Batteries tutorial

When I first wrote this in 1998 almost of the batteries used in electric flight were NiCd (pronounced Nick-ad or Nye-Cad, take your pick) rechargeable cells. For many purposes these are still the best cells to use. These all provide a voltage of 1.2V and come in a range of sizes and capacities. Because 1.2V is pretty low we normally use packs made up of a number of cells. E.g. 7 cells will give 7 x 1.2 = 8.4V. When we talk about batteries or packs it is normal to refer to the number of cells rather than the voltage e.g. a 16 cell pack.

Since cadmium (the Cd in NiCd) is extremely poisonous there has been a concerted effort to replace NiCds in the consumer market. The main rivals are NiMH cells (Nickel Metal Hydride). These are similar to NiCds in voltage, size, shape etc. Their advantage is that they tend to have a higher capacity for the size but they are usually not able to provide as much immediate current as NiCd and then cannot be recharged as fast. Both of these "problems" are being addressed by the manufacturers though you should remember that these batteries are not made specially for us, the real high volume market for them is power tools and other consumer devices. If that market demands the ability to recharge in 15 minutes then we'll reap the benefits.

There are various different shapes and sizes of NiCd/NiMH cell in common use. These also have varying capacity as shown in the table below. The capacity of a cell is measured in Amp Hours (AH) or more commonly milliAmp Hours (mAH).
1 AH = 1000 mAH = the ability to deliver a current of 1 Amp for 1 Hour
or 10 Amps for 6 minutes (1/10th of an hour)
or 30 Amps for 2 minutes etc.
Note that this is not related to voltage (all the cells give 1.2V nominal). It a characteristic of the cell type and size.

The data in the table is for Sanyo NiCd cells. These are the most widely used and the most reliable. There are some other reputable names including Panasonic and Gold Peak (GP). In my opinion it is not advisable to trust your plane to the no-name batteries sometimes available from Tandy/RadioShack etc. But it's up to you.

Battery Coding

Sanyo make many different types of rechargeable cell. They are coded so you can tell what sort you have (most of these codes apply to cells made by Panansonic too).

The first few characters in the cell designation, eg "KR", "RC" or "N", represent different types of internal construction. Cells coded "RC" are optimised for RC use, eg RC2000, RC1700.

"N" and "RC" cells have a lower internal resistance than an equivalent capacity "KR" cell. This makes them suitable for our fast charge/high discharge-rate type of application. The low internal resistance means less heating of the battery and therefore less wasted power.

The numbers give the cell capacity in mAH e.g. 1000, 1700.

The letters immediately following describe the case diameter.
(Increased capacity is obtained by making the cell longer.)

Case Code Diameter Std Length

AAA 10mm 45mm

AA 14mm 50mm

A/AF 17mm 50mm

SC 23mm (sub-C) 43mm

C 26mm 46mm

D 34mm 58mm

The final character/s R, E, K, etc represent the cell performance or capability. Of these, it is "R" which is normally of most interest to us as this means the cell is suited to rapid charge/discharge. Some "E" cells are also used, "E" standing for extended capacity.

Thus 1000SCR = 1000mahr in a sub-C Case and is an "R" type (high rate) cell.

1200AE= 1200mah in an "A" case and is a E-high capacity cell.

A listing of most of the currently available NiCd/NiMH cells can be found on Sanyo's own website at Battery Data.

Other battery types

There are plenty of other battery technologies which are used, generally for specialised areas of our hobby. Lithium Metal (Tadiran) batteries came and went again. They are no longer produced. Lithium Ion rechargeable cells look promising but they have some unfortunate characteristics, like a tendency to explode if you abuse them and an inclination to simply die if you overcharge them or discharge them too far.

The latest cells which look very promising are Lithium Polymer cells (abbreviated to Li-poly or even Lipo). These are basically Lithium Ion technology with some of the problems ironed out. In particular they are in strong "plastic bags" rather than metal cases which both makes them lighter and means that instead of blowing up into lots of shrapnel when abused they just blow up like balloons. Much safer.

These cells deliver 3.7V rather than the 1.2V of NiCD or NiMH so you need less cells connected in series to give the voltage required. They will not deliver very high currents but they are very much easier to use in parallel to increase the load. Hence you will see pack designations like 3S2P meaning 6 cells connected as 2 parallel sets of 3 in series. If one cell gives 3.7V and a maximum of 5A this would give you 11.1V and 10A.

However they do still need VERY careful charging with a specilaised charger if you want them to have a reasonable life. They are very easy to destroy by overcharging or by discharging to too low a voltage. IMPORTANT you must never use a standard NiCd/NiMH charger on Lithium Ion/Polymer cells. It will kill them in a very short time. At the moment (April 2003) they are mainly being used in small Indoor models and Park flyers, anything requiring fairly low currents, but the manufacturers like Kokam are working on improving the size and current delivery to make them suitable for larger planes/motors.