Practical Experience of Voltage Vs C Rating in Lipo Batteries?

[rotor92000]

Disclaimer: The 'C' values stated in this post are simply hypothetical in an attemp to explain the thinking behind the question.

 

If you increase your voltage from 7.4v to 11.1v, your ROF increases.  If you change your battery's C rating from say 15C to 30C, your ROF could also increase by some degree.

 

So, here's the question(s)...

 

What's the difference in performance between, let's say 7.4v 30C and an 11.1v 15C?  Is one easier on the motor than the other?  You could assume that if, hypothetically, the ROF were the same, the load on the motor must also be the same and therefore, technically no different?

 

Anyone got some actual pratical experince on this one?

 

Cheers all!

[EDcase]

The C rating is the batteries maximum current output so if a motor needs high current to drive it then a battery with low C rating would struggle.  If used in full auto for extended periods it would get hot and may burn in the worst case.  Using a battery with low C that gets warm will have a shorter lifespan.

 

Note that this is more of a concern with RC motors as they can get quite large and power hungry.  Most batteries can cope with AEG motors quite happily.

If your battery feels warm after using full auto then it may be struggling to deliver the power needed by the motor so get a higher C battery or a more efficient Neodymium motor.

 

 

 

Edited May 17, 2021 by EDcase

[Arwen]

My G&G TR16 (stock gearbox) double feeds a lot when using the top battery in this photo (the nuprol battery)

Under advice from Sam at KoA I started using the bottom battery, with a higher C rating. Cured the double feeding completely with no noticable additional wear to the motor. 

 

Unfortunately I cannot remember if it was the burst or the continuous C number that Sam said was important 😅

Edited May 17, 2021 by Arwen

[rotor92000]

Hmm Once I found out how to calculate the discarge from the battery, ohm's law led me to this...

 

For the 7.4v 30C battery, we can say for a 1000mAh capacity (for ease of calculation) the battery is capable of discharging 30amps.  The power to the motor would be 222 watts (watts as a unit doesn't really mean much to us in airsoft but it's easy to see the difference between the two examples).

 

Now if we have an 11.1v 15C battery with the same 1000mAh capacity, it's capable of discharging 15amps.  Due to the higher voltage but much lower current, we can calculate that the motor would recieve 166.5 watts.

 

The higher the power, the faster the motor will turn.  Therefore, in my calculation, the 7.4v 30C should turn the motor faster and produce a higher rate of fire.

 

There must be some nuance I'm missing or am I simply misunderstanding something?

[EDcase]

The complication is that with the lower C the voltage will drop so the motor will not get the full 11.1v

It will turn fast because the battery will try to keep up the wattage but it will damage the battery long term.

 

Essentially the C rating is the safe maximum constant power output for the battery before damage will occur.

 

Some batteries have a second higher C number which is the Burst rating.  This this is the highest momentary power rating.

 

Edited May 17, 2021 by EDcase

[Adolf Hamster]

as above, c rating x capacity gives the rated current.

 

for example the above batteries in arwen's pic the nuprol has it calculated for us, but the turnigy is rated at 1.5x35=52.5A continuous draw 105A burst draw.

 

the motor will try to draw what it wants based on the voltage regardless of what the battery wants to do, and a motor that tries to over-draw is equivalent to mildly shorting the battery. as EDcase says an over-drawn battery will drop voltage but that's not a situation you want to be relying on.

 

in mild cases this will reduce battery life, but in extreme cases can cause the battery to burn.

 

 

as a general rule if the battery physically fits in the gun there is zero downside to over-rating the battery in terms of capacity (more mah generally = bigger battery) or C-rating (which generally means more expensive battery). if the gun is too fast then you can either reduce the voltage, or change the motor/gears but you do not want to reduce the current capacity as a means of dropping rof.

 

 

[rotor92000]

43 minutes ago, EDcase said:

The complication is that with the lower C the voltage will drop so the motor will not get the full 11.1v

It will turn fast because the battery will try to keep up the wattage but it will damage the battery long term.

 

Essentially the C rating is the safe maximum constant power output for the battery before damage will occur.

 

Some batteries have a second higher C number which is the Burst rating.  This this is the highest momentary power rating.

 

I'm not sure the comment about the drop in voltage due to the lower C is entirely correct.  The voltage should remain around 11.1v unless the current draw from the motor exceeds the capability of the battery, at that point I would expect to see some sag in the voltage and RPM until the trigger is released.  

 

The missing part of the jigsaw now seems to be the specfications of the motors.  What input do they require to achive a given RPM? There seems to be a lack of actual specifications on any of the websites I've been able to find.  If you were buying a motor for a drone, look at the specifications you can find:

 

I'm probably overthinking all this but it's just a very basic electrical circuit and therefore, if you have all the specs, it should be easy enough to compare apples with apples.  Now I get that once you move from the theoretical to the real world, there are a bunch of variables that you can't calculate for but an understanding of the theory is what I'm struggling with lol  I should probably just shut up lmao

 

 

35 minutes ago, Adolf Hamster said:

as above, c rating x capacity gives the rated current.

 

for example the above batteries in arwen's pic the nuprol has it calculated for us, but the turnigy is rated at 1.5x35=52.5A continuous draw 105A burst draw.

 

the motor will try to draw what it wants based on the voltage regardless of what the battery wants to do, and a motor that tries to over-draw is equivalent to mildly shorting the battery. as EDcase says an over-drawn battery will drop voltage but that's not a situation you want to be relying on.

 

in mild cases this will reduce battery life, but in extreme cases can cause the battery to burn.

 

 

as a general rule if the battery physically fits in the gun there is zero downside to over-rating the battery in terms of capacity (more mah generally = bigger battery) or C-rating (which generally means more expensive battery). if the gun is too fast then you can either reduce the voltage, or change the motor/gears but you do not want to reduce the current capacity as a means of dropping rof.

 

 

Yep, you make some good points there.  Mainly that if we knew what the motor required, we could better match a battery to a motor with less of 'finger in the air' that seem to have to do.  I guess the easiest thing to do it just get a load of batteries and try them but "there should be a better way!" LOL

 

Thanks for your input everyone. 

Edited May 17, 2021 by rotor92000

[Adolf Hamster]

8 minutes ago, rotor92000 said:

I'm not sure the comment about the drop in voltage due to the lower C is entirely correct.  The voltage should remain around 11.1v unless the current draw from the motor exceeds the capability of the battery, at that point I would expect to see some sag in the voltage and RPM until the trigger is released.  

 

think that's what he's referring to- the battery flagging due to being overloaded.

 

8 minutes ago, rotor92000 said:

The missing part of the jigsaw now seems to be the specfications of the motors.  What input do they require to achive a given RPM? There seems to be a lack of actual specifications on any of the websites I've been able to find.

 

welcome to airsoft. remember we are talking about a hobby where nimh is still common, tamiya connectors are bloody everywhere and brushless motors are only just now becoming a thing.

 

part of is is simply that a lot of the draw will depend on a load, which varies wildly depending on the build of the gun.

[Iceni]

On 17/05/2021 at 21:44, rotor92000 said:

Therefore, in my calculation, the 7.4v 30C should turn the motor faster and produce a higher rate of fire.

Trouble there is your looking at W. W is a static measurement for energy consumption. The measurement you need is EMF -  Electro motive force. Faraday's law of induction. EMF is voltage.

More voltage is more RPM, because more voltage gives a stronger magnetic field in the windings. More magnetism means more force pushing against the case magnets. More force is more speed in this instance. With no load V will always = a set RPM due to the EMF. :: A 7.4v can never equal an 11.1 for speed on the same motor.

More Current (Amps) means more torque. Torque is the work you can do as in energy. So more A gives more energy, As a static measurement you can represent that as W or you can represent it as W/second - Joules. Lenz's law. Current is important because if you can't do enough work to make your object move then the motors tries to draw more and more A. Hence why fuses are Amp rated. It's also why an under-rated battery will fry, Your motor asks for more and bullies the battery for it to the point of damaging the chemicals - They effectively boil and break down. The heat generated by this increases the resistance of the battery, This acts as a potential divider and drops the voltage as a component of that effect.

You mentioned you were getting better performance with higher Amp batteries. Well did you consider that both the load on the motor, and the Amps play a part. If the motor is overloaded due to a big spring or resistances then a battery that can supply more Amps is going to give a performance boost. If you start to see this then either the lower performing battery is overloaded, Or the load is so great that the bigger battery with more amps is allowing the motor to overload, Either way there is an imbalance and neither situation are good places to be at.

In airsoft your V is pretty much regulated. Unless you made a voltage control circuit, And I've never seen one of those implemented, we just use whatever the battery has.

C is just a multiplier. It doesn't do anything by itself. And isn't a figure where higher is always better, You always have to work out the final value. If anything C is a reasonable representation of the battery chemistry, and the ability of that chemistry to charge and discharge.

1000mAh 30C = 30 Amps
1500mAh 20C = 30 Amps
2000mAh 15C = 30 Amps
3000mAh 10C = 30 Amps

 

Quote

What's the difference in performance between, let's say 7.4v 30C and an 11.1v 15C?

Who knows. Without the capacity you can't make any assumptions. All you can say is the 11.1v will want to spin the motor faster with no load. It's one of those questions that looks right, but it's actually just showing a lack of understanding.


If you are wanting to pick a decent motor combination for an AEG it's easier just to ask. A question like - I want to make my gun shoot faster -- It's completely stock --- It's this model ---- What upgrades will give me better ROF or a ROF of 25 RPS?

You'd get a far better set of answers, Because more than a few of us have already spent the money on motors, batteries and gear sets, and will share information freely.

 

Edited May 18, 2021 by Iceni

[Samwise]

23 hours ago, Arwen said:

My G&G TR16 (stock gearbox) double feeds a lot when using the top battery in this photo (the nuprol battery)

Under advice from Sam at KoA I started using the bottom battery, with a higher C rating. Cured the double feeding completely with no noticable additional wear to the motor. 

 

Unfortunately I cannot remember if it was the burst or the continuous C number that Sam said was important 😅

 

I probably said the burst was the most important rating
For most of the use a battery sees in Airsoft, it'll not be getting too stressed for so long that it can't recover so the burst rating is the most useful - if it is accurate of course. 

I've found that the ratings on Nuprol batteries aren't particularly useful, usually overestimated by a decent margin whereas the Turnigy ones are generally pretty good. 

 

 

21 hours ago, Iceni said:

Trouble there is your looking at W. W is a static measurement for energy consumption. The measurement you need is EMF -  Electro motive force. Faraday's law of induction. EMF is voltage.

More voltage is more RPM, because more voltage gives a stronger magnetic field in the windings. More magnetism means more force pushing against the case magnets. More force is more speed in this instance. With no load V will always = a set RPM due to the EMF. :: A 7.4v can never equal an 11.1 for speed on the same motor.
 

The current in the wire creates the magnetic field, not the voltage. 
The current moves due to the voltage so you could say it's indirectly responsible but it is the current that actually creates the magnetic field around the wire. 

This is also why the magnetic field weakens as the motor gets faster, which has a range of useful side-effects for Airsoft. 
When the coils pass by the static magnets, they act like a generator producing Back-EMF (reverse voltage). 
The power used by the motor is the supply voltage minus the reverse voltage times the current. So when the motor speeds up, the reverse voltage closes in on the supply voltage and lowers the current use. 
There will always be a load applied in an Airsoft gearbox so it'll tend towards a certain figure but on full-auto, it can reach a fairly steady state. 

In terms of voltage drops during load, it's complex and varies greatly between different chemistries and various other factors. 
I can't say I've memorised all characteristics of all batteries in all situations 🤣but I do have a rough understanding of some of the effects.
The power capability of a battery pack is dependent on its voltage and impedence - which is kinda like resistance except that it changes based on 'stuff' (you can quote me on that) due to chemistry and other things. 
The impedence of a pack tends to rise as current flows, if the pack is rated for a low discharge (C - which is not coulombs but a battery specific symbol) and a higher load is applied then the impedence will rise even faster, causing a voltage drop. 
Interesting, the flow of current will heat up the pack which can actually improve the capacity. On a cold day, using your gun in longer bursts (eg 3x10 second bursts) can make the battery last longer than a similar number of shots fired more intermittently (eg 10x3 second bursts). 

To answer the OP's question, think of it like this: 
The voltage sets the maximum speed the gun can achieve
The C rating (or more accurately, the total pack discharge) is how close to that speed you actually get. 

If your gun needs a burst of 40A on startup and 30A for continuous full-auto then having a battery that can only supply 20A will limit your performance all round. 
In the same setup, having a battery that can provide 30A will give you better (but still sub-optimal) performance on semi and will allow full-auto to perform at full speed. 
Having a battery that can provide more than 40A will allow everything to operate as fast as it 'wants' to but without stressing the battery. 
The higher the number, the less stress the battery will get and the longer it will last, in terms of charge/discharge cycles. 

I know I've rambled on a bit so feel free to ask any questions and if I've gotten anything wrong, please do let me know. 
There's only one thing more fun than geeking out and that's finding someone else with more knowledge 😃

[Iceni]

1 hour ago, Samwise said:

The current in the wire creates the magnetic field, not the voltage. 

Incorrect but also correct.

The voltage allows the current to charge the coil to an energy potential. This is why current always lags voltage in inducted circuits. Without the voltage you can't get the amps in place, but you can still over amp a motor that is running at a lower voltage by adding load. The added amps in that case are not going to make the motor faster as the motor is trying to work harder.

Try to explain that voltage doesn't really do anything, but at the same time it's controlling your motor speed, but what you really want is amps, but not an overload of amps due to load, is frankly a little arse about face... It's easier just to say volts = more force, directly of not. If you re-read what I wrote you'll see there's no reason given as to why more volts give a stronger magnetic field, It's enough to know they do, and not get into the fact a higher voltage allows the current to flow faster and more freely. It's not needed. I did however point to both laws that govern this, so the op can go a delve into it as their leisure, nothing was hidden.

It's basic AC motors physics. And before you say but our motors are DC... No they commutate.

Look at partially reactive loads

https://learn.openenergymonitor.org/electricity-monitoring/ac-power-theory/introduction

 

It's hard to say adding more amps will speed up the motor.... It won't. Adding more volts will. You can limit a motor with both volts and amps to control the speed, but this isn't about control, we want max effect.

Back EMF isn't really a thing that needs much note. It exists, it limits the motors top speed, and causes heat.

Edited May 18, 2021 by Iceni

[Samwise]

Err, are you sure about that? 

Magnetic fields are created by charged particles when they move through a medium.
Static charged particles create no magnetic field. 

Current is defined as the net rate of flow of charged particles through a medium. 
Voltage is the potential difference between two points, it's like the pressure on the charged particles. 

Voltage is the push but something has to actually move to create the magnetic field. 

Edit: 
I've gotta add this here to show that this comment was originally in reply to a much simpler comment that has been expanded somewhat.

Edited May 18, 2021 by Samwise

[Adolf Hamster]

1 hour ago, Iceni said:

Back EMF isn't really a thing that needs much note. It exists, it limits the motors top speed, and causes heat.

 

I dunno, back emf is why i'm not allowed in the electrical labs any more.....

[Samwise]

Back-EMF is best EMF 🤣

Also, it doesn't limit the top speed nor generate heat. 

That's the current. 

As the motor speeds up, it generates more Back-EMF which causes the motor to draw less power. 
This happens when the momentum of the gearbox builds up and lessens the load on the motor ie. during full-auto. 
Or when the load drastically reduces eg. a stripped piston. 

You could say Back-EMF is actually a good thing for DC brushed motors. At the very least it is a necessary comonent without which, DC brushed motors would not work. 

[Iceni]

51 minutes ago, Samwise said:

I've gotta add this here to show that this comment was originally in reply to a much simpler comment that has been expanded somewhat.

I am an habitual editor, If I post something I'm generally moving things about for a good 30 minutes. Hell I was still editing the one from yesterday up to about an hour ago! I'm still not happy with some of the wording.

 

Quote

because more voltage gives a stronger magnetic field in the windings.

I'd probably edit that to say allows rather than gives. But I'll leave it be since it got commented on!

I've been caught by it before, Brain engages I write and post then rethink and rethink it. I've caught others out who do it as well. It's part of been on a forum.

Edited May 18, 2021 by Iceni

[Samwise]

Haha, yeah same here. 

The way it's worded now is much better so my comment is largely unneeded. 

I tend to add edits to the bottom so the flow of the conversation is a little easier to follow. 

[Iceni]

It's what forums are for, If you can't have a discussion then there's no point. If anything is to be taken away from this it's that I shouldn't go for a shower halfway through editing a post that was posted then moved directly to an edit window. You only commented on what was visible at the time you posted. It's my bad.

It's been bugging me all day Edit:

 

Quote

Also, it doesn't limit the top speed nor generate heat

In a motor with no load BEMF does limit the speed. As the motor gets faster, you generate more BEMF, This causes the EMF and current to the motor to drop, but the speed will remain constant. In effect it acts to govern the motor.

The heat aspect comes from a motor under load. Because you are running a loaded motor the speed the motor will get to will never be high enough for the BEMF to totally govern the motor speed. It's always going to be trying to speed up. The effect of that is the motor reaches an equilibrium that isn't optimal, Less BEMF than could be generated and thus the lack of BEMF means the motor is always going to end up drawing more current. Yes you will see a current drop once the motor gets to that equilibrium, but the motor is always going to be trying to go faster.

The question can be asked in a different way. Can a motor under load ever reach it's maximum theoretical speed, And what are the consequences to heat generation as load increases and speed decreases?

It's practical application V/S theoretical. It's why BEMF is not important. The motor is loaded and the speed of the motor is going to be dragged down by mechanical action, In that situation BEMF generation is lower possibly to the point where you will see near supply voltage over the motor.

Edited May 19, 2021 by Iceni