How to Choose and Where to Buy Lipo Batteries

Given the wide range of actuators and electronics which go into a RC model, choosing the right battery may not be an easy task. This article guides you through the thought processes involved in choosing one or more 1s lipo battery for your RC models. 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 RC car batteries lipo 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.

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.

Let’s look at the Gens ace brand li-poly battery that is available at Genstattu.com.  The specs read as follows:

11.1v 1600mAh 12C

If you are using RC servos which operate at 4.8V to 6V nominal, an 11.1V LiPo will kill them, so you either need a separate battery or a voltage regulator. The C rating is the capacity and the discharge rating is normally provided as a multiple of the ‘C’ rating. The capacity relates to how long the robot will last. For example if your 11.1V battery has a capacity of 2000mAh (2Ah) and can discharge at 5C, that means it can discharge at 10A, which is more than enough for your motors.

Now let us look at a 7.4v li-poly made by Gens Ace 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!

Be careful when using 7.4V LiPo batteries with RC servos (which normally work best at 4.8V to 6V). LiPo batteries can normally discharge at high current (a meaure of the capacity). The higher the capacity, the longer the robot will run, but also the heavier the battery will be. For hexapods similar to Lynxmotion’s, a 2-4Ah battery is best, and you might get 15-20 minutes of use. 800mA won’t last too long.

Conclusion:

LiPo batteries have the potential to overtake NiMH batteries in general use in the next few years, quicker than any battery in history. It is certainly an exciting time for the hobby, and things are changing on a frequent basis. Just remember to have fun, and if you don't know something, ask questions! The only dumb question is the one you don't ask!