A Buyer's Guide to RC Batteries
There are many different types of rechargeable batteries used to power radio controlled models. Here I will take you through some of the terms that are used to describe the different characteristics of these batteries as well as each of the common types of batteries - their features, pros, and cons.
WARNING: Whichever battery type you are using for your radio controlled model make sure to use a compatible charger. Using an incompatible battery charger can cause damage to the battery and potentially lead to leakages of toxic chemicals, fires and even explosions.
Battery Chemistry/Types - This means the chemicals used in the construction of each battery type, such as nickel cadmium (Ni-Cd).
Capacity (mAh) - The capacity of a battery will be indicated in a number of milliamperes/hour. A battery with double the capacity used in the same appliance under the same conditions would last for around double the time. Some types of batteries have a much larger capacity for their volume than others, which means that the batteries can be made much smaller while having the same capacity as is the case with lithium batteries.
Battery Life/Life Cycles - The life of a battery is indicated as a range of life cycles, which are the number of times the battery can be drained and recharged before it becomes inoperable. The life of a battery can vary depending on how it is being charged and used, so even within a single battery type the battery life could vary by as much as several hundred life cycles.
Memory Effect - This is a term used to describe a battery 'forgetting' its full capacity. This can happen when a battery is not fully charged, or more rarely when it is not completely discharged before charging. The battery can 'forget' the amount that isn't charged. For example, if a battery is completely discharged then charged to only 75% of its full capacity, it may then only charge to 75% from then on.
Deep Discharge - Batteries are designed to discharge to a certain voltage. Deep discharging describes a battery being discharged beyond this point. Damage to the resistance of internal cells caused by deep discharging can impair the ability of a battery to charge. Some batteries are more resistant to deep discharge damage than others, and these are usually batteries that are also good for high output applications.
Thermal Runaway - When the heat generated within a battery exceeds what can be transferred to its surroundings thermal runaway occurs, where the internal temperature continues to rise faster and faster until it becomes critical. If not corrected the temperature can cause the battery casing to distort and breach leading to leakages and potentially catch fire. This is one of the main reasons why it is important to get the correct charger for your batteries to avoid overcharging.
Weak-cell Syndrome - This is a condition where one or more sells within the battery are no longer able to hold power. This condition is usually discovered when you go to use a freshly charged battery and it dies right away, despite testing showing that it is fully charged.
Ni-Cd - Nickel Cadmium
Nickel cadmium batteries have the longest battery life with up to around 1000 cycles. These batteries are able to operate across a wide rage of temperatures so will work just as well in both hot and cold weather. Ni-Cd batteries also have good high output and deep discharge properties and have excellent protection from thermal runaway.
On the downside Ni-Cd batteries have a much smaller capacity than other battery types and suffer from the memory effect. Availability of cells for this type of battery have become limited with many of the less common sizes having been discontinued by Panasonic after they acquired Sanyo a number of years ago.
Ni-MH - Nickel Metal Hydride
Ni-Mh batteries have about double the capacity of Ni-Cd batteries, but also weigh twice as much. These batteries do not suffer from the memory effect. They have good performance at moderate temperatures but can struggle in extremely hot or cold weather. Like Ni-Cd batteries they have good deep discharge properties, and Ni-Mh batteries have good protection from thermal runaway. Availability of cells for Ni-Mh is very good, which keeps the price relatively low. They generally have shorter battery life than Ni-Cd batteries at up to around 800 life cycles. The biggest drawback of Ni-Mh batteries is that they are more prone to weak-cell syndrome than the other battery types.
Li-Ion - Lithium Ion (Lithium cobalt oxide & lithium manganese oxide)
There are several type of Lithium-ion batteries. Lithium polymer (Li-Po) and lithium iron phosphate (Li-Fe) batteries are also lithium ion batteries but have sufficiently different properties that I will discuss their differences separately below.
Lithium-ion batteries have extremely high capacity, meaning they can be made very small and light, making them very popular in mobile devices such as mobile phones and are the most common types of batteries for radio controlled aircraft where weight is an issue. Li-Ion batteries do not suffer from the memory effect and have the best protection from deep discharge. These batteries generally have similar battery life to Ni-Mh batteries. Lithium batteries have much slower self-discharge rates so they will go flat much slower if you leave them unused for a time. Charge and discharge is very good, meaning they can be charged quickly and can provide high power output. Availability of materials for lithium-ion battery cells is very good.
There are a couple of downsides for lithium batteries. Firstly they have poor performance in extreme temperatures, particularly in cold weather. They are also the poorest in terms of safety as they have very poor thermal discharge and require an integrated circuit to avoid thermal runaway. The stories you may have heard of laptops and mobile phones having caught fire were caused by thermal runaway in the batteries. The chemicals contained in these batteries are highly flammable, and are potentially explosive if exposed to water so it is very important to check for damage regularly.
The other consideration with lithium-ion batteries is disposal as the chemicals they contain are toxic to the environment, particularly to water sources, and must be disposed of properly.
Li-Po - Lithium Polymer
Li-Po batteries are even lighter than Li-Ion batteries, but have a lower capacity. Their internal structure is gel-like so there is far less risk of leakage than with lithium-ion batteries. Over time the gel will start to harden, reducing the battery life, and these batteries can also suffer from the memory effect.
Li-Fe - Lithium Iron Phosphate
Li-Fe batteries have a longer battery life compared to other lithium batteries. They are also much safer as the chemicals they contain are incombustible and these batteries are not prone to overheat when in use or when overcharged. The chemicals are not toxic to water supplies in nature so disposal is easier.
Into the future - SSB - Solid State Batteries
Technologies for batteries made of all solid materials, with no liquid components, have been around since the 1830s but it has been only in recent years that there has been a real push in development of viable batteries driven by the rise in electric vehicles. Being made of solid materials they have the potential to be much safer than other batteries with higher capacity. There are a number of current complications with SSBs including durability, stability, and high cost.
Once SSBs become more common in mobile devices they will make their way into radio control models. With their potential to outmatch other batteries for capacity, weight, and safety they will likely become a very popular option.