Very Basic AEG Motor Electrical Theory

[mcguyver]

OK, a motor is basically an electromagnetic coil with another magnet system to offer repulsion/attraction to the coil. In the case of airsoft guns, their motors are a permanent magnet motor. This means that the only source of current draw inside the motor is the coil. Now, when a motor first starts, it will be drawing it's highest amperage. The industry term used to describe this is inrush current. As the motor starts to turn, it generates current as the magnetic poles of the motor coil cut through the magnetic lines of force of the magnets permanently attached to casing of the motor. This creates current inside the coils, current that is exactly opposite (180 degress out-of-phase) with the current used to start the motor. This is called counter electromotive force, or c-emf for short.

You have to remember that nearly all motors in nearly all types can also be generators. If you were to turn your motor fast enough, like clamp the shaft in a drill and spin it, it will generate voltage and current to a supplied load. So, as the motor is spinning to drive your gearbox, it also is generating voltage and current. And this current is high, almost as high as the current used to start the motor, but not quite. For example, the motor might draw 30 amps at inrush, and the c-emf could be 24 amps. The math looks like this:

30 amps (inrush) - 24 amps (c-emf) = 6 amps.

The above formula describes what is actually happening. The motor draws 30 amps (and always does while running, no matter what), but then generates 24 amps back to the battery. So, the net draw from the battery is actually 6 amps.

Now, when you talk about the capacity of a battery to discharge current, you must also take into acccount it's capacity to take that current back. The formula above describes the very basic operation, and does not take into account the pulses of current occurring in both directions due to pole reversal frequency. Even though this is a DC (direct current) system, there is a frequency and phase element present in the motor's cycle that makes it resemble an AC system, albeit with a more square-wave look to it.

Now, on to wiring and fuses. Just because your motor draws 30 amps at inrush does not mean you need wiring rated for 30 amps. This would be #10 AWG wire, and not very installation-friendly. Nor does it mean you need a 30 amp fuse. The Canadian Electric Code (which is law BTW) requires that wiring for AC and DC motors be rated in ampacity (current capacity) for 125% of the full load amps (rated running current as supplied by the manufacturer). For a motor rated for 6 amps FLA, this means 8 amps rated, or #16 wire. The maximum fuse would be 300% of the FLA or 3 x 6 = 18 amps. Now, airsoft guns hardly fall under the enforcement of the CSA, so this is not a legal issue, but the theory behind ratings is quite evident here. You can instal a 20 amp fuse and still protect the motor and wiring, but there may be the case of a blown fuse if you increase the load. Increasing the load will increase the FLA of the motor. The motor should be able to handle 125% load increase with no ill effects. An EG700 motor in a stock Marui is capable of handling 200% load increase to give you an example.

Are you lost and confused yet? You should try 3 phase motor theory and throw in a good helping of power-factor correction.

So, to sum it all up. Use a battery that is capable of large discharge currents (25+ amps at least). A fuse is your friend and is there to protect your wiring and motor. It IS NOT a source of resistance and DOES NOT reduce ROF or current flow to your motor (unless you use a current-limiting fuse, but that's not very common at all in the real world). #16 guage wire is sufficient even for the most upgraded of guns, and #18 is fine for stock or modestly upgraded guns. If a fuse blows, it has done it's job and saved you $$$ and time for repairs. If wire burns or melts, it's not a capacity issue for feeding your motor, but a short-circuit that needs to be repaired.

[mcguyver]

Basic Motor and Battery Performance:

Now, as many user have noticed over the years, when you change battery voltage, often you notice a change in your rate of fire. For example, when you use a gun with an 8.4v battery and switch to a 9.6v battery, you notice the gun fires faster. This holds true especially when you use batteries of a similar size, let's say, an 8.4v 3000 mah and a 9.6v 3000mah. This is not always the case however, if you go from an 8.4v 3000mah to a 9.6v 1700mah for example. This is due to cell size and current discharge capacity.

The battery cell size and type will determine your discharge capacity. This is the amount of current that a battery cell (and in actuality the whole pack) can actually discharge at it's rated voltage. All batteries can, in theory, discharge infinite current, if time is infinitely small. But as time increases (this is in milliseconds BTW), the battery chemistry and plate surface area will only allow so much current to be generated by the chemical reaction inside the battery. This is why an 8.4v 1700 mah sub "C" (large cells) will outperform an 8.4v 1700 mah "AA" battery pack (like in a PEQ or nunchuck for example) by a country mile.

Battery Chemistry

There's been alot of talk about this over the years, as the old tried-and-true NiCad batteries have been shoved aside in favour of NiMH or Lithium-based packs. There are differences in chemistry that bear out performance and size differences as well. The old NiCad batteries generally used to top out at about 2400 mah for a sub "C" vs now about 5000 mah for a NiMH battery in a sub "C". The capacity appears to be doubled between the two, and the performance seems to be advantageous enough to use the NiMH and not the NiCad. But appearances can be deceiving. The NiCad, if tested in current discharge, is almost always going to deliver the best performace in maximum discharge current, as it's chemistry allows for this. The drawback is in it's longevity. When used in a gun with low current demands placed on the motor, like a stock gun for example, it won't last as long (number of shots) as the larger-capacity NiMH.

The other major reason to use the NiCad over the NiMH is cold-weather performance. When you use your AEG in winter play, all batteries perform poorly in cold weather. They can't produce as much current at -5 as they can at +25, that's no secret. The chemistry for best cold weather performance is NiCad, then NiMH, then LiPo, a very poor cold-weather performer.

I've been using power tools for many years, with all battery types. They stay in my truck, and I use them at temperatures from -40 to +30, and I'm very unforgiving on them. LiPo is unusable at anything colder than freezing, NiMH is OK, but as temperature falls, it becomes useless at temperatures not much colder than freezing. I can use the NiCads all the way into the -30s with poor performance, but they do perform. I am forced to stick with NiCads as my main power tools because I can't change the weather, nor can I change the job I require of them.

Now for LiPo, the newest battery technology. LiPo is the best current discharge to size battery currently available. They are the reason that you can get tiny cell phones and laptops, as well as portable video games, the electric car, etc. But this performance comes with a price. Cold weather performance is terrible (my cell phone dies in a matter of minutes if exposed to -20 temperatures) and availability is not as easy (but getting much better) as the more traditional chemistries. The main disadvantage currently is charging and stability. You must have charger designed for them specifically, any old off-the-shelf charger won't work, and you risk causing damage to the battery or having it explode on you. LiPo packs have circuitry built in to prevent catastophic discharge (which can also lead to KABOOM!!) and charge, but they are not 100% failsafe. Some guys are a little leary of having a battery millimeters from their cheek that could explode on them if they had a wiring problem or whatnot, even if it's a small risk.

Charging

This is often the most overlooked aspect of batteries and their use. Many guys will spend $800 on a gun, $75 on a battery, and then spend $20 on a wall charger. This is terrible, not only for your battery, but inevitably for your on-field gun performance. I don't know how many times I've heard guys complain about a crappy battery, that a 3000+ mah battery only gets them half a day's use in a stock gun.

A good charger will offer good peak detection circuitry to detect when a battery is fully charged, and also to be able to detect false-peaks. A false-peak often occurs when a battery is pretty dead, like if it's been used until dead, left in a constant current draw gun like a PTW or similar device. When you first put it on a charger, it may charge for a few minutes (or sometimes a few seconds), and read fully charged, when in fact it's still almost fully dead. Many chargers have a tough time with this, even some expensive ones, and I've never seen a charger that is 100% foolproof on this. But, good chargers do a pretty good job of picking this up and properly charging the battery. Sometimes, you need to reset the charge cycle a few times to bring a dead battery back to life and a full charge. Most of the time with regular airsoft use, this doesn't happen, but is more for extreme cases.

Along with charging also comes discharging. The battery most susceptable to building up a memory is NiCad. Even then, it can take years and hundreds of charge cycles before this happens, with there never being a discharge done to the battery. For most guys, this is a non-issue, as they don't use NiCads at all. The quality of your battery may also play a part in this, as poor quality chemistry is not as reliable as a better battery by a reputable manufacturer. Then generally accepted battery life for a NiCad is 300-500 cycles, for NiMH is 1000 cycles, and for LiPo can be anywhere from 300 to thousands, depending on the pack.

As a rule, I discharge my NiCads at the start of the year if they've been sitting charged over the winter, and after their use, before recharging for storage. I use them until they're spent before I change them, and in cases of light use, may store them partially charged. I never store them when completely drained, not for any hard scientific reasons, but more for usability on short notice if needed. I have never found the need to discharge NiMH batteries (and it's not really recommended), and LiPo should never be discharged, as a matter of safety.

For chargers, getting one with a discharge function is ideal, but not necessary if you don't use NiCads. As a general rule, you should buy a charger with adjustable charge rates if you have differing battery sizes, and spend as much on the charger as what a quality battery costs, usually this means $50+. Digital displays, and adjustments up the wazoo are fine, but not always necessary, and if you don't know how to properly use them, can be harmful as well. Use what you're comfortable with, but never cheap out.

How you charge your battery also affects how long it will last and how it will perform. Contrary to popular opinion, heat is your enemy. If you charge a battery and it gets hot, you are doing it wrong. Heat is a battery killer, both on charge and discharge (use in your gun or discharging for maintenance). If your battery gets hot, that's a sign that you are using too much current, and forcing the electrolyte to store electrical energy as chemical energy at too fast of a rate. Reduce your rate, and keep your battery cool. Some more expensive chargers have thermal probes that tell your charger the temperature of your battery and allow an automatic shut off if the temperature exceeds certain limits. NiCads are the most susceptable to this, but NiMH is not immune at all. Over many years, I've had the opportunity to kill alot of batteries of both types, and sometimes it's been 1 cell that has failed in a pack due to heat, rendering the whole pack useless.

Battery Quality

Many guys out there think that all batteries are the same, just go where the best deals are. This is absolutely false and bad advice. Batteries are not created equal, not only are the cells themselves different in quality, but so in the construction techniques and materials. Most batteries are made with cells either "tabbed" (spot or laser welded metal strips are used to connect the cells together) or soldered. The best method is soldering, but only if it's done properly. Too much heat applied to the cell during soldering will damage the cell, and not enough heat will lead to a "cold solder joint" and the connection will fail mechanically. This is only recommended for those very experienced with soldering, and not for tinkerers. This is why tabbing is the most prevalent means of construction. The better quality batteries will have thick bussing (the metal strip connecting cells) with many weld points on each cell. Poor quality batteries use really thin bussing and barely enough welds to hold it all together, let alone pass alot of current.

Many batteries fail, either during use or charging (most common) as the construction is poor enough that sustained current during charging causes excessive heat to form at the poorest welds and has been know to destroy a whole pack. It's recommended that you buy the best batteries to power your gun, and I can tell you that $20 mass-produced batteries from e-retailers are not the best there is. Usually, the price and manufacturer will give you a clue. No-name cells is not a good indicator of a quality battery.

That's pretty much all the most common and generalized info I have available for basic motor and battery performance.

[diamond_SEA]

perhaps this should be stickied in the repairs section?

and +1 on the good charger, absolutely a good investment for the long run

[THE CHIEF]

this is really helpful you just fixed like 2 of my problems.thanks

[tycho]

awesome thread - would read again

[Ktown Militia]

+1 Mcguyver. straight from the textbook

[Syn]

Read this when it was first made, great stuff. Also good basic battery info here: http://www.airsoftretreat.com/forums...p?topic=4524.0