Intro #

The batteries we use in airsoft come at different sizes, shapes, configurations, prices, or even chemistry. Even though this industry likes to sell parts lacking proper specification (motors which are fully specified by just two words - “High speed” / “High torque”, precision barrels without the specified tolerances, generic parts that follow a “TM specification” which does not actually exist, etc), battery manufactures do tell us something about their product. All of them at least specify the capacitance and nominal voltage of their product. But is it enough? Well, turns out - no!

A battery can be done in different ways and it may have different characteristics thanks to that. If you look at commercially available batteries (like the standard 18650 ones, for example), they have real datasheets that specify a bunch of values and graphs showing different dynamic characteristics. You will not find anything like that for our hobby batteries. It proves, however, that two batteries with the same nominal voltage, capacitance, and chemistry might still have very different properties. In other words, one 11.1V 1500mAh Lipo might be different than the other 11.1V 1500mAh Lipo, and by a lot!

C-rating for the rescue #

The Lipo batteries (I guess all NiMH battery producers know they suck for our use-case anyway so they don’t bother in specifying this) include C-rating. This magical 15C or 25C or 40C (or some other) label. They often even comes with two values - 15C/30C or 25C/50C, etc. And suddenly you know that a 11.1V 1500mAh 25C battery is better than the 11.1V 1500mAh 15C one! Except.. well, many people have voltmeters, a lot of them can measure the capacitance with their chargers but did you hear about someone having a C-rating-meter? I didn’t! So, if you can sell more batteries by just changing the 15C to 25C on the label, why wouldn’t you do that? Customers will not notice anyways!

And no - it’s not a conspiracy theory. You will see why I don’t trust this value and how I assess the value of a battery below.

What is C-rating #

But first - what is C-rating? It is not just a magic value without any meaning, after all. This is an embroiled way of specifying the electrical current limits of the battery. Instead of specifying this in Amperes, we use C which is a unit derived from the nominal capacity of a battery. 1C means the current equal to the numeric value of the capacitance. So 1C, in case of a 1000mAh battery, is 1000mA = 1A but in case of a 2200mAh battery, 1C is 2200mA = 2.2A. This works for both charging and discharging. If we say that it is safe to charge the 1200mAh battery with 1C of current, we mean you can use up to 1200mA = 1.2A.

Now, what if we have two C-ratings, like 25C/50C? Then the smaller number is the constant rating (you can draw that much current constantly or for a long time) and the larger one is inrush rating (you can draw that much current but only for a moment). So a 2000mAh 25C/50C can safely deliver 50A of constant current and the short bursts of up to 100A are also OK.

Easy and clear, right? The C-rating is, then, well defined and there is no place for abuse.. well, not quite.

What is the problem with C-rating #

At the first glance, C-rating is clear. But as soon as you start asking some questions, you realize there is no way to answer them. So what does it mean that the battery can handle 25C? Does it mean that as long as we do not cross this value the battery will not age/break? What will happen if we exceed this - will it explode? How come I get better results on a 1500mAh 25C battery than on a 1500mAh 40C one but from another manufacturer? How long can I draw the inrush current from the battery without having problems? Is it 1ms, 1s, 1 minute?

That is the problem - there is no standardization around the C-rating so each manufacturer can (and do!) specify it differently. And if we consider the fact that many people base their purchase decisions based on it, we have a lot of room for marketing abuse. One thing that we should at least feel certain about is that it won’t set catch fire as long as we keep current below the rating.. or do we? :)

2 problems occur when a high current is sucked from the battery:

• The heat is generated - we must ensure the battery does not overheat as this may age it much quicker and above some temperature, it might even catch fire!
• The voltage drops - each battery has so-called Internal Resistance. It represents the voltage drop in the function of the current drawn. So an IR=10mOhm means that for every 1A of current, the voltage will drop by 10mV. Since the speed of a motor is proportional to the voltage and the motor is taking (a lot of current)[/docs/experiments/motor_current/] when starting, this may cause performance degradation. A lot of batteries have IR over 50mOhm so if the motor takes 60A of current while starting, making the voltage drop 3V from the nominal 11.1V… Now if we use a good battery that only has 10mOhm of IR, the voltage drop will be only 0.3V which means the motor will spin up faster.

Now the RC community, in contrast to the airsoft community, is more concerned about the continues current as their toys often have the motors running for the whole time. In our case, we cannot keep the motor running for too long as we have a limited number of BBs in the mag. They did show, however, that if you will fully discharge your battery at the C specified range, some batteries might overheat. So at least in some cases, the C-rating does not even guarantee safety.

I, on the other hand, can show you that the higher C battery might have a bigger IR causing the actual performance of our replica to be worse.

Measurements #

I recently bought 4 new batteries and measured their parameters before using them. Here are the results:

As you can see, even though the GPX Extreme is rated to 40C, it has 5 times bigger IR than the Rhino battery (per cell). Its nominal current is 72A but at this current, it would drop 1/2 if its voltage. If we compare it to the Rhino battery, with the IR of 16.7mOhm at the nominal current of 110A we only have a 0.61V drop, which is just 5.52%. The intuition tells us that the GPX Extreme (with its 40C) would be much better than the iPower battery (that not only has two times smaller C-rating but also smaller capacitance) but the actual difference in IR is only 25 mOhm vs 29 mOhm per cell, which is quite low. The iPower drops its voltage by only 22% on a nominal current so its C-rating is much saner than the GPX. But still, it’s 5 times bigger compared to Rhino.

If we compare the GFC which is much smaller (only 1000mAh and 30C) than the GPX Extreme (1800mAh 40A), the actual voltage after drop on 30A current would be make the output voltage 6.86V compared to 5.87V. So you would get better RoF from the battery with much worse parameters!

Conclusions #

All we need our battery to do is to rotate the motor, do it fast and without overheating. But the speed of a motor is proportional to the voltage we use, so the bigger the voltage drop, the slower the motor. Just by looking at the C-rating of the batteries presented above, we should expect that:

• the GPX Extreme will be only slightly worse than the Rhine since we have 40C vs 50C,
• The iPower should be considerably worse than the GPX as it only has half the C rating. But the actual measurements show something different - the GPX is indeed better than the iPower but the difference is very small. And Rhine is indeed better than GPX Extreme, but the difference is really big.

Can we then conclude anything about the battery performance based on its C-rating? Not really! Maybe if we compare two batteries from one manufacturer then we can expect the one with a higher C to be better? That is something I should check in the future!

I also briefly showed that Internal Resistance is a much better metric (although it’s also not ideal, but this is a topic for another article), but almost no manufacturer of hobby Lipo batteries specify that.