Batteries for Model Aircraft Radio Control Systems

When I first entered the hobby and for a long time afterward, I had no idea what a Milliamp Hour (mAh) is, how to figure out what capacity battery I needed for any given application or how many flights it was good for.  I do not think anything confused me more than batteries.  In fact, I still do not fully understand it, so take that into consideration while you read this.

If you really want a thorough understanding of batteries, then you will have to look elsewhere.  This page will cover the basics as I understand and what I have gotten by with.

The scary thing is that there is no formula to determine battery size.  There is a good reason for that.  I am talking about onboard batteries now.

Milliamps (mA) are the capacity of the battery.  One mA = 1/1,000 of an Amp.

Think of mAh as a fuel tank.  More milliamp hours = bigger tank (more fuel).

Think of Volts as the octane rating of the fuel.  More volts = more power.

If you have a 600 mAh battery and you think you need more capacity then changing from a 4.8 volt pack to one having 6.0 volts is the wrong answer.  Get a pack having a higher mAh rating instead — an 800 mAh pack, for example.

If you want your servos to move faster or have more power and you do not want to buy faster or more powerful servos, that is the time to step up the voltage.

Milliamp Hours is the milliamp draw a battery can sustain for an hour (in theory) before the voltage drops to an useable number.  For example, a 600 mAh battery can sustain a 600 mA draw for one hour.  In theory, this means it can sustain a 1,200 mA draw for 30 minutes or a 300 mA draw for 2 hours.

The actual relationship is not linear, however, due to other factors such as internal resistance of the battery.  What this means is that as you load the batteries more, the voltage will drop off faster.  In the above example, a battery may only be capable of providing 1,200 mA for15 -20 minutes rather than the 30 minutes you might expect.

Battery Capacity

Battery capacity is dependent on several variables:

• The amount of current drawn by the equipment.  All else being equal, more servos = more current draw.  Additionally, servos vary considerably in their current draw and depending on the loads placed on them, the current draw can go up significantly.

  If the servo is stalled (loaded to the point where it can not hold its position), the amperage draw spikes and can drain the battery very quickly.

  This can occur for two common reasons:  Either the servo is too small for the job or there is a serious bind in the controls.  Both of these are easy to fix, but the former requires you to recognize the problem and replace the servo with one that can handle the load.

• How you fly your aircraft.  If you continuously yank and bank your plane around the sky then the servos are doing more work and will drain the batteries faster.  If you fly a slow floater and move the controls to correct the heading occasionally, then the battery will last much longer or a smaller battery can be used.

Choosing the "right" battery

So how do you determine how large of a battery to use?  Simple.  Ask somebody with experience.  Any standard Radio Control set you buy will work with a standard trainer and that includes the batteries and servos.

Standard servos and included battery packs are intended for models up to .60 size.  Again, this depends on the type of model.  If you fly a .60 size model that imposes high loads on the control surfaces, then standard servos will most likely be inadequate on the primary control surfaces.  But they will be fine for most sport models (trainers and Stiks) as long as you fly them within the realm of reason.

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Copyright © 2004 Paul K. Johnson