Those Things We Call Batteries – Part Two

The radio system’s power comes from electrical storage devices known as batteries. Without power, any radio system is less than worthless. If battery power fails during a flight, your best airplane is also now just as worthless. To me, that makes batteries the as critical a component for safe RC flying as receivers and airborne switches.

If an aileron servo fails, it might be possible to safely land using the rudder instead. A bad elevator servo can sometimes be compensated for using airspeed or knife-edge tricks. There is even a video out there showing an airplane being safely flown and landed after an entire half wing fell off. If the engine quits, the airplane will still glide. But no battery power means having no control at all and no hope of saving anything.

In the first part of this battery series, we talked about all the different battery types, testing devices to determine how long they will last in the air and when to dispose of them. We also discussed field charging and when to do it. If you haven’t read part one, it is still available in this section: Those Things We Call Batteries Part One.

Here in Part Two, we will look more closely at Nickel Cadmium (Ni-Cd) and Nickel Metal Hydride (Ni-MH) batteries. We will look at such subjects as why batteries age, how to fix them and how to store them.

More on Ni-Cds

Did you know that both Ni-Cd and Ni-MH batteries begin to age once they have been charged for the first time? Even if they are never used in an airplane, they do get old. I didn’t know this back when I first started in RC. I thought batteries lasted forever, I mean, they can be recharged, right? So if you recharge them, they will be like new, right? WRONG!

I lost a beautiful P-39 Aircobra on its maiden flight because I used a receiver pack that had gone bad. I just didn’t know that could happen. I charged it overnight, went to the field and watched the P-39 snap roll into the ground right after takeoff. That was when I started to learn about Ni-Cd batteries.

Photo 1

Battery testing equipment was not available at that time. The first one, called a “Flite Life” came out two years later. I bought one immediately. It was not very sophisticated by today’s standards. It discharged any battery at 250 mAh. You connected an electric clock to it. When the Ni-Cd being discharged, 8-cell transmitter packs or 4-cell receiver packs only please, reached 1.05 volts per cell, the Flite Life turned off the electric clock. If you kept a record of the various discharge times per pack, it was possible to tell when a battery had reached the 80% point mentioned in Part One.

While primitive, that old Flight Life tester saved me more than a few aircraft. Since then, testers have become very sophisticated as noted again in Part One. Whichever battery tester you choose from Part One, please be sure to get one.

During those early testing days, I began to notice that batteries permanently lost capacity even if they were not used. If a battery was several years old, had been put away fully charged and then tested, it gave poor results no matter how many times it was recharged.

I didn’t know what was happening so I asked Larry Scribnick of SR Batteries fame (Website) what was happening. Without getting too technical, this is what happens to Ni-Cd batteries as they age. “AA” size Ni-Cd batteries look similar to the standard “AA” dry cell. But internally, they are very different. They are composed of many small individual “plates” with tiny spaces in between. Each plate produces voltage by chemical means. The size and number of these plates determine the battery’s capacity, not its voltage which is always 1.2 volts (nominally) per cell.

As the entire battery ages, chemical deposits build up between these plates. These deposits reduce the plates’ efficiency to produce current. This reduces the battery’s overall capacity. If there are too many deposits, the battery loses the ability to maintain capacity long enough for a safe flight. Yes, this is a very superficial explanation but it is important to know about the deposits to understand the next step.

Charging a battery that has many deposits on a regular wall charger does not help much. The usual charging current for Ni-Cd batteries is C/10. “C” means the capacity of the battery in mAh. Therefore, the proper slow charge for a 700 mAh Ni-Cd battery is 70 mA. The constant slow charge does little to remove the deposits.

To restore a battery that has deposits, it is necessary to use other than a slow, constant charger. Chargers like the Sirius Pro Charger use “pulse” charging to dislodge the deposits. Again without being technical, these restorative chargers provide higher current rates and vary the charge modality to help “remove” the problem deposits.

Photo 2

In Part One, we showed the graphic discharge curve for an old 800 mAh Ni-Cd battery pack. You may have wondered why we used an old pack for this example. Because, now we can show how a restorative charger works on this old pack to revitalize it. Look at photo 2 above. The black curve is the first discharge of the 800 mAh pack that yielded a capacity of just 560 mA. The pack has just 70% of its original capacity remaining and would be on my “ground work only” list.

After the discharge illustrated above, the same pack was put on a Sirius Pro restorative or “conditioning” charger. This “fast” charger recharged the battery in less than two hours. Then it switched to its “restorative” mode that pulse charges the pack somehow. (Each such charger uses slightly different, proprietary restorative techniques.)

The old pack was left on the Sirius charger for two full days. Then the CBA II (Website) was used again to discharge it. The result is the red curve in photo 2. The battery pack now had a capacity of about 810 mA! That is probably just what it had when new. The Sirius Pro restorative charger made the old battery almost new again. I not only saved some money, but probably an airplane as well.

But nothing is for free and this restoration “miracle” has a cost. Once a restorative (also called “conditioning”) charger is used to give life back to an older pack, then it needs to always be charged on that type of charger. If charged with the slow wall charger a few times, the problem seems to quickly return. This is usually not a problem for a very human reason. Once a pilot gets used to not having to charge overnight before flying but rather just on the way to the field, it becomes a very easy habit to acquire.

But some caution here as well. Regular “Peak Detecting” fast chargers should not be used as the only charger. Peak Detecting chargers build up some heat during the charge process and sometimes push the battery too far. Heat damages a Ni-Cd pack and removes some of the life from it that can never be restored. If you are constantly going to fast charge, then use a restorative charger like the Sirius Pro. Less expensive “Peak” chargers are designed for field use to extend flying time, not for constant maintenance.

Photo 3

During the above discussion, the C/10 charge rate was mentioned for Ni-Cd. packs. All the battery experts seem to agree that this is the best slow charge rate for Ni-Cd battery packs. As proof, enlarge photo 3. The left side wall charger came with a radio set that used a 550 mAh receiver and transmitter battery. The one on the right was packed with a radio set using a 1 amp hour (1000 mAh) receiver battery and a 700 mAh transmitter battery. It is no coincidence that these two wall chargers from different companies both use the C/10 charging rate.

Why slow charge at all when fast chargers are available? There are probably many reasons but two stand in the forefront. Fast chargers discontinue the charge when the entire pack reaches about, stressing “about” hard here, 90-95% of the rated voltage. Stopping “early” like this usually protects an individual cell in the pack from being overcharged while the other cells are still coming up to full charge.

Yes, each individual cell in a pack is different. They charge and discharge at individual rates and do not act as a team. You may have heard about “matched” battery packs. This means that the company or individual constructing the pack has made an honest effort to include only those individual cells that have similar charge/discharge curves. Note the key word here is similar as no two cells are identical in every respect. Matched packs live longer and produce more usable capacity, but are more expensive.

But slow charging a pack, especially the first time, allows all the cells to reach full capacity without damaging any individual cell. The charge rate is low enough that the fully charged cell is not damaged by the small amount of extra current being used to bring the other cells up to full capacity. Slow charging a pack at the start helps to prolong its useful life because all cells are undamaged and more balanced to each other.

Secondly, slow charging does not heat the cells as does a fast charger. Lower temperatures mean longer battery life. Slow charging also insures that the battery pack is at 100% capacity before you leave for the field. My suggestion is that you slow charge before most flying sessions, if you do not use a restorative charger, and fast charge those few times at the field when you just have to get airborne a few more times than usual. (See Part One for suggested flight times per battery capacity.)

What about charging at less than C/10? Getting too low a charge rate, C/100 for example, approaches the “trickle charge” area. This is used to maintain a battery pack at nearly 100%. (Some pilots swear this is the best way to preserve Ni-Cd batteries. Many experts agree.) But C/100 only maintains the battery at its present voltage, whatever that may be. A Ni-Cd battery will self-discharge at a given rate while in storage. The trickle charge is designed only to make up that lost charge. It will not charge a half-charged battery pack back up to its full capacity.

I know a few pilots that keep all their batteries on trickle charge. Their claim is that it prolongs the battery life by many years. One has batteries more than 10 years old and they still work great. I am not recommending this, but it may be worth a closer look. Trickle chargers are available and could save both money and airplanes. But I suggest still checking the battery pack before flying by using a full discharge/recharge cycle.

Even more critical than trickle charging a half-full pack is the opposite situation where a pilot buys a larger capacity battery and uses the radio system wall charger to charge it. This happens very often and it is not a good idea. Trying to charge a 1500 mAh battery pack on a 50 mA wall charger just doesn’t work. Most of us would think that we could just charge the pack for 30 hours and have it reach full capacity. For various reasons, this does not work. What happens is that eventually the 1500 mAh battery begins to act more like the 500 mAh pack for which the charger was originally designed.

Photo 4

When purchasing larger capacity batteries, you will also have to purchase an after-market charger capable of charging the new battery at least at C/10. This includes transmitter and receiver packs. Since most aftermarket chargers are fast chargers, it makes sense here to buy a conditioning or restorative charger, like the two shown above) for the new larger battery.

What about storage of Ni-Cd batteries? Again I checked with Larry Scribnick about this. His answer surprised me. I had always been taught to store unused Ni-Cd battery packs fully charged. But the answer is that Ni-Cd batteries self-discharge which means that there is always some chemical action taking place. The more charge in the pack, the more action happening. Therefore, it is best to store the batteries discharged, but at the safe level; not under 0.9 volts per cell.

This means checking the pack’s voltage on a regular basis and then using a slow charger to bring the pack back up to about 1 volt per cell. This provides the longest battery pack life but is the difference large enough to make a difference? Today, it costs between $15 to $25 to buy a new receiver pack and about $20 for a new transmitter battery pack. For me, and this is just my opinion, I would rather cycle (charge, discharge then slow recharge back to ~1.05 volts pre cell) the pack every 90-120 days. If that means the pack lasts four years instead of five, I can almost afford that.

It is a good practice to date each battery pack, transmitter and receiver, with the month and year of purchase. This helps to keep track of the older batteries for more careful checking. Keep a list of the date for each battery “in the plane” and check the older ones before flying. How old can a pack be and still be good? This depends as much on the number of charge/use cycles as on the years involved.

It is not feasible to record every cycle, including flight cycles, for every battery pack once your hangar grows much beyond five aircraft. Some can do it, but I have never managed it. Instead, I rely on the 90-day testing outlined in Part One. My own notes show that once a battery pack reaches around 400 cycles or about four years, it starts to decline. The conditioning chargers help here, for a while. But eventually, the batteries wear out.

In aircraft that fly a lot, the batteries are replaced every two years regardless of test results. “Fly a lot” means more than 200 or more flights per season. Those that don’t fly much are cycled and tested every 90-120 days using a conditioning charger. If the batteries still test above 80% capacity but are more than seven years old, they are replaced anyway.

Finally, does the Ni-Cd “memory” phenomenon really exist and what is it? The “memory” concept says that if a Ni-Cd pack is always fully charged and then always discharged the exact same amount, say a full charge and then always only two 10 minute flights made each time, then eventually the battery will only stay charged for just 20 minutes and then drop to zero.

Does this actually happen? It is possible that this concept started back when there were no testing devices available to RC’ers and older packs couldn’t be restored. But many batter experts say this is a very real concept while others say not true. It is not up to us newer pilots to evaluate this problem. We only need to know that if we charge and then safely discharge a Ni-Cd pack every 90 days as recommended in Part One, then “memory” just can’t happen and we are safe from its ravages.

Nickel Metal Hydride Batteries

Nickel Metal Hydride (Ni-MH) batteries are identical in appearance to Ni-Cd batteries and similar in construction, but use a different chemistry. Ni-MH batteries have two distinct advantages over Ni-Cds. First, for given cell size, “AA” for example, Ni-MH batteries have more capacity. About the largest capacity Ni-Cd “AA” cell is around 1600 mAh. But some Ni-MH “AA” cells reach over 2000 mAh (2 amp hours). The battery packs weigh about the same but the Ni-MH pack will last longer.

Photo 5

But there is another, even more vital, Ni-MH advantage that can sometimes save aircraft. This advantage is its discharge curve. Like a Ni-Cd, the Ni-MH battery discharges at a constant rate for most of its charge. Yes, it does have a “surface” voltage, here over 7 volts. Like a Ni-Cd, the voltage drops quickly to its useable amount, here about 6.1 volts. Also like a Ni-Cd battery, the voltage stays in the useable range for a long time.

The important difference is at the end of the discharge curve. Remember how the Ni-CD battery’s voltage “fell off the cliff” at the end (see photo 2)? But the Ni-MH final discharge curve is much less abrupt. This can, and has, saved airplanes.