About batteries

[Sewdhull]

I thought I would share what I've learned about batteries, lithium ones in particular.

Regardless of the type of lithium cells we are using there are a few parameters that make your battery better or worse.

 

 Voltage is just the cells voltage added together. This varies with increasing load, downwards.

The capacity (which should be taken with a pinch of salt) is what it says , how much charge the cells can give you once charged.

This varies with increasing load, downwards.

 

The C rating, something a manufacturer puts on, hopefully a sensible number reflecting the cells ability to supply current.

Remember it's current that makes your motor spin and voltage that decides how fast it will go.

You cells have an internal resistance that matters because this is predominantly why a good cell will give high currents with lower loses, and a cooler cell. You do not want your battery to be the limiting factor, it will not like it.

 

Your battery pack of however many cells should you short it will provide many hundreds of amps, even the smaller lipos, esp those with a high C. Put a fuse (size it for your max motor current plus 10A) in to protect your wiring by all means, but what you really want is the battery disconnecting asap. 

Your connectors will desolder in a few seconds and you don't want that incase the wires reconnect and then you have something incendiary.

 

The reason that Voltage reduces with increasing load is the cells own internal resistance, it's a combination of chemistry and other resistance in the cell.

 

As you draw more current the output voltage drops because of the IR and heat is produced. These losses are not linear. Eg at 10A the loss may be 1W but at 20A it would be 4W. At 40A it's 16W.

A poor battery will have more losses, get hot, puff up, catch fire .

A reason to use high torque motors you can get more power out of them or give the battery an easier life.

 

It's the same for the wires and the MOSFET. Oh and your connectors too,  altho your wiring and connectors should be fine if you've moved on from tamiya.

 

The last thing I'll say about batteries for the time being is Thier mechanical robustness. 

The pouch cells, non round ones can be easily punctured (the pouch containing the chemistry is thin plastic, wrapped in more thin plastic then covered in the brands thin plastic)  which makes them useless once penetrated.

The round ones have good mechanical robustness but usually scary chemistry.

Look after the pouch cells, don't throw them around like people do.

Your round ones, usually lion, are in a metal can so can take more abuse.

 

Get a good charger, the cloned B8 I think it is works well and has cell IR measurements as a health guide.

 

[Leo Greer]

Adding on to your post, you can calculate the actual discharge amperage by multiplying the "C" rating by the Ah listed. So a 1200 Mah (1.2 Ah) and 25C would have a rating of 30 Ah. Most stock RIFs need 15-25 Ah continuous to run properly without straining (and eventually puffing/ruining) the battery.

 

On another note, most manufacturers lie through their teeth about C and Mah rating. Over on AirsoftSociety, the main American forum, we had a very electrically savvy member run actual tests on both, and the results are impressively bad for most packs tested throughout the thread:

 

https://www.airsoftsociety.com/threads/objective-battery-test-titan-lion-vs-kypom-lipo-vs-hv-lipo-valken-others.161379/page-6

[Sewdhull]

Capacity drops alot then you draw different loads.

At 1A for example you may get the rated capacity but at 10A you will get less.

If C rating was some sort of standard it would be more useful.

C X the mAh gives you Amps not Ah. 

It's not a real thing tho as the C is made up.

 

Edited November 13 by Sewdhull
More thoughts

[Leo Greer]

I mean Amps when I write “Ah”.

[Pseudotectonic]

Ah = Amp-hour (capacity), but more commonly in mAh (milliamp-hour) which is 0.001 Ah

 

A = Amp (current)

[Lozart]

16 hours ago, Sewdhull said:

If C rating was some sort of standard it would be more useful.

 

 

It is an industry standard term and describes the charge rate of a battery. If it's good enough for MIT, then it's good enough for airsoft.

 

Quote

· C- and E- rates – In describing batteries, discharge current is often expressed as a C-rate
in order to normalize against battery capacity, which is often very different between
batteries. A C-rate is a measure of the rate at which a battery is discharged relative to its
maximum capacity. A 1C rate means that the discharge current will discharge the entire
battery in 1 hour. For a battery with a capacity of 100 Amp-hrs, this equates to a discharge
current of 100 Amps. A 5C rate for this battery would be 500 Amps, and a C/2 rate would
be 50 Amps. Similarly, an E-rate describes the discharge power. A 1E rate is the discharge
power to discharge the entire battery in 1 hour.

 

(taken from the attached file)

summary_battery_specifications.pdf

[Rogerborg]

17 hours ago, Leo Greer said:

https://www.airsoftsociety.com/threads/objective-battery-test-titan-lion-vs-kypom-lipo-vs-hv-lipo-valken-others.161379/page-6

 

Thanks, that's very interesting.  I also saw - on Facebook of all places, which is why I can't find it again - testing of lipos sold for airsoft vs those sold for RC use, which found much higher internal resistances in the airsoft packs, even new out of the box.

 

I've formed a belief that we get the trash-tier cells that don't make the cut for RC use.

 

But I've learned to stop worrying about it since my toys do what I need them to do, up to cycling faster than the mag can feed from a modest 11.1V / "1400mAh" / "30C" battery[*].

 

The numbers may lie, but, meh, they do the job.

 

[*] Although worryingly I've just done an inventory and only found one of the pair of these that I know that I have. Now I have a little firebomb hiding away somewhere waiting to have something hard and heavy dropped on it.

[Colin Allen]

34 minutes ago, Lozart said:

 

It is an industry standard term and describes the charge rate of a battery. If it's good enough for MIT, then it's good enough for airsoft.

 

 

(taken from the attached file)

summary_battery_specifications.pdf 72.9 kB · 0 downloads

Sadly, purveyors of batteries for airsoft toys do not seem to have received the memo about the C rating being an industry standard term, which I suspect was Sewdhull’s point.

[Lozart]

50 minutes ago, Colin Allen said:

Sadly, purveyors of batteries for airsoft toys do not seem to have received the memo about the C rating being an industry standard term, which I suspect was Sewdhull’s point.

 

The issue is that while C ratings are very much a real thing, battery manufacturers lie about what they actually achieve from their battery. 

The attached actually explains C and its relationship to current and capacity better than the MIT thing.

What-is-a-battery-C-rating.pdf

[gavinkempsell]

1 hour ago, Rogerborg said:

[*] Although worryingly I've just done an inventory and only found one of the pair of these that I know that I have. Now I have a little firebomb hiding away somewhere waiting to have something hard and heavy dropped on it.

Liposafe...

[Lozart]

The idea that an airsoft AEG works with a constant current is complete rubbish too. If you're firing in full auto then you'll get a high inrush current that will settle to a lower running current. If you're spamming semi then you'll just be getting endless inrush current spikes.

[Rogerborg]

53 minutes ago, gavinkempsell said:

Liposafe...

 

That's where it should be, but isn't.  The game's afoot.

[Colin Allen]

4 hours ago, Lozart said:

 

The issue is that while C ratings are very much a real thing, battery manufacturers lie about what they actually achieve from their battery. 

The attached actually explains C and its relationship to current and capacity better than the MIT thing.

What-is-a-battery-C-rating.pdf 533.74 kB · 1 download

I suspect that it is more likely that the branders lie about the C ratings rather than the battery manufacturers, which is what I commented.

I understand C ratings rather too well.  Much of my current work is on alternative energy sources for transport; while I am more focussed on hydrogen, I have had to learn about battery technologies.

[Sewdhull]

C ratings for the packs we use are unreliable.

I'm sure it has a place for scientists and engineers etc.

When I make batteries it's useful info from a manufacturer, altho they quote current and IR too.

Mass produced batteries from unknown sources are hard to stomach C wise. 

The Nanotech batteries have been good tho, altho I've not bought any for a while.

Airsoft currents are quite low but often we have use small thin batteries which isn't ideal, because of where they have go.

Anyhoo few batteries will be rubbish, most will be fine.

 

[Dan Robinson]

16 hours ago, Lozart said:

battery manufacturers lie about what they actually achieve from their battery.

 

I've yet to buy  a powerbank that remotely resembles the badge capacity.  at least in terms of mah's.  I was actually pleasantly surprised that my older batteries are pretty close once you allow for errors and safety overheads.  I have a gameday this weekend so might do a check of my batteries for cparacity - does mean doing them individually though.

 

 

[Lozart]

6 minutes ago, Dan Robinson said:

 

I've yet to buy  a powerbank that remotely resembles the badge capacity.  at least in terms of mah's.  I was actually pleasantly surprised that my older batteries are pretty close once you allow for errors and safety overheads.  I have a gameday this weekend so might do a check of my batteries for cparacity - does mean doing them individually though.

 

 

 

 

It's probably like the good old 56mpg "urban cycle" figures from car manufacturers that are done with the engine out on a bench with a simulated load. Given a fixed load with a known current they possibly do come close (ish) to the numbers on the pack, but in actual use in an AEG? Not so much.

[Dan Robinson]

Tried hypermiling my Transit van once - fully loaded I got over 600 miles from it.  That was the most boring week I can remember - I'll stick to my regular 290 - 320 miles per tank thank you very much.

[Sewdhull]

The issue for capacity is what current you discharge at and what voltage you stop at. 

Ofcourse there is blatant mislabeling, but you will get more mAh at lower currents and cells will empty at different rates, the poorer cell dictating when the battery dies.

The actual numbers matter less than if the battery suits your needs.

 

[Lozart]

48 minutes ago, Sewdhull said:

but you will get more mAh at lower currents

 

 

That's not true.

 

The mAh capacity of the battery is not affected by the discharge rate. A 2000mAh battery is a 2000mAh battery regardless of if you drain it at 2mA or 2A, it will discharge for longer at a lower current but the overall capacity remains the same.

 

Cell discharge rate is affected by chemistry and quality of manufacture, that is true. LiPo batteries can discharge at much higher currents than Li-Ion for example, but will sag more under load. And yes, a cheap shit battery will always perform poorly compared to a high quality one (although given the spectrum of quality in airsoft batteries that is pretty much moot because they all suck balls to some degree).

 

The actual numbers DO matter if you're trying to make sure that the battery you want will meet your needs. For example if you have limited battery space and can only fit a physically small battery in then it makes sense to go for the highest quality you can afford so that you get the best performance from that physical size. If size is not a restriction then yes, go nuts - as long as the current capability of the battery meets or exceeds your demand then you'll be golden.

[Pseudotectonic]

6 hours ago, Lozart said:

 

That's not true.

 

Ackchually that is true, for example:

 

[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]

14 hours ago, Pseudotectonic said:

 

Ackchually that is true, for example:

 

 

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

[Colin Allen]

2 hours ago, Lozart said:

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]

 

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.

1 hour ago, Colin Allen said:

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.