Airsoft Batteries and Electrical 101

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Airsoft Batteries and Electrical 101

Post by DHM078 on Wed Jul 06, 2011 12:02 am

The AEG's electrical components may seem rather complicated, but the circuit is actually incredibly simple. But first, there are a few terms to become familiar with:

Electrical current - electrical current is simply the flow of electrons:



A useful analogy for current would be water flowing through a pipeline. Electrical current can be quantified. The amount of current is measured in Amperes, or Amps for short.

Electric potential energy - This is essentially the force that "motivates" electrons to flow as current. Electric potential energy is measured in volts, and the term "electric potential energy" is usually substituted with the term voltage to make life easier. If we were to think of a circuit as a pipeline filled with water, the voltage would be analogous to water pressure. Like water pressure "motivates" water to move through the pipe, voltage "motivates" electrons/electrical current to move through the components of a circuit.

Electrical resistance - Resistance is exactly what it sounds like; it is the degree to which a component resists the flow of electrons. Resistance is measured in ohms.

Closed vs open circuit - In a closed circuit, electrical current can readily pass through, but in an open circuit, where there is a break, current cannot flow. A closed circuit is sometimes referred to as a complete circuit. An open circuit is sometimes referred to as a broken circuit. The following represents an closed circuit:



The following represents a open circuit:



The components of an AEG circuit:

Battery - The battery is made up of what are called electrochemical cells. Inside these cells, a chemical reaction generates a voltage. Going along with our water flow analogy, the battery generates the voltage, much like a water pump generates pressure, so the two could be considered analogous. The battery is thus in a way pumping electrical current through the circuit. It continues to do so until the chemical energy inside the cells is used up. At this point, the battery no longer has the chemical energy needed to produce a voltage, and the battery is discharged, or "dead". It must be either replaced or recharged if possible for that battery type. All batteries used to power an AEG circuit are rechargeable. For now I am giving a general explanation of an AEG circuit, so I won't go into details about the different battery types now; they will be discussed in depth later in this guide. Below is a picture of a typical airsoft battery pack:



Wires - Electricity flows through them. Not much else to say.

Switch/trigger assembly - This is the part of the circuit that we control when firing the gun. A switch is anything that allows us to open or close a circuit at will. When you pull the trigger, the trigger moves a "trolly", if you will, with a metal bar. This is pushed by the trigger into contact with two smaller metal pieces sticking out of the stationary part of the switch assembly, to which wires are attached. The metal parts that come into contact are appropriately called the trigger contacts. The trigger contacts act as a switch. When in contact, they close the circuit, allowing electrical current to flow through from the battery to the motor. When the trigger is released, the contacts are separated by a spring, and and the circuit is opened. No more current can pass through. Another component that isn't really part of the circuit or switch assembly you should be familiar with is the cutoff lever. On semi-auto, the cutoff lever is engaged by the sector gear with each shot, and when the cycle is completed, the cutoff lever disengages the trigger contacts, opening the circuit and preventing other shots from being fired. The trigger contacts and cutoff lever are labeled in the picture below:



Motor - I am not going to go into a detailed discussion as to how passing current through a motor allows the electrical energy to be converted to mechanical energy, as it is somewhat complicated, and most of it isn't too relevant. For the purposes of this guide, all you need to know is that when current is passed through the motor, the pinion gear spins, allowing it to cycle the gearbox. Below is a picture of a motor:



Here is a simple circuit diagram representing the components in arranged in the AEG circuit:



The parts are labeled, for those not familiar with circuit diagrams. But even still, it is easy to see how electrical current flows. Starting at the negative end of the battery, current passes through the trigger assembly when the trigger is pulled, and then flows to the motor, after which it returns to the battery. Very simple.

Now I will discuss the different types of batteries that can be used to power an AEG. The first thing you should know is that your gun doesn't give a damn where its power comes from. LiPo, NiMH, or hamster wheel-powered turbine, electricity is electricity. That does not mean that each type of battery is actually the same in practice; all I am saying is that there is nothing magically powerful about electricity from a LiPo, or nothing "safer" about electricity from a NiMH. But that said, each battery type offers some distinct advantages and disadvantages.

The battery types as it pertains to airsoft:

Nickle-Cadnium (NiCD) - These batteries often come with stock Chinese guns. They offer several advantages. Firstly, they are relatively low cost. They do not require any special chargers. They are also very hard to damage. They can be stored for long periods of time discharged. They also hold their charge for a long time. But they do come with drawbacks. For one, they have poor cell energy density. This means it takes a rather large battery to hold an amount of energy that other batteries with better energy density can do in a much smaller space. They also drop voltage rather quickly under load, and have poor discharge rates. They also suffer what is know as the memory effect, which, to spare you a long, complicated explanation, basically means you need to discharge the battery completely before charging it again, if you want the battery's entire capacity.

Nickel-metal hydride (NiMH)[ - These batteries offer essentially the same advantages as the NiCD batteries. They have similar cost, do not require special chargers, and are relatively hard to damage (not as hard as NiCD though). However they do not hold their charge as long as NiCDs. They are also more susceptible to damage from over-charging. But they do not have any memory effect, and require no discharging. They, like the NiCDs, also have poor cell energy density, drop voltage quickly under load, and have low discharge rates.

NiCD and NiMH batteries are functionally very similar. They have similar discharge rates, similar cell energy density, and have similar voltage behavior under load. They also have the same nominal cell voltage (1.2v/cell). They can also be charged on the same chargers. For these reasons, they are almost the same when it comes to how they function in our guns. Quality NiMH batteries configured for AEGs are far more available however.

Also, if you plan to use NiCD or NiMH batteries, get a smart charger. It is a great investment that will preserve the life of your batteries. Just do it. And you also must discharge before charging with NiCd. Many smart chargers will also discharge, but you can buy dischargers separately. You do not need to discharge NiMh batteries.

Lithium-ion polymer (LiPo) - Ah, the batteries that have gained infamy for making guns run as if magical pixies enchanted them, for shredding gun internals, and for causing world-ending explosions. There is so much misleading and outright wrong information floating around the internet. I will start by discussing them as I did the other two types. They are very cheap if you know where to get them. Namely, hobby/RC sites and shops. If you buy them from an airsoft store, expect to pay several times the cost, and expect exaggerated battery specs. They have the highes cell energy density available. The also have the highest cell voltage, meaning fewer cells. Few cells plus high cell density means you can fit a powerful battery into a relatively small space. They have very slow voltage drop. Their voltage does not drop much under load. They can be charged relatively quickly, and have the highest discharge rates available. They do require special chargers though, and a working knowledge of how to use them safely, as they can pose fire hazards.

Lithium-iron phosphate (LFE/LiFePo4) - These batteries are also lithium based batteries that are emerging into the airsoft scene. They were developed for the RC world to be essentially a "safer" LiPo. It is and it isn't, as I will discuss later. They have better cell density then NiMH and NiCD, but not as good as LiPo. They share the LiPo's slow voltage drop/flat power curve. They also have much better discharge rates than NiMH and NiCD, but not as good as LiPo. They can also be charged at greater speeds than any other battery type (for airsoft at least). They also require special chargers, and while they are considered "safer" than LiPos, understanding how to use them safely is still very important.

Lithium based batteries are not idiot proof! If you're going to use them, you should have an understanding of them and how to use them safely. The first thing I will say is that the reports of them being world-ending nuclear disasters waiting to happen are rather exaggerated. That said, there are a few things that you should know about them to use them safely and properly. The first is that they must be charged on a LiPo-specific (or LiFePo4) charger. Some LiPo chargers also do LiFePo4s, and even NiMhs with some. You cannot just plug them into a standard wall charger or smart charger. The cells also must be balanced. Many chargers will do this too, but not all. Separate balancers are available. Also, don’t cheap out on chargers and such. You want something good that will do a good job properly charging your battery and balancing. You also cannot just run a lithium battery until it is dead. As you drain any battery, the voltage goes down a bit. LiPo cells peak at about 4.2volts, and their nominal voltage being 3.7v/cell. Do not let them drain below 3volts in any cell, or you could cause permanent damage to the battery. Because LiPos have a fairly flat power curve, many say just recharge when you the RoF has decreased notably. But the best way is to use a cell-monitoring LiPo alarm which when plugged into the balancing plug in the battery will either sound some sort of alarm or light up an LED to let you know that one or more cells has gone below 3v. This same type of over-discharge is possible with LiFePo4s too, but they can tolerate a bigger voltage drop. This, and their hard cell casing which makes puncturing more difficult, make the batteries "safer" to use, but all types of lithium batteries still burn violently and should thus be handle with relative care. LiFePo4 cells average 3.3volts/cell by the way. Do not short out a lithium battery, or you will cause permanent damage to the battery, or a fire/personal injury. A bad short to a LiPo can end explosively. To that end, always run a fuse with them. If you are worried about the added resistance, consider using automotive fuses. Their fuse boxes and fuses have much less resistance than the tube-shaped ones that come with most AEGs. You can also get self-resetting fuses that you just wire into the circuit and are permanent, and reset themselves. They are sold by AWS, HS Armories and Extreme-Fire, and have practically negligible resistance. Also, don’t buy LiPos from airsoft sites for the reasons mentioned earlier. Buy them from hobby/RC sites. For instance, I just bought an 11.1v 20C 4000mAh battery for about $26, and a 7.4v 20C 3000mAh for about $14 from Hobbypartz. The latter would have probably cost over $40 at an airsoft site, and the former over $70, if you could even find one to those specs, if purchased from an airsoft site. And that's from Hong Kong sites! In the US, the latter battery would cost over $90, the former well over $100!

How a battery will affect your gun/how to choose a battery for your gun:

First thing, what determines how fast the motor is going, and thus RoF, is power, measured in Watts.

Power = current X potential

Or in other words:

watts = Amps X volts

A few more things to be familiar with:

C-rating is the discharge rate. This number alone is relatively meaningless. Add in the battery's capacity however and it will get you somewhere.

Discharge in Amps = discharge rate X capacity in Ah

Or in other words:

Amps = C-rating X mAh/1000

LiPos and LiFePo4s will give you a C-rating. Most NiMHs do not, so when comparing NiMH batteries to LiPos, consider that most quality NiMH cells have a discharge rate of about 8C or less.

That equation gives you the battery's continuous discharge rate in Amps. The thing is, the motor will only draw what it can use, regardless of the output. So a battery that could potentially supply 200amps will only ever supply 30amps if that's what your motor draws.

Now there is the issue of how much current your gun draws. This isn't constant though. The startup draw of the gun always spikes much higher, then goes back down, and increases as the load force increases, and then goes back down after the piston is released. So hooking up a multimeter and measuring it on a battery that will surely provide enough current, such as a 7.4v 4000mah battery at 30C (that's 120amps to work with, pretty much no gun will draw anything like that), won't just give you a specific number to aim for, but it will give you a general idea. One other thing, expect a current spike at startup. This is normal. With LiPos, even if that draw is more than what the continuous discharge rate will provide, most LiPos also give you a "burst" discharge rate which allows for quick bursts of higher discharge. That will usually account for this spike. NiMH batteries are rather limited in their ability to give current bursts, only expect a few more C, as opposed to the sometimes 20+ you can see with LiPos. That is why even though a NiMH battery may get a similar RoF to a LiPo in some cases, the LiPo gives faster trigger response; the LiPo can simply bring the motor up to speed faster.

So with the amount of current a given battery can supply, the battery's voltage, and your gun's current draw in mind, you can get a pretty good idea of how much power will be going to the motor, and what battery will suit your needs best as far as finding the balance between reliability and performance. More power is going to equate to faster wear, but I hope you could figure that out by yourself.

Other things to bear in mind, especially when moving to a more powerful battery:

Trigger contacts: There are two ways using a beefier battery can cause issues here. The first is by producing so much heat through resistance that they actually fuse together (yes, I have actually seen this happen, albeit on a 18.5v LiPo in a very current demanding setup that was tried for shits and giggles). If you are actually going to be running that stressful, you are already probably know enough to use a MOSFET that can handle that. The vast majority of people can ignore that. The main issue is arcing between the contacts leading to corrosion/oxidization of the contacts. Higher voltage increases potential energy and may make arcing more likely, but it is the amount of current that really does the damage. Corroded contacts can be sanded down a bit, but generally lose some efficiency, and are likely to arc again with the same battery anyway. In this case you have two options: use a lesser battery, or get a MOSFET.

What is a MOSFET? MOSFET stands for "metal oxide semiconductor field effect transistor". A MOSFET has four terminal: gate, source, drain, and substrate. The substrate terminal is connected to the source within the MOSFET, and has no external terminal. The ones we worry about are source, where current enters, drain, where current exits, and the gate, which uses the metal-oxide insulation from the channel between the source and drain to act as a capacitor, which stores charge at a specific voltage. Essentially, it is a part that allows you to control the passage of a large quantity of current with by varying the voltage at the gate. It only requires an infinitesimally small amount of current to charge the gate capacitance. By using the gate to control the flow of current through a suitable MOSFET for our current applications, and using the trigger contacts to control the opening and closing of the circuit at the gate, we ensure that only an infinitesimally small amount of current passes through the contacts, making arcing/corrosion neigh impossible. This also will lower the electrical resistance of your electrical system, essentially making the gun more efficient. This is the most effective solution to corroded contacts should this become a problem, and aren't a bad idea in general because of the lower resultant resistance. A simple MOSFET-based AEG circuit looks like this:



This is a very much over simplified circuit though. For practical use, MOSFETs will need some sort of protection from voltage and inductance spikes and need to have proper resistance. That is why the MOSFETs available for purchase have other components and even programming. The best MOSFET-setups available for purchase are made by Extreme-Fire, AWS and HS Armories.

MOSFETs available for purchase come in many varieties with many different features. Many of them are programmed features that allow customization of some sort such as burst fire functionality, and semi-only operation. Some regulate rate of fire. The different features that they come with are listed on the webpages of the sellers (at least for the ones I mentioned above), and are for the most part self-explanatory. One feature, however, is a bit trickier, and warrants some explanation here: active braking. When your gun fires, it stops because you release the trigger, and power to the motor is cutoff. However there is considerable rotational inertia, that is, it has momentum, and the gears don't stop spinning unless something stops it. What usually ends up doing that is a combination of magnetic resistance from the motor, force from the spring, and friction. This is enough for most guns, but for some guns, particularly those running at high rates of fire, and with weak springs, this is not enough to stop the gun before it cycles again. This is why some high rate of fire guns fire multiple times on semi-auto. Active braking is an attempt to electronically prevent this. With an active braking MOSFET, when the trigger is released, there is a brief surge of current backwards, which slows the motor to a stop much faster by essentially pushing it to run in reverse. It does wear the brushes on the motor slightly faster though (lose about 3-4% of motor life generally, about 5-7% in semi-only guns).

Battery connectors: The small-type Tamiya connectors that come in most guns are rated for 10amps. That's it. However even most NiMH batteries used in airsoft can supply that, and pretty much all AEGs will draw more than that. The reason they don't burn out on us is that we do not drawing power continuously, only the shorter bursts when firing. But if you're going to run a LiPo in a gun drawing say, 30 amps, mini Tamiya connectors are a bad idea. They actually have about 20 times more electrical resistance than genuine Deans connectors or XT60s. The larger Tamiyas are better than the small ones, and are rated for 20amps. But they still have about 10-12 times the resistance of Deans, and are quite large. Tamiya connectors are a pain in the ass anyway, with their poor connections. Considering that Deans are smaller than mini connectors, and a pretty cheap upgrade, you might as well pick some up if you're going to be using a beefy battery, NiMH or LiPo.

Lastly, if you are running the gun at a higher speed, things are going to break faster. Expect it, and if you're running the gun at a high speed, build it to last as such. Check that your gears are properly shimmed, that your motor height is correct, that your piston's angle of engagement are correct, or find someone who can do it for you, when putting in a powerful battery. If you are running the gun at a high RoF or power, choose your parts accordingly. And remember that it is not the battery breaking your gun, but rather the speed at which you are choosing to run it that is breaking it.

Hopefully that clears up any myths and answers any questions about batteries and electronics in airsoft guns. If you have any specific questions that aren’t addressed here, feel free to ask me.
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DHM078
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