Understanding & Caring For NiMH Rechargeable Batteries
Those of you who have chosen to go down the Glow/Nitro power source for your trainer will have to use a battery to power their receiver. This is most likely to be a NiMH Rechargeable Battery.
On the other hand, if you’ve chosen an electric motor option then it is very likely that your receiver will take its power from a ‘Battery Elimination Circuit (BEC) integral with your Electronic Speed controller (ESC). Some ESCs are supplied without a BEC option in which case you may choose to power your receiver via a separate battery.
Many people new to rc flying have very limited knowledge of NiMH batteries and how to care for them. This post will provide all the information you need to get the best out of your NiMH rechargeable batteries.
A Bit Of History
The first miniaturised rechargeable batteries suitable for radio control aircraft applications were Nickel-Cadmium (NiCd) types. The technology for these batteries was perfected in the 1950s. The only alternative at the time was the throw-away alkaline batteries which, although having greater capacity than the NiCds, could not be recharged.
These NiCds would only run for 1/10th to 1/5th as long as alkaline types. The big advantage was that when the batteries did run down, they could be recharged several hundred times.
Unfortunately the Cadmium used in the cells is poisonous and harmful to the environment. Disposal of NiCd cells presents an environmental issue to such an extent that several countries in Europe have banned NiCd batteries for just this reason.
The introduction of Nickel-Metal-Hydride (NiMH) batteries during the 1990s provided a safe alternative. With very similar properties to NiCds, but with higher capacity, and more importantly, no super-toxic components NiMHs rapidly became popular with rc modellers.
Electrical Characteristics of NiMH Rechargeable Batteries
The characteristics we are concerned with are as follows:-
a) Voltage b) Maximum Current c) Capacity d) Self-Discharge Rate
Let’s take these one at a time.
Although a NiMH battery voltage starts out at about 1.4V it drops to 1.2V almost immediately. As it discharges in use, the voltage remains relatively constant, dropping only to about 1.1V just before it is fully depleted.
Although a battery produces a voltage, the equipment it is powering requires that voltage to provide current, measured in Amperes (A). Think of voltage as water pressure and the current as the rate of flow. The water pressure won’t be any good if the tap ( I believe that this is called a ‘fawcet’ in the USA) holding it back is closed. Batteries are limited as to how much current they can produce by their internal resistance. Your tap can’t provide as much water as a fire hydrant, even though the pressure may be the same.
NiMH batteries are able to deliver significantly more current than disposable alkaline batteries. This makes them well suited to high-current devices like our more powerful servos and even smaller electric motors such as those used in smaller planes.
Using our analogy of the water pressure and rate of flow as Voltage and Current, Capacity is the total amount of water available from the container it is stored in.
A good AA NiMH battery has a maximum capacity of about 2.5Ah although they are available in smaller capacities. Theoretically a 2.5Ah NiMH can deliver 2.5A for one hour. Such a battery could, for example, deliver 0.5A for 5 hours (since 0.5 × 5 = 2.5) or maybe 1A for 2.5 hours, even 10A for 15 minutes. The common factor here is that Amps x Hours = 2.5
Self Discharge Rate
Traditional NiMH batteries have one serious disadvantage: self-discharge. This means that a fully charged NiMH battery will lose more of its charge the longer it is stored before use.
A good NiMH battery’s self discharge rate is about 1% per day. Thus, after about a week it would be at 93% charge and after a month, 73%. After three months it would only be holding 40% of its fully charged state.
This is not an issue so long as one gets into the habit of recharging these batteries the night before a flying session. The 1% overnight loss is of no great significance.
Low Self-Discharge NiMH Batteries
NiMH technology has progressed recently and we now have a new generation of low-self-discharge batteries which can retain up to 85% of their charge after sitting idle for a whole year. This is a major breakthrough for rc flyers as it now enables us to charge a transmitter and have sufficient power for several weeks use before having to re-charge.
Many modern transmitters have very low current consumption so that even modestly rated NiMHs will last well. The voltage level of transmitter batteries is usually shown on the front status display. Receiver batteries are required to drive several servos and a receiver so cannot be expected to last so long. Even so, several flying sessions should be possible with these NEW GENERATION NiMHs.
Because it is not necessary to recharge after every flying session, it is all the more important to check the remaining capacity in the battery at the beginning of each session.
I strongly recommend you invest in a Battery Capacity Checker as shown here. USA visitors can order this on-line by clicking the image right. UK visitors can purchase by clicking this link below
How to Charge NiMH Batteries
There are two types of charger available suitable for charging NiMH batteries. They are commonly described as “DUMB” or “SMART” types.
“Dumb” Chargers or Overnight Chargers
This type charges the battery very slowly, typically taking 14 to 16 hours to fully charge a dead battery. It is very important to understand that when the battery is full, the charger will continue trying to charge. As the battery is already fully charged, the surplus charge is turned into a small amount of heat. This low heat won’t harm the battery as long as it isn’t allowed to continue for too long.
If you use a “dumb” charger it is important to remove the batteries from the charger when the charging is complete. Most dumb chargers are designed to charge at a rate of C/10 (battery capacity ÷ 10). This takes 14 to 16 hours for a full charge. However, this is only the case if the batteries were fully depleted before beginning the charge and the charger is actually rated to supply the batteries full capacity divided by 10 (e.g. a 200mA charger will charge a fully discharged 2000mAh battery in 14 to 16 hours. Partially depleted batteries will reach full charge sooner. The problem is knowing when this has happened.
You may be wondering why, if the charge rate is C/10, the charge time is 14 to 16 hours. one would expect this to be 10 hours. It is recommended , at this charge rate, that the time should be 1.5 x 10 – hence the anticipated time of 14 to 16 hours.
IMPORTANT: If you use the charger with batteries of a higher capacity than it was designed for, a full charge will take longer than 14 to 16 hours. Thus, in the case of a 1000mAh pack, using a “dumb” charger with an output of 50mA the charge time will be around 30 hours for a totally discharged battery (1000/50 x 1.5).
In short, properly charging with a dumb charger is a guessing game.
“Dumb” chargers became popular because they are inexpensive to make. Although overuse of such a charger can damages the battery, this damage takes the form of a gradual reduction in capacity rather than a complete failure.
Fast “Smart” chargers have become very popular. In addition to not overcharging batteries, they charge much faster, typically in one to five hours. The reason fast chargers are not “dumb” types is that overcharging at these higher rates can result in critical damage to the battery.
A good fast charger normally uses one of two methods to determine that the charge is complete:
- a) −ΔV (negative delta-voltage) This method detects the voltage drop that a NiMH battery exhibits if you attempt to keep charging it when it won’t take any more. (The ∆ symbol represents “change”). The graph right shows the charge characteristics of a typical NiMH battery using a ‘constant current’ charger which has cut off having sensed the delta peak voltage drop.
- b) ΔT (delta-temperature ) This method detects the temperature increase once the excess charging current starts getting turned into heat.
Some chargers are designed to use both methods. −ΔV as the primary method and ΔT as a backup in the event −ΔV should fail.
Memory Effect – Myth or Fact
As NiMH batteries age the time between recharges gets shorter. A phenomenon called “memory effect” is blamed for this. It is considered to have been caused by the repeated partial use of the battery’s capacity before recharging it. It seems to “remember” that only part of its capacity was used before it was recharged, and thus refuses to deliver more than that.
The only circumstance under which true memory effect can occur is in cases where the charge and discharge cycles are identical every time. One of the few situations where this occurs is in satellites that are orbiting the Earth. These charge their batteries using solar power for some period of time, and then operate from them during the night. These cycles are exactly the same length every time.
NiMH Battery Life
NiMH batteries looked after properly will last for years. They do eventually deteriorate and there is a limit to the number of charge/discharge cycles that they can tolerate. Manufactures typically quote 300-500 cycles but these are FULL charge/discharge cycles and normally ‘Fast’ charges of 1xC or more.
The NiMH rechargeable batteries in our radio system should last far longer, probably for 1000’s of part discharge cycles. The more carefully we use and recharge these batteries the longer we can expect them to last.
I hope this post has been helpful and informative for you. If you have enjoyed reading it you may get further help from my full website at www.rookiercflyer.com especially if you are just getting started in model plane flying. Please feel free to share it with groups or individuals you think could benefit from it.
Catch you again very soon.