Lipo Battery questions (KM4 SR-10 & M249 by Echo 1)

Discussion in 'Gun Building, Modifications & Repairs' started by howie3601, Aug 3, 2015.

  1. howie3601

    howie3601 New Member

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    Any of you guys that have a lot of experience I really need some help from.

    I just bought a KWA SR-10 used (Great Condition) Shoots great.
    And I bought a used M249 SAW Para version by ECHO 1

    The M249 came with a 7.4 Lipo battery. I decided to hook it up to my KWA KM4 SR-10. The trigger response and rate of fire is amazing! (I can put it in my KM4 because it's to wide for the crane stock)
    I realize that lipo batteries can put more stress on your gear box but I'm sure it takes a long time if you are not blasting it on full auto all the time.

    What I would like to know is how long will a 7.4 Lipo battery (Random "Normal" mAh) last me on the field at a milsim event for 2 days? How many Lipos do I need to take? I would love to know. also what mAh would yall recommend and this thing called a "C" rating. Still confused on the C rating.

    And just to kill 2 birds with one stone
    How many Lipo batteries will I need for the M249 (Since it's fully auto only)?
    I plan on bringing it too.

    If you're interested in the event I am going to it's Operation Black sheep. The one event being hosted in Perry GA at the Guardian Center.

    Thank you and have a great day!
     
  2. Tyrnek

    Tyrnek Member

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    Lipos can strain the GB by increasing the RoF, but the main issue comes from the added strain on your electrical system. With a 7.4v, it should be roughly the same amount of stress as a 9.6v Nimh, but you should install a MOSFET if you decide to bump up to an 11.1v... and tune your GB to handle it.

    (Funnily enough, while full auto might strain the mechanical components of your GB more, semi is very strenuous on your electrical system.)

    As for battery life, Lipos generally last AT LEAST as many rounds as their mAh, and often go far longer than that. Generally, the rounds-to-mAh ratio gets higher as the mAh capacity gets bigger, so in regards to your question about mAh capacity, it's "The biggest that you can get to fit". As for how many you need, consider what type of shooting you'll be doing (full auto or semi?), how much ammo you expect to fire, and the size of your batteries. Find the number of batteries that meets these requirements, and then toss in one extra.

    For the SAW, to find the round count for a given mAh, multiply the mAh by 1.75. This is extremely conservative, but considering your past experience with Lipos (none), being overly cautious is better than having a Lipo catch fire in your gun.

    Speaking of which, get a dedicated Lipo balance charger and a voltage monitor, and then read this: http://www.rchelicopterfun.com/rc-lipo-batteries.html. Lipos give you better performance. The tradeoff is that they require greater care in handling.

    Oh, and one more thing: Always balance your cells, and never let the voltage in any cell get below 3.7V.
     

  3. BoogerMc

    BoogerMc Airsoft Jedi Master Supporting Member

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    All good advice, but now for some experience in field. I use a 7.4v 1,600mah lipo for my SAW (follow my builds in my links in my signature below) and my M60 and I will run out of ammo before I run out of battery. On average, I can get between 1,000 and 1,500 rounds on a single charge. I've never gone beyond three or four boxmags on either gun in a single, eight hour day of play.

    However, I do not recommend a MOSFET for the SAW even with an 11.1v battery because the sustained full auto tends to overheat the MOSFET and fry it. Even with an enlarged heatsink, it's just too much for the MOSFET. The trigger switch in the SAW will handle the current just fine, after all, they are designed for use in automobiles and other mechanical systems and can handle much higher voltages.

    Although, I do recommend doing some upgrades to the SAW's gearbox to accommodate the lipo over a long period of time.
     
    Last edited: Aug 6, 2015
  4. Tyrnek

    Tyrnek Member

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    ^ Good to know, did not know specifics about SAW anatomy.
     
  5. Airborne101

    Airborne101 Well-Known Member

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    While they may be rated for the higher voltages, they are not rated for the currents that our guns put through them, especially when upgraded.

    http://extreme-fire.com/Micro-Switch-Bounce.pdf
     
  6. AirsoftRyan778

    AirsoftRyan778 Active Member

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    Voltage and amps are 2 different things, extreme fire makes some fantastic mosfets, but a basic nukefet should get the job done also. If your mosfets are burning out you are getting some cheap mosfets, my BTC can handle 18v and 540 amps, and I run it on a 11.1v 4000mah 45-90c lipo, which is huge and weighs almost a pound, so saying that mosfets will burn out is far from accurate, although there are plenty of mosfets that will
     
  7. Airborne101

    Airborne101 Well-Known Member

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    Booger does have a point. The ratings you posted, 18v and 540 amps, is in a perfectly ideal setup. That means you have a fully effective heat sink attached to it. Simply put, if you ran the MOSFET at those numbers, you would fry it, because it isn't properly heat sunk. If it was, it would be quite a bit larger. If you can't channel the heat away from the chip, the rating doesn't mean squat.

    Further more, a gun only pulls the amps it needs. I don't care how big or powerful your battery it, the battery doesn't "push" the amps. The gun draws the amps. So even if your battery can generate 180amps continues, your gun likely isn't drawing that, other than at startup. With volts being set, that produces a power in watts, which can be converted to heat. That is the heat that you have to move away from the chip.

    I believe a well made mosfet is capable of operating with your run of the mill support weapons. I know booger however, runs a very upgraded M60, which I can see him frying MOSFETs since they aren't very well heat sunk.
     
  8. AirsoftRyan778

    AirsoftRyan778 Active Member

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    Your hun will draw more amps if you are using stronger springs, are using a high speed motor, having bad shimming, pretty much, tbr more strain you put on the motor, the more amps it will draw
     
  9. TheInfidel23

    TheInfidel23 Resident Derp Supporting Member

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    That's not the point. Even if a BTC product could handle multiple hundreds of amps, your wires and connectors would melt far before anything else had a chance to.

    Unless you're doing something crazy, continuous amps should never top 30-35.
     
  10. Airborne101

    Airborne101 Well-Known Member

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    I don't think it was claimed otherwise.
     
  11. BoogerMc

    BoogerMc Airsoft Jedi Master Supporting Member

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    Last edited: Aug 8, 2015