(article updated June 2011)
There are three main types of rechargeable batteries in common use for electric powered radio controlled model aircraft: Nickel Cadmium (Nicad or NiCd), Nickel Metal Hydride (NiMH) and Lithium Polymer (Lipo or Lipoly). All three types are very commonly sold on eBay. This guide explains the advantages and disadvantages of each type, how battery capacities and maximum safe current loads are expressed and how to compare battery packs.
Batteries for model aircraft usually come in the form of "battery packs" made up of several "cells". The cells are joined together by soldered metal strips, and then the whole pack is usually heat shrink wrapped in tough plastic material. The cells can be arranged in different patterns in the pack, to give many possible pack shapes, to fit neatly into the battery bay of whatever plane you have.
It is possible to buy individual cells on eBay, solder them and heat shrink wrap them yourself, but this requires skilled soldering and I don't recommend it for beginners. If you are not careful the heat from the soldering iron can damage the cells.
Here are a couple of examples of battery packs, on the left an 8-cell NiMH pack, nominal voltage 9.6V, capacity 1000mAh, on the right a 3 cell Lithium Polymer pack, nominal voltage 11.1V, capacity 2200mAh.
There are several numbers to know about in order to compare batteries.
The voltage of the battery pack will depend on the number of cells in it and the type of cell. For example, lithium polymer cells have a nominal voltage of 3.7 volts, so a battery made of 3 of them connected in series will have a voltage of 11.1 volts. NiMH cells on the other hand have a nominal voltage of 1.2 volts so an 8 cell pack such as the one above will have a voltage of 9.6 volts. Bear in mind though that these are nominal voltages - a freshly charged pack will have a higher voltage, so don't be surprised if you measure your 8 cell NiMH pack with a voltmeter and find it is actually around 11 volts!
The performance you get from your motor will depend on the current going through it, measured in amperes (A) or amps for short. A milliampere (mA) or milliamp is a thousandth of an amp.
So you can compare batteries to estimate flight times etc. batteries are rated in Amp hours (Ah) or milliamp hours (mAh). For example, if a battery has a 2000mAh rating, this means that you can draw 2 amps (2000mA) from it for an hour, OR 1 amp for two hours, OR 6 amps for 20 minutes - you get the idea. So if you have a 1000mAh battery and you know your motor draws 5 amps from it at full throttle, then the theoretical flight time at full throttle would be 12 minutes (or one fifth of an hour). For batteries of the same voltage (or number of cells), the flight time will be roughly proportional to the mAh rating of the battery.
The capacity number of a battery is often referred to as "C" when describing the maximum safe current you can draw from it. For example, a 2200mAh battery has a "C" of 2200mA. The seller of the battery may describe the maximum safe current draw for this battery as 10C, which means that you can draw 2200mA x 10 = 22000mA or 22 amps from it safely. Another battery with the same capacity may have a 15C maximum current rating, meaning you can draw 33 amps from it. This battery will not give you any better flight times with the same motor, BUT you can use a more powerful motor, because the battery can deliver a greater current. The battery with the higher C rating will therefore usually be more expensive, everything else being equal. Sometimes batteries will have a continuous current rating, and a higher burst current rating, which can only be sustained for a short time, say 20 seconds. e.g. 15C continuous current, 25C burst for less than 20 seconds.
It is unsafe to exceed the maximum current rating of a battery.
1. Nickel Cadmium (Nicad or NiCd)
These were the first batteries to be used in battery powered planes, and they now represent old technology compared with the other two types. For a given weight, the other two technologies now have much higher capacity, so Nicads are now mostly used for transmitter batteries or other applications where weight is not an issue.
A big disadvantage of Nicads is the so called "memory effect". If you recharge a Nicad battery that has not been fully discharged, this can permanently lower the capacity of the battery. This is inconvenient, because it means you shouldn't "top up" the battery with your charger before flying.
The nominal voltage of a single Nicad cell is 1.2 volts.
2. Nickel Metal Hydride (NiMH)
NiMH cells are available in the same shapes and sizes as Nicads and have the same nominal voltage of 1.2 volts. Recent improvements have increased their capacity so they typically offer at least twice the capacity for a given weight as Nicads, with comparable maximum current values. And generally they have much less memory effect than Nicads, so you can recharge regardless of how charged up the battery already is. A disadvantage of NiMH batteries is that they have a high "self discharge". This means that they go flat even when they are not used, typically losing about 1% of their charge each day. For model aircraft use you will normally be charging them not long before you use them, so this is not a big disadvantage. If you put them in a torch for emergency use though, where you might not use it for months or even years after you charge them, this is a severe disadvantage.
There are now NiMH batteries of a newer type that have a much lower self discharge, such as the Sanyo "eneloop" and other similar items from other manufacturers. Unlike the earlier NiMH batteries, these ones are usually supplied already charged so you can use them immediately.
If you need the low self discharge type NiMH battery you will need to check with the seller whether the batteries you are buying are of this type.
3. Lithium Polymer (Lipoly)
Lithium polymer batteries offer another huge jump in performance, but you must be very disciplined in their use, or they can be very dangerous (they can catch fire or explode if mistreated). The nominal voltage of a single cell is 3.7 volts, so there is less flexibility available to build a pack of a desired voltage compared with NiMH cells. The nominal voltage when the cell is fully charged is 4.2 volts, so most chargers will automaticlly stop charging when this level is reached.
If a lipoly pack is discharged below about 3 volts per cell, it will permanently lose some or all of its capacity. For this reason, if you are using an electronic speed controller (ESC) with an automatic cutout on the motor circuit when the battery gets low, it is important that the cutout voltage matches the lipoly you are using. If using a 3-cell lipoly the auto cutout should happen at about 9V(=3x2.7) and for a 2-cell lipoly at about 6V. Many ESCs are programmable for different cutoff voltages. The idea is that the automatic cutout switches off the motor, but still keeps the servos for the control surfaces alive, so you can do an elegant dead-stick landing before the battery runs out of energy completely! Some ESCs allow the motor to be restarted after the cutout happens, and some don't.
Another consequence of the intolerance of the lipoly to discharging below 3 volts/cell is that you can never "cycle" the battery - i.e. fully discharge it and recharge it, the way you can with the other technologies. This means that over time, the voltages of the cells in the battery can become unbalanced. For example, if you are charging a 3 cell lipoly, your charger will not charge it beyond 3x4.2 = 12.6 volts. However, one cell could be at 4.6 volts, and the other two at only 4.0 volts. The cell charged at 4.6 volts risks damage through overcharging.
To counter this, many batteries now have an additional small white plug called a "balance plug". If your charger supports it, you plug this into the charger and it monitors the voltage of EACH cell as well as the total charge. If one cell has a higher voltage relative to the others, it will temporarily suspend charging that cell to allow the others to catch up. That way the battery cells can be kept in balance and the battery will last longer, as no cell will be overloaded. Note that the accompanying photo of a lipoly shows one with no balance plug, but I will update it when I get a chance! The first version of this guide was written when lipolies with balance plugs, and chargers that could support them, were much less common than they are now.
Remember always to disconnect the battery from your plane or charger when you are not using it, so there is no risk the battery can be accidentally discharged to below 3 volts per cell.
Another useful thing to know is the convention to describe how the cells are connected in a battery. For example, a battery with 3 cells connected in series may be referred to as a 3s battery. The "s" is for "serial", and a battery constructed this way will have a nominal voltage of 3 x 3.7 = 11.1V. Similarly, "p" is used for "parallel", so a 2p battery will have two cells connected in parallel, and a nominal voltage of 3.7V. It is possible to have a 4s2p battery - this would have 8 cells, and consist of four pairs of 2 cells, each pair connected in parallel but connected in series to the other pairs. Such a battery would have a nominal voltage of 4 x 3.7 = 14.8 volts. To labour the point, sometimes 3s batteries are referred to as 3s1p.
Lipolies have relatively low internal resistance, so less of the battery's energy is wasted in heating the battery itself, compared with the other technologies.
Often overlooked by buyers is the type of connector between your battery and the rest of your equipment. However, the connector must be appropriate for the job, as the safety of your model depends on it. I can tell you from personal experience it is not a nice feeling to suddenly totally lose control of your model, just after you have started to fly it upside down, because a connector stopped connecting!
My own personal favourite is the Deans Ultra T shaped connector shown below, and I now use it for any battery that I expect to deliver a current over 10 amps. It is good for at least 40 amps. If you are in doubt, ask the battery seller to tell you what type of plug the battery has and what current it is rated for. The Deans plug is starting to be superseded by others such as the XT60, that is said to be better, but mine are still all Deans, because there would be a considerable soldering effort, as well as cost, in changing all the batteries and ESCs to the new plug.
Any battery is potentially dangerous if wrongly charged or abused, with dangers coming from fire, explosion or injury from leakage of toxic chemicals. You should become an expert in each type of battery you use, how to charge and handle it safely, and how to dispose of it without damage to the environment when it has reached the end of its life. This guide is not meant to be a comprehensive one on battery safety, but here is a list of some of the precautions you should take:
- ALWAYS use the correct charger for the type of battery you are using
- NEVER charge a battery near flammable materials
- NEVER leave a battery charging unattended
- NEVER short circuit a battery or exceed the maximum current rating
- If a battery is damaged, do not continue to use it, but dispose of it safely
- Lithium polymer batteries have particular hazards associated with them. DON'T use them unless you are prepared to learn and practise the appropriate precautions