewdhull]

Yeah I understand, I wasn't clear.

 

It's not as frequent as auto fire for example, but the loads will go from zero to 100% as the drone races around. Batteries only last 3 or 4 minutes.

 

Our Airsoft batteries are pretty poor by comparison.

[Sewdhull]

Here have a look at this battery.

 

https://www.quadcopters.co.uk/lipo-batterys/3s-lipo-batteries-111v/tattu-1550mah-111v-3s-75c-lipo-battery-2114

 

I'm unsure why our Airsoft batteries are so poor.

I realise it's out of stock.

Edited November 24, 2023 by Sewdhull