Lithium-Polymer Batteries — Part 3

I hope you’re hanging in there, as we board the train to happy town and head on to the land of li-poly euphoria.  In part one we dispelled the myth that li-poly batteries are prone to go nuclear.  We also examined the real reason why guns with li-poly batteries go KABOOM!  After clearing the air of those misconceptions we ventured on to part 2 where we went over the mechanics of charging and caring for your new found friend.  All aboard as we head on over to the other other side of the battery world for what might just be our final stop.

So what now?  Well, let’s talk a bit about picking out a li-poly battery and where to buy them from.  After KWA came out with their “Li-Poly Ready”, M4 (which is a bit misleading), airsoft retailers began to claw their way into the li-poly market.  The most common offering I see is one by Firefox, but Evike and others are marketing their own in-house brands as well, and it looks like King Arms is about to start in as well. In the end these all probably come from the same factory, but that is neither here nor there.  Let us dissect the nomenclature of the li-poly battery.  We’ll start with an example:

7.4v (2S) 2200mAh 25C

Seems confusing, but it’s not.

7.4v: This refers to the voltage of the entire pack. Since each cell holds 3.7v, this pack must have 2 cells wired in series.  Common pack voltages you’ll see for airsoft use will be 7.4v and 11.1v.

2S: This refers to the number of cells in the battery pack, and the fact that they are wired in series.  If they were wired in parallel it would have said 2P instead.

2200mAh: As usual, this is the capacity of the battery in milliamp-hours.  The larger the capacity, the longer the battery will last before needing to be recharged.

25C: We talked about this in part one.  It’s the discharge rate.  If you don’t remember what the discharge rate is all about please go straight to part one, do not pass Go, and do not collect anything but nasty looks from the gnomes who write these things.

Selection

Now that we have the nomenclature down, we need to concern ourselves with the choices that are out there.  First let’s see where we’re putting our pack and determine how much space we have to work with.  With that sorted out, we know our physical space limitations.  Now let us look into what it is that we want out of the AEG we’re putting this into.  If you’re looking to create an ROF beast, then an 11.1v battery is your goal.  If you’re simply looking to replace that mini 8.4v or 9.6v NiMH or NiCad battery, then a 7.4v li-poly should fit the bill nicely.  Your next concern is capacity.  How do you determine the capacity you need?  Well, it depends on a lot of things, and there is no real concrete answer to this.  If you have a gun that runs at a high fps, you’re going to need more capacity as it’s going to be used up more quickly.  Now looking at reality, I’ve seen an m14 DMR running semi-auto only, go through 700 rounds and the 2200mAh li-poly battery was still almost full.  Your real limiting factor here is space.  You can use a battery as big as the space you have to fit it in, but you might want to consider using this battery in more than one gun, so you have to fit the smallest denominator.

Now onto the discharge rate.  This is where the money is.  We’ve done the math on this a few times, but we’ll do it one more time, but with an actual battery.  Let’s look at the Firefox brand li-poly battery that is available at several HK retailers.  The specs read as follows:

11.1v 1600mAh 12C

Doing the math we find out that this pack has a continuous discharge capability of 19.2A.  What a failure. Why spend the money on a charger, a balancer, and a li-poly for a discharge rate of 19.2A?  Now let us look at a 7.4v li-poly made by Elite for just $6 more:

7.4v 2200mAh 30C

Doing the math, you get a 66A continuous discharge capability!  So sure, we drop back to 7.4v, but we’re back to supplying the motor and drivetrain the current it needs to run at 100%!  So in this case, the 11.1 wasn’t a win because of its low C rating.  Once you’ve decided on capacity and voltage, you need to consider the discharge rate carefully.  As proven here, the low discharge rate is a real deal-breaker for the Firefox batteries!

So how big of a C rating do you need?  Good question.  Really you just need to make sure that your battery can put out at least 30A CONTINUOUSLY.  Ignore peak ratings, as they don’t play a part as much as the continuous rating.  If you can supply more than 30A with the battery of choice, that’s fine.  Remember, you’re motor only takes what it needs.

Another important aspect of buying li-poly batteries is the retailer.  Me personally, I prefer to buy my li-poly batteries from RCLipos.com, as their service is top notch and they are knowledgeable about the product they sell.  Buying from some of the HK stores could leave you high and dry if you have a question about the batteries, or need help after the sale.  Stores that supply RC hobbyist tend to provide the best service and be more knowledgeable.

One more thing…

One thing that I neglected to mention is that most li-poly batteries come without a power connector.  While they have power wires, and a balance connector, they often are power plugless. This is easily remedied by soldering on a pair of Deans connectors (my connector of choice), or crimping on some Tamiya connectors (the bane of my existence).  Honestly, if you’re using Tamiya connectors, you’re hurting yourself by using a poor quality connector with a high quality (and output) product. <commence gnome attack on Tamiya connectors now>  Now, if you’re afraid to put on your own connectors, or don’t want to deal with it, many RC battery stores will put the connectors of your choice on the pack for an additional charge.

Pricing

Okay, I guess you guys have been good on the trip so far, so I’ll put together a little price list as a reward.

First up, the low cost setup (with a high quality battery) ($110.84+shipping)

Product Retailer Price
Elite 7.4v 2200mah 30C www.RCLipos.com 42.00 (free shipping)
Lipo Sack www.RCLipos.com 24.95 (free shipping)
ACM Balance Charger RSOV.Com $43.89 + shipping

Now for a setup almost identical to what I use with my M249 ($274.95+shipping)

Product Retailer Price
Elite 11.1v 2200 mAh 30C www.RC.Lipos.com $60.00 (free shipping)
Triton 2 Charger any R/C hobby shop $130
Power Supply (from a PC)* any computer store $20
Equinox Balancer any R/C hobby shop $30
Lipo Sac www.RCLipos.com $24.95 (free shipping)
Lipo Low Voltage Monitor Ebay $10.00

Okay, don’t let these prices fool you.  There are even cheaper options out there as well as more expensive.  Li-poly systems run a huge range of prices, so don’t be afraid to look for one that fits not only your gun, but your budget as well!

Conclusion

Well, we’re pulliing into the station here at Li-Poly Euphoria, and boy has this been a trip.  We know now that most of us are running our AEG’s at less than 100%, are using underpowered power supplies (batteries), have found out that li-polys are innocent of AEG murder in the first, and now know how to pick out a li-poly.  We’ve seen the cost of switching to li-poly batteries and realized that it’s not unobtainable.  But wait…. yeah you in the back.  What’s you’re name?  Doubting Thomas? Well, I thought our little trip to Li-Poly Euphoria would convince you that NiMh and NiCad technologies were a thing of the past, but that’s fine.  Join me at the bar in part 4 as we take a look at li-poly and NiCad and NiMh batteries one more time and see who the real work horse is in terms of watts.  Won’t you join me at the bar and drink some of that li-poly Kool-aid?

Part 4 coming soon!

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5 responses

20 11 2011
Zach Wittenberg

can you tell me what would be the best items to purchase for this gun and what upgrades would be necessary for it to run a lipoly? http://www.evike.com/product_info.php?products_id=29642

20 11 2011
Zach Wittenberg

can you tell me what would be the best items to purchase for this gun and what upgrades would be necessary for it to run a lipoly?? http://www.evike.com/product_info.php?products_id=29642

16 02 2012
Valentin Cubillos

Hi, i’ve like very much this article series.
I’m in a big trouble about the current issue, because a buyed a new KMA SR10, ans the specs says that accepts “9,6VNiMh, 7,4V or 11,1V 15C LiPO/LiFe”.
I play in the desert (here in the north of Chile) so we need batteries with a big mAh to play without going powerless and walk half a mile to pick another (2000mAH +).
Surfing in the KWA forums i found some guys who reccomend no to use a battery with a mAh x C result bigger than 24A, but i disagree, because knowing that the resistance (the airsoftgun internals) is basicly constant, there might be a difference between a 11,1V and a 7,4V with the same mAH and C rate.

4 02 2013
RJ45

How old is this blog? I don’t Elite as an option any more.

18 09 2013
indianajbird

I’ve been drinking the lipo koolaid so far but while waiting for my fourth cup I have a question.
If we have a battery.
7.4v 2200mAh 2s 25c.
And we are allowed to drain it to 3volts per cell. That would only give us 1.4 volts of actual usage until we here the faint sound of mice screaming from the stock of my g&g combat machine rk47. (65 bucks off ebay) anyway that’s only 20% of the actual total voltage. Am I to assume that we can also only use 20% of the 2200mAh as well ? If so it sounds like it really wouldn’t last that long considering I have to leave 80% of the voltage or untouched. Also what are the advantages of the 11.1 v over the 7.4v consedering I’m only going to be pulling 20amps or so of koolaid power anyway.

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