battery problem, smaller battery won't cycle

[Fox Hound]

Originally Posted by mcguyver

Nope. You are wrong. I've been down this road before and figured this out years ago, just through trial and error. A 9.6v 2600 mah battery in the PEQ of an old M4 I had would not run an M110 spring, but as soon as I tried an 8.4v 2400 mah battery, it would run the gun all day. The 9.6v used AA cells, and the 8.4v used sub-C cells.

The discharge rate does not cover in-rush, which for an 8 amp draw can equal many times that, maybe 30 amps or more easily. 14 amps does not equal 30, and that's why your motor can't fully start and cycle the gun.

i guess i could be wrong, but i have no idea what you're talking about in your last sentance but at 30 amps the wires would fry. Thought i'd throw that in.

[mcguyver]

Originally Posted by Fox Hound

i guess i could be wrong, but i have no idea what you're talking about in your last sentance but at 30 amps the wires would fry. Thought i'd throw that in.

No, the wiring won't fry. The inrush is cyclic, only occuring at start up and diminishing very shortly after to your actual running load. The motor will draw a large pulse of current during this time, current that the battery must deliver. But that cell size in incapable of doing a large discharge like that, and larger cells are required. The wiring in your gun can handle current into the hundreds of amps for milliseconds, and many tens of amps for several seconds or longer, more than enough for your motor.

[mcguyver]

Even the fuse will last for a time with amperage in excess of the rating.

It seems a little "Motors 101" is needed here.

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 do 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.

[Styrak]

/epic motor & wire post

[Drake]

Seriously. It's one of those things I knew happened, but didn't know why it actually behaved that way. This should be stickied for mcguyver's excellent explanation.

[m102404]

mcguyver...very nice explanation...posts like those should be clipped out and posted as a FAQ/Sticky (maybe not this entire thread and certainly not the crap that I write). A non-electricians guide to using a multimeter for AEGs would be helpful to.

Further to the battery cell comparisons...here's a workbench example that I was working on/testing last night. Just my own tests...noting scientific or lab quality...just layman observations.

I have three packs...freshly/fully charged...and 2 M4 guns, one with a full m120 spring (400+fps), the other with a m100 (for whatever reason it shoots 384fps, probably a mis labeled spring)...and one mechbox with a stock CA spring (haven't chronied this spring in ages, but if I remember right, it was shooting 320-ish).

Pack #1 is a butterfly 9.6v 2300mah AA cell type battery. Pack #2 is a 9.6v 1500mah custom 2/3A cell sized pack (fits a MOD stock) that I made a couple of nights ago. Pack #3 is a 9.6v 2xxx mah crane stock sub-C cell size pack.

Pack #1 will cycle the CA-spring mechbox without problems, labours a bit with the m100 and does not do well with the m120. It'll crank it a couple of times but it's boarderline (i.e. wouldn't use it in a game or even for testing). Objectively...a 8.4v 600 or 1400mah 2/3A size pack works much better. Also, this 9.6v AA cell pack would not crank, not even budge, my old G36 with a M120 spring, but it would cycle my AK with a 2-3coil cut m120.

Pack #2 is pretty fast on the CA-spring mechbox, runs stong and fast. It'll crank the 384fps gun no problems, but I don't know how long it'll last in a game (my "normal" minis, 8.4v 1400mah intellects, last until just before lunch). It seems fine for the full m120 gun but I suspect it'll fade before lunch at an all day game. It's not really fair to test it now while it's cold outside.

Pack #3 is really fast on the CA-spring mechbox...so much so that I stripped something (bad, bad, grinding sound) after about a nonstop 600-700 round burst (no bbs, just my finger over the nozzel). I was actually trying to test the "light" mechbox to destruction, so I this was actually the last pack I tried on that mechbox. Pack #3 will run the 384fps gun with no problems at all and will probably go all day and then some. It'll run the m120 gun all day and had lots of juice to spare at the end of a hot/heavy game (e.g. Boarder Wars 2007...went through at least 50 mags throughout the day).

So...what this all means...to ME...is that small cells may get you the same voltage rating (i.e. 8.4 or 9.6) but they don't have the same "guts" to sustain that power output. Pushing them hard, either because that's all you can fit in the gun or just for the hell of it, will just kill them quicker. Also, there can be significant overspin with some mechbox builds. I believe that if the piston comes to a significantly compressed state (e.g. 70-80-90% cocked) with a strong spring, small cells may not have the discharge capacity left in the pack to "start" up the motor and finish the cycle. Holding down the trigger trying to get it to go just overheats the wiring, motor, blows fuses, etc... It's funny, if it didn't work the first time, it's probably not going to work a 0.5sec later...but everyone pulls the trigger again, just human nature.

Hope that helps all the non-electrical people like me out there...

BTW...it's a lot of fun to run a mechbox so fast that it tears itself apart, the amount of heat build up in the mechbox shell was "cool" to observe...now I've got a build to do for the weekend! Running out of spare parts though...

Tys

[MADDOG]

Originally Posted by Fox Hound

i believe the size doesnt matter as long as the specs are correct. If its 9.6V it has a potential of that, and the milli amp rating would only show how long it would last.

the Discharge rate of a 2/3A battery is 14amps, that is more than enough to work the motor. the motor will draw as much as it can and im pretty much confident that its the battery being weak since i can only draw about 8 amps now from 10amps.

Hey Fox,

You defintely need to run sub-c with this setup or a Lipo 11.1
I tried the AA cell packs and it will not work. Learned the hard way from spending money on multiple batteries because I thought they were all duds.

Lipo is your best bet for the ready mag.

MD

[Styrak]

Originally Posted by MADDOG

Hey Fox,

You defintely need to run sub-c with this setup or a Lipo 11.1
I tried the AA cell packs and it will not work. Learned the hard way from spending money on multiple batteries because I thought they were all duds.

Lipo is your best bet for the ready mag.

MD

I'm not sure about that motor, but I'm running a Systema M120 on a 2/3A 8.4V mini and it works fine. Does the type of motor affect it much? As in, would the motor he's using require a bigger battery?

[McKee]

Just a note on LiPo, im sure it's been posted before however,

YouTube - lipo battery overcharged and blown up

DONT OVERCHARGE THEM FOR GOD'S SAKE.