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 How to build your plane and install your gear.

BATTERIES & CHARGERS

| Types of batteries| Battery Capacity | Batts for Tx & Rx | Batts for Elec Planes | Fast charge-discharge | CP cells | BEC | Charging & Chargers | Charging Rates | Memory Effect | Cell Reversal |

Read the complete article for more items and more detail. 

SUMMARY

  • Dry cell BATTERIES (Energisers and Duracells) only usually used for some unpowered gliders - usually 2 channel Nicad batteries used for everything else, including powering electric planes. Cells welded or soldered together as packs. Nickel Metal Hydride (NiMh) used occasionally - have advantages and limitations
  • Store your batteries almost flat - but not totally. Don't keep giving them overnight charges without discharging them. Article includes info on Memory effect.
  • Peak detector CHARGERS detect voltage "peak" which indicate full capacity - then stop or slow charging. Not all chargers cope with NiMh cells
  • Nicads can be charged in 30-20 mins with a peak detector charger, NiMh need at last 30 mins. (Newer more resilient NiMh chargers are now coming onto the market.)
  • Batteries labelled as SCR or SCRC are used particularly in electric flight and can be charged and discharged faster (Charge 15 mins, discharge as little as two!)
  • The newcomer to the Battery family is the Li-poly battery. At this time (1/2004) they are transforming what small planes can do - but are also rapidly advancing what can be done for larger and higher performance planes. AT THIS STAGE, THIS SITE DOES NOT INCLUDE details of the special characteristics or special CHARGING or BEC requirements of these cells.

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TYPES OF BATTERIES

Alkaline Batteries - Duracells, Energisers and their generic equivalents (which can be good value) - are not rechargeable (except in special chargers), and are often used for the transmitter and receiver of 2 channel transmitters and also some cheaper multi-channel units. Although they have plenty of capacity, they are not used for powering electric powered aeroplanes as the power cannot taken out of them quickly enough.

The voltage of each cell is 1.5 volts

Nickel Cadmium (Nicad) Batteries - are rechargeable and usually used in transmitters, receivers and for powering electric planes.

The voltage of each cell is 1.2 volts however their capacity can vary. In the AA size they start at 600mah and can be as high as around 1100mah.

They are able to be charged and discharged very quickly, and in electric flight a lot is demanded of them.

Nickel Metal Hydride (NiMh) - are also rechargeable and again, each cell is 1.2 volts.

For their size and weight, they are able to hold more power in them than Nicad batteries but generally do not take well to being charged or discharged at extremely fast rates. (See Batteries used in Electric Powered Planes section below.)

They also discharge more quickly just sitting on the shelf.

Lithium Batteries - these are generally specialty batteries and one of these may live in your computer radio to maintain its settings even when the main pack is taken out.

They have a very large capacity for their weight but can only give out small amounts of current. Special rechargeable lithium batteries are sometimes used for light indoor and park models. A special charger (or function on a complex charger) is required to charge them - and at a very low rate.

Lead Acid Batteries - the are the big things like you have in your car. They are not used for electric flight but are often used to power winches for launching gliders or for charging up the batteries mentioned above.

Gel Cells - are lead based batteries but they contain a jell rather than a liquid, and are used by some fliers as a portable pack for charging up small packs.

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BATTERY CAPACITY

The capacity of a battery is measured in amp-hours (AH), or milliamp hours (MAH). 1000 milliamps = 1 amp.

A one amphour (1000mah) cell would take one hour to discharge at the rate of 1 amp (1000 milliamps) - or half an hour at 2 amps.

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BATTERIES USED IN TRANSMITTERS AND RECEIVERS

Alkaline cells are often used in 2 channel and cheaper multi-channel radios. Individual rechargeable cells may also be used, but it is important that they are not recharged whilst in the transmitter. The reason is that as they charge, they give out gas which leaves a residue and this can very easily reduce the conductivity between the battery holders and the cells - leading to loss of control of your plane.

Similarly, if individual cells used in receiver packs, each cell should be removed for charging.

Individual cells in receiver packs are not allowed in some clubs, particularly in internal combustion engined planes, as the vibrations may shake them loose or may cause the connections between the cells and the holder to fail.

In multi-channel transmitters, and their receivers, Nicad batteries are generally soldered or welded together in packs.

NiMh cells are becoming a little more popular as receiver packs, but remember that they lose power quickly after charging just sitting around than Nicads do.

Most receivers require 4 cells. Some large gliders or powered planes require 5, because of the number of servos being run.

The safest way to avoid losing your plane as a result of your receiver battery running down is to check it regularly using an on field battery tester (such as made by Hitec - around A$45), or (my preference) with an on-board voltage indicator - with a line of LED lights - about A$25-A$30.

Because the voltage drops off quite sharply when the battery runs flat, as soon as you notice a drop off, don't fly again(!!).

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BATTERIES IN ELECTRIC POWERED PLANES

Cells used in range in size from very small (50mah) to quite large (3000mah). The size of cell used depends on the size of the plane, the current draw of the motor used and the performance requirements.

Nicad cells used are almost always Sanyos. The price you pay will vary, depending on your supplier.

The Sanyo batteries used for less demanding applications are generally YELLOW. Most aerobatic planes give at least a 4 minute motor run on full throttle. The yellow batteries can just cope with this.

A docile plane can give up to 12 to 15 minutes (some longer).

A basic electric glider will give around three minutes - perhaps 4 of motor run - but will use that up in several climbs, giving 15-30 minute plus flights. Requires SCR or SCRC cells.

A hotter glider will only give a minute or two of motor run, but the climb will be steeper and faster and so less is required. However this may not equate to longer flight time, as much of the battery's energy has been turned into heat. This is not a problem to the competitive glider pilot as the aim of these machines is to minimise motor run and the battery will not be dead at the end of a 5 minute 7 cell glider event. (The shortest motor run I have seen is 4 seconds, I've heard of as little as one second - straight into a thermal!)

A 1000mah cell would take one hour to discharge at the rate of 1 amp (1000 milliamps) - or half an hour at 2 amps. A 1700mah pack would take 1.7 hours to discharge at 1 amp.

My hot motor draws around 45 amps and so would discharge a 1 amphour (1000mah) cell in 1.3 minutes. (Calculation: 1/45 x 60), or my 1700mah pack in 2.26 minutes (calculation: 1.7/45 x 60).

Some competition electric motors are drawing over 100 amps(!!) but the beginner won't be flying them!

NiMh cells are becoming a little more popular for propulsion of electric planes - but it is important to note both the advantages and disadvantages of them until now - but to also be aware that there is a new generation of these coming onto the market.

(Note - I do NOT claim to be an expert on this - so speak to a number of experienced people - and please feel free to get them to give me feedback. No responsibility accepted for any damage to batteries or gear caused by the reliance of the contents of this site!)

NiMh cells do not generally like fast charge or fast discharge. The common consensus is that normal NiMh cells will require at least 30 minutes to charge, this compares with about 20 minutes for safe charging of Nicad cells.

Discharge on the larger cells (in the sub-c size - generally 2000mah upwards) would be limited to about 20 amps. It IS possible to discharge at higher rates, but it may shorten the lifetime of them.

Some newbies think - "There's a flier with a 150 gram pack of 8x600 AE cells - I have some NiMh 600mah cells that are far lighter - I'll use those." Unfortunately it doesn't work like that - because 600mah NiMh won't allow the current to come out nearly as quickly as Nicads, and when it does come out it will come out at a lower voltage. Since power (watts) is a combination of current (amps) time Volts, the power getting through to the motor is less than may be required for the plane to fly well - or at all.

It is generally accepted on the indoor scene that if a motor required 7 Nicads, it would probably require 8 NiMh cells.

A new generation is in the process of coming onto the market which cope better with fast charge and discharge - and these may give nicads a run for their money.

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FAST CHARGE / FAST DISCHARGE CELLS

The modern Sanyo fast charge batteries (SCR & SCRC, including the CPs) are generally (but don't bet on it!) GREEN. They used to be RED or BLACK (and slightly heavier).

These are generally referred to as "SCR" or "SCRC" and are capable of handling large currents in and out. They can be charged with a peak detector charger in as little as 15 minutes.

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"CP" CELLS

This is a new generation of high capacity Nicad cells which are lighter than earlier models. It is worth making the effort to get a pack or two of these to take advantage of the weight savings.

Popular sizes for electric gliders include the 1300mah CP 7 cell pack, weighing about 250 grams, and the 1700mah pack weighting about 330 grams. That's a considerable difference in weight, meaning less weight to drag into the sky, less to carry around (so less sink) and slower landings.

There are many schools of thought but there is a consensus that the current output of the 1300 CP pack is limited to 45 amps. However the raw beginner could not handle a craft which draws this sort of power so you can ignore this for the moment.

HINT - Whenever you put a battery on charge, wrap a rubber band around it. That way, you'll remember it's the charged pack!!

Put rubber bands on the packs as they are charging - so you can differenciate which packs are charged.

DO I NEED AN ELECTRIC FLIGHT PACK AND A RECEIVER PACK FOR MY ELECTRIC PLANE?

A BEC (Battery Eliminator Circuit) is a facility in your speed controller or on/off switch. It allows you to use the power from the main battery to also power your receiver, this saving the weight and expense (and possibility of battery failure) of a separate receiver battery. On most units the power will be completely cut off from the motor after the voltage drops below a certain level, leaving sufficient power for your receiver and servos for a reasonable flight. How long, depends on many factors.

Early BEC was not reliable and so had a bad name - but the current BEC speed controllers are very reliable.

From my experience, provided you have appropriate capacitors installed across your motor (see below) and your motor and controller are at least 6 inches from your receiver and aerial, BEC should not be a problem. I fly with BEC and my view is that there is a greater danger from the running down of a separate receiver battery and flying a heavier plane has disadvantages of its own.

It is important that your controller/switch has a definite BEC cut-off. I recently learned of a unit which allows the motor to completely drain the battery, so that the pilot is obliged to throttle off when a reduction in motor power is noted. The drop in power is not easy to detect 300 feet into the air and has the potential to leave you without any control whatsoever. This seems like a recipe for disaster sooner or later so avoid these.

However if you are drawing large amounts of current or running high voltages (lots of cells) you may not be able to use BEC.

 For some good info on Batteries and Chargers, go to

Electric Flight in Australia

Sanyo batteries are the most popular for transmitters, receivers and for powering electric planes.

 

 Now that's a lot of cells!

 

Standard 4 x AA cell receiver pack - as used in unpowered gliders.

 

AN EXAMPLE OF PLAYING WITH DIFFERENT PACK SIZES AND CELL NUMBERS

(This is not essential reading but it provides an interesting insight.)

Batteries have different capacities and voltages - but their weight can also heavily affect the performance of your plane.

My current speed 400 direct drive aerobatic plane goes exceptionally well on a 8 x 600mah AE cell pack, which weighs 150 grams.

But with a pack of only seven of the same cells my flight time actually increases by up to 50% because I am drawing less current and carrying 17 grams less weight. In little or light wind it is extremely docile with the 7 cells, in fact it is like having a second plane.

The trade-off is that being slower it is not as aerobatic (e.g. able to do fast rolls) or climb as steeply or maintain its speed in a dive unless full power is applied. With 8 cells the total weight of the plane is 550 grams and with 7 cells the weight is 533 grams.

 I have another pack of 8 x 1100mah AA cells which weighs 200 grams. They have plenty of staying power and the extra 50 grams in weight increases the penetration and decreases the buffeting in turbulent winds.

But with the plane now weighing 600 grams (and because the layout of the plane dictates that the pack be at the front - with a corresponding move forward in the centre of gravity (CG) it is not as reactive, lands faster and although there is more initial power coming out of the pack (its initial voltage is slightly higher that the 600AEs) this is not fully reflected in the climb rate. Nor is the longer motor run from the extra capacity reflected to the extent expected in total flight times in lesser winds as the plane is now sinking faster at nil or low throttle.

My pack of 7 x1100 AA cells weighs 167 grams and I had expected it to give a particularly long duration. The weight of the plane with these cells is 567 grams. However I was disappointed as this 7 cell pack was now required to push a plane that was 17 grams heavier than when it had the 8 x 600mah cells and as a consequence its climb rate was particularly slow (and slower than with 7x 600mah pack) and its sink rate was faster than with either pack. All this overtook the advantage that I expected to obtain of it being a higher mah pack.

The above illustrates that sometimes it is not possible to predict how well a plane is going to fly with different equipment when there is a trade-off between the performance of the component and weight. It also illustrates that a characteristic which is an advantage is in one set of conditions can be a disadvantage in another

 

  

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BATTERY CHARGING AND CHARGERS

In order to maximise your flying time, you will need to be able to charge at the flying field using a fast charger running from your car battery.

Three main types of charger:

  • the first is leads with built-in resistance but no timer, which rely on you remembering to disconnect them. Sooner or later you'll forget and destroy your expensive battery pack and possibly cause an explosion.
  • The second type uses a timer to automatically turn itself off to avoid overcharging. The big disadvantage is that you will not know if your battery is undercharged, fully charged or overcharged. The best way to destroy a battery is to overcharge it.
  • The third, and very much preferred alternative is the "peak detector" charger. As batteries charge their voltage increases but then drops slightly when fully charged. The charger detects this peak and safely switches off or drops to a lower "maintenance" charge rate. Most peak detector units have variable output and the opportunity to also charge transmitter and receiver batteries. Expect to pay A$75 upwards.

There is actually a fourth - but not generally used in electric flight - as used in electric drills. A thermo-cutout is included in the pack in one of the charging leads, this rests against one of the cells and when it heats up (because the battery has reached full charge) it stops the current going through.

Missing the peak - note that peak detector chargers sometimes fail to detect the "peak" if the battery is hot or if used in the sun or near a hot car engine, leading to overcharging.

Charging rates - charging at "1C" means charging the battery at its rated capacity over 1 hour. For example, charging a 600mah battery at 600 milliamps. In theory it will take 1 hour to charge.( In practice it may take more like 1.2 hours because more needs to be pushed in than comes out.)

Charging it at 1200 milliamps (twice as fast) means charging at 2C. Most people are used to seeing batteries charged over 10 hours (C/10).

Nickel Metal Hydride (NiMh) cells have a very faint "peak " which is sometimes not detected by some charger - or if the cells or charger are hot.

As stated above, NiMh cells do not like fast charge or fast discharge. The common consensus is that NiMh cells will require at least 30 minutes to charge, this compares with about 20 minutes for safe charging of Nicad cells.

Complex chargers (such as my Supernova 250s - which is excellent and popular and cost about A$200 - $250 and I understand is in the process of being superseded by the 3000 model) - include in their instructions that NiMh cells should not be charged in AUTOMATIC mode as this would push the charge in at a greater rate than NiMh cells can handle. Ignore these instructions at your peril.

In Manual Set mode - my Supernova 250s will not allow NiMh cells to be charged at more than 1C, ie in less than on hour. It is possible to override this by telling the charger that your battery is twice the capacity it really is. For example, to charge a 100mah NiMh cell at 2C (ie in 30 minutes), input the cell capacity as 200mah. This will not lead to overcharging, as it is the peak that tells the charger to switch off, not the "capacity".

Is it necessary to have two charger outputs? Some chargers have 2 outputs, one being for receiver while you charge your flight pack or transmitter battery. But it is rare for them both to be peak-detect.

Peak-detect is preferable, because (a) if it's still charging it ain't full, (b) if it stops charging (and that cam take as little as 20 mins) it should be full and (c) overcharging causes damage.

Charging off car battery can drain it if used a number of times in one sitting, if the battery is tired, so always have jump-start leads in the car and don't be last off the field or you may find yourself there all night(!).

When to charge - I was in the habit of charging my batteries the night before flying but later learned that that I will have better power if I charge them immediately before flying, ie on the field.

In order to occasionally charge cells at a slow rate, one theory is to charge them at a slow rate the night before flying, accept that your first flight will be of lesser performance, charge them on the field for subsequent and better performance flights. Another theory is to simply charge them on the field every time and accept the possibility that they may not last as long(!).

As mentioned earlier, if you're using individual rechargeable cells, DON'T charge them in the transmitter (or the four cell holder for the plane) and then just take off. The reason for concern is that as they charge they vent slightly - and the stuff that they vent can cause poor connections between the battery and the battery holder. So take them out, charge them, then put them back in. Otherwise you could have your plane in the air and loose control!

"Memory" problems with Nicads are mostly caused by overcharging but this can be avoided by using a "peak detector" charger which turns off (or trickle charges) when the batteries have reached their full capacity. (Top of page)

Memory effect

There are lots of different views on this - I will take the liberty of stealing from

http://www.ezonemag.com/pages/efaq.htm#battery4

"To avoid Nicad memory, simply fast charge your batteries regularly and provide an occasional deep discharge to 1.0 volts/cell under a light load. Also, avoid trickle charging batteries for long periods. Fast charging negates the effect of Nicad memory."

Note the recommendation to only go down to 1.0 volts per cell - NOT dead flat."

NiMh cells are not subject to memory effect.

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What is Cell reversal?
Borrowing from the same source again: Answer. (Jim Bourke, jBourke@ezonemag.com)

"In a battery, not all cells are created equal. One will be weaker than the others. So, as the battery is discharged, the weakest cell will use up all its active material. Now, as discharge continues, the current through the dead cell becomes a charging current, except that it is reversed. So, now reduction is occurring at the positive terminal. As there is no more nickelic hydroxide, it reduces the water, and produces hydrogen. Cell pressure builds, and it vents. The cell has lost water and the life of the cell has been shortened

"This is the big danger of battery cycling to prevent memory. Invariably, unless one is very careful, one ends up reversing a cell. It does much more harm than the cycling does good. Also, keep in mind that cells do have a finite life. Each cycle is a bit of life."

Charging packs together

This is not really for the beginner - but I had better tell you to stop you from doing damage to your gear experimenting.

It can only be done if the packs are of the same capacity AND of they are both equally discharged.

The must be charged IN SERIES - so this means that you will need to have a charger that will provide the voltage required.

Your average torch has the cells IN SERIES - ie the positive end of one touches the negative end of the next. This increases the voltage but the amps stay the same. It is the same with charging multiple packs - the positive from the charger attaches to the positive of pack A. The negative of Pack A attaches to the positive of Pack B. The negative of Pack B attaches to the negative of the charger.

If you try to do this in PARALLEL (ie both positives attached to the positive output from the charger and both negatives attached to the negative output from the charger) then it is likely that one pack will take all the charge - so don't do it.

Note that some chargers which allow charging of multiple cells (at high voltage rates) will have limited amperage output at these higher voltages, because of the limitations of the inbuilt inverter.

WHAT ARE MATCHED CELLS?

A pack is only as good as its weakest cell, and different cells, even from the same batch, may differ slightly. To ensure packs are made up of equally good cells, the performance (charge/discharge) of each cell is measured, and those which have similar performance are "matched" into packs.

(Think this through - it is hoped that you do not get to buy cells which are found to be of lesser performance than those which were chosen to go into matched packs!)

The beginner doesn't need to purchase matched cells.

WHAT ARE "PUSHED" CELLS?

Individual cells are put into a device and a large voltage is put through them for an instant. This changes them so that they have a higher voltage on discharge. This is excellent for competition but it is also generally conceded that it reduces the ultimate life of the cell. So leave this to the competitors for the moment.

USING A 12 VOLT CAR BATTERY CHARGER TO POWER A PEAK DETECTOR CHARGER.

Quite simply - DON'T - or you will risk blowing up your peak detector charger. Get expert advice as to what is needed to supply the 12 volts you require.

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HAVING MADE YOUR PURCHASE
YOU 'LL NEED TO KNOW.........
HOW TO BUILD YOUR PLANE AND INSTALL YOUR GEAR.

I have a Supernova 250s, it cost me A$200 (normally about A$250) and it is very popular with electric fliers.

It is possible to buy peak detector chargers for as little as around A$65. These are fine - they have a variable charge rate but won't read out the amount of charge put in.

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