About batteries

[Dan Robinson]

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.

 

[Pseudotectonic]

That was just a generic LiPo discharge curve from https://www.dnkpower.com/lithium-polymer-battery-guide/

You will get less overall mAh regardless of temperature with bigger current, as well as a bigger voltage sag the entire way

So in any case, bigger capacity battery is always bigger because it represents a smaller C at any given current and will have less voltage sag, which is really what can affect the feel of the motor

 

[Lozart]

Ackchually that is true, for example:

If you extrapolate the trace down to zero, they all converge.

 

[Colin Allen]

If you extrapolate the trace down to zero, they all converge.

Indeed they do; the mAh rating is a measure of capacity, which does not change based on the discharge rate.

 

[Sewdhull]

View attachment 119217

You cant extrapolate to zero, only to the useful voltage for your application. At just 4C you've lost 10% of your capacity.

In Airsoft 3v is probably the lower useful voltage as your motor will slow down.

Running a power bank for example, the electronics will produce the required voltage as long as the battery can supply current, well to a point.

Running a lithium battery to low voltages will damage it long term too.

Indeed they do; the mAh rating is a measure of capacity, which does not change based on the discharge rate.

There are losses in discharging a battery, those losses increase with current based on the IR of the battery.

Capacity is quoted at specific discharge rates because of this.

Perhaps not in Airsoft however, but at a cell level you need this information to make a pack for whatever your application is.

 

[Sewdhull]

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.

 

[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]

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

 

[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]

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? ?

 

[Lozart]

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]

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]

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]

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.