To gain a basic understanding of the requirements and use of high-performance batteries commonly used to power small unmanned aircraft.
At the end of this briefing you will be able to:
- Discuss the characteristics of a lithium polymer cells.
- Consider the vulnerabilities and dangers associated with lithium polymer batteries.
- Explore the measures we can employ to maximise service life.
- Discuss battery selection methodologies explore ‘safe’ disposal of depleted batteries.
Lithium Polymer Batteries
DO NOT ATTEMPT TO DISASSEMBLE A BATTERY!
Lithium Polymer (Li-Po) batteries are a type of high-performance (high current output) power storage devices. This comes at a cost, as they have a much lower power density (i.e. lower capacity, low Ah/mAh) compared to batteries of a similar size.
Think of a battery cell as a “ideal” cell in series with a resistance.
Internal Resistance & Voltage Drop
Effect of Internal Resistance – Charging
Result: Cell appears to be more fully charged than it actually is.
Effect of Internal Resistance – Discharging
Result: Cell appears more discharged than it actually is!
Inside a Li-Po “foil pouch” cell you will find a long piece of very thin plastic film – the polymer!
Laminated onto the polymer are the thin lithium carbon coated aluminium & copper anode & cathode electrodes. These are laminated in an alternating pattern on the front and back side of the polymer separator film.
Everything will be saturated with a greasy solvent based organic electrolyte.
The long internal polymer film (which is over 7 feet long in the case of a 5000 mAh cell), is folded accordion style back and forth upon itself. The entire folded material is then heat sealed into the foil pouch, along with the gelled electrolyte.
As a matter of interest, the gelled electrolyte has a very sweet solvent smell, not dissimilar to nail polish remover, or acetone.
Charge & Discharge Cycle Chart
Typical charge and discharge cycle charts for Li-Po batteries at room temperature (20°C). These particular charts are based on Sanyo data.
The cell voltage is reasonably constant out to about the 20% capacity state, and then falls off precipitously, similar to NiCad and NiMH cells.
Lithium-Based Battery Charge Regime
Service Life as a User
Fact: We are battery users, not battery manufacturers.
This means that we can only use the battery in accordance with manufactures directions!
From the user perspective, battery life can usually only be extended by preventing or reducing the cause of unwanted parasitic chemical effects which occur in the cells.
Shelf life & Cycle Life
Fact: Battery performance deteriorates over time regardless of whether the battery is used or not.
Shelf Life refers to how long a battery can remain on the shelf – in other words not being used – before it is no longer serviceable.
Cycle Life refers to the number of charge/discharge cycles a battery can be subjected to before it is no longer serviceable.
The effects of voltage and temperature on cell failures tend to be immediately apparent, but their effect on cycle life is less obvious. We have seen above that excursions outside of the recommended operating window can cause irreversible capacity loss in the cells.
The cumulative effect of these digressions is like having a progressively debilitating disease which affects the life time of the cell. In the worst case, this can cause sudden death if you overstep the mark.
At about 15 ºC cycle life will be progressively reduced by working at lower temperatures. Operating slightly above 50 ºC also reduces cycle life but by 70 ºC the threat is thermal runaway.
The battery thermal management system must be designed keep the cell operating within its sweet spot at all times to avoid premature wear out of the cells.
Beware: The cycle life quoted in manufacturers’ specification sheets normally assumes operating at room temperature.
Li-Po Battery Service Life
Fact: Batteries have a finite service life.
This is due to the occurrence of unwanted chemical or physical changes to, or the loss of the active materials from which they were manufactured.
(Otherwise they would last for ever!)
Some Li-Po batteries have been rated by the company for 300+ cycles, but those numbers are usually in a ‘perfect’ setting. It is reasonable to expect around 200-250 cycles from any good/decent Li-Po manufacturer.
Battery life greatly depends on how the battery is used and stored. There are well documented cases of over 300 cycles with little drop off in measured performance (voltage under load, IR, etc, although capacity will tend to drop some with time).
Inappropriate care can make that number just about as low as you want. At the very least it will accelerate the capacity loss.
Limit the amount of discharge, store properly and routinely, etc.
Managing Battery Stock
Batteries – like all other aircraft components – are a controlled item. Batteries are a critical component, and come under the controlled maintenance program.
As a minimum there must be:
- Battery pack numbering and identification:
- Serial Number
- Voltage and Capacity
- Maximum Charge Rate
- A Record or Log for each battery:
- The Record or Log must be accessible
- The Record or Log must be useable
- The Record or Log must be sensible
Other useful information that could be recorded (where appropriate):
- Record (Log) Usage Information:
- Date / Time of Day / Flight Duration
- Start capacity – end capacity = consumption in mAh
- Record (Log) Charge Information:
- Start time and finish time
- Charge rate
- End Capacity
- Add operational information to Record (Log):
- Ambient temperature
- Maximum altitude (calculate operating temperature)
- Installation considerations:
- Vibration and Shock Absorption
- Storage considerations:
- Charge as per manufacturers instructions
- Identify and isolate from other equipment as necessary
The alphanumeric designation for battery cells – as used in radio control applications – refers to both the number of cells in the battery pack, and the way in which those cells are interconnected.
Series (S): Denotes batteries connected in series. For example, 3S denotes a three-cell battery wired in series, thus multiplying the voltage to three times that of a single cell.
Parallel (P): Denotes batteries connected in parallel. For example, 2P would be two batteries connected in parallel to give twice the current capacity, but the voltage of one cell only.
Thus, a 3S2P battery contains 6 cells, and has three times the voltage and twice the capacity of any individual cell.
All cells should be identical if they are to be manufactured into a single “battery pack“.
Specific Notes on Managing Lithium Polymer Batteries
Main Points of Li-Po Batteries
Lithium ion polymer batteries, or lithium polymer batteries (abbreviated “Li-Po“) are rechargeable batteries which have technologically evolved from lithium ion batteries. Li-Po batteries contain a dry polymer suspended in an electrolytic gel (no metal-based conductive elements).
Li-Po Batteries are extremely volatile. The chemical compounds within the battery are flammable, and if not cared for correctly the battery can catch fire, or even explode.
Because of the volatile nature of Li-Po batteries, chargers specifically designed for use with Li-Po batteries must be used. Failure to do so incurs the very real risk of explosion and/or fire.
Battery output leads must never be allowed to short together, as immediate damage to the Li-Po battery will result.
Li-Po batteries must never be allowed to discharge below a certain point. It is therefore critical that low voltage cut-out devices are employed to protect the battery.
A battery that has been involved in a severe crash should be isolated for a reasonable amount of time (a few hours). It should never be immediately loaded into a vehicle, or transported from site until it is confirmed safe. This is because of the elevated risk of fire.
Li-Po in Comparison to Lithium-Ion
Lithium-Ion Batteries began their development in 1912, but did not become popular until they were adopted by Sony in 1991.
Lithium Ion Batteries have high energy-densities and cost less than lithium-polymer batteries.
- Do not require priming when first used
- Have a low self-discharge
- Suffer from aging – even when not in use
- Usually come in a rigid plastic case
- Have a nominal voltage of 3.7V
- Don’t like near-freezing temperatures
Charged initially with a constant current, has a gradually increasing voltage. When the voltage limit per cell is reached, then the charger switches to a constant voltage with a gradually decreasing current flow.
There have been very few cases of batteries suddenly exploding when they are not being used, abused or charged (i.e. during transport and storage).
Lithium batteries are commonly air-freighted protected by a few layers of bubble wrap and small versions are carried around in mobile phones. Should it be required to ship a battery pack, care must be taken when packing to ensure that it cannot be physically damaged.
It has been reported that some Li-Po battery fires have been caused when a dog was attracted to the smell of a lithium battery – and bit it.
Charging & Safety
The majority of lithium battery fires occur during charging, therefore charging should only take place where a fire cannot spread. Fire safety must always be a prime concern when working with Li-Po batteries.
Additionally it is most strongly recommended that Li-Po batteries are not charged inside a vehicle, and in particular a moving vehicle.
Charging in an purpose built and approved fire/explosion resistant bag is recommended. Alternatively a heat-resistant ceramic container with a loose fitting lid as flames, smoke and gas are released should the battery “vent”. Metal containers can be used, but ensure the charging wires cannot be cut or shorted.
Keep batteries separated so that a fire cannot damage other batteries. The charging container should be kept away from anything flammable.
Battery Charging & Charge Balancing
Many Li-Po batteries, particularly larger packs, come with a second, smaller, multi-wire plug – which is to be used for balance charging.
Balance charging simply ensures that all individual cells within the battery pack are at the same voltage. If a battery is not balanced, some cells may be overcharged, others may be over-discharged. In either case the life of the battery pack will be compromised.
Lithium batteries are not automatically balanced by applying a small “overcharge” in the same way that nickel-cadmium or lead-acid batteries may be balanced.
Note: Not all balancers are compatible with all chargers – some research into compatible types will be necessary in order to avoid battery damage.
For example, some balancers are only able to balance a small amount of amps, which, depending on the Li-Po cell, may not be enough to achieve a balanced state. Some of the ‘newer’ Li-Po chargers are able to individually charge each cell using the balancer plug, thus eliminating the need for a separate balancer, and at the same time ensuring sufficient power to complete the balancing process.
Storage Tips & Techniques
Extracting the longest service life out of a battery packs is of prime concern. One of the key factors contributing to the service life is correct storage.
The greater portion of the service life of many battery packs is spent in storage. The conditions that a battery pack is exposed to during storage directly impact on the achievable service life from the battery pack.
Additionally a unique characteristic of Li-Po batteries is that their life span is dependent on aging from time of manufacture, not just a number of charge/discharge cycles. An older battery will not perform as well as a new one, due solely to its age.
This limitation is not widely publicized, and consequently is not well known. This is because as Li-Po battery ages, it’s internal resistance increases.
Under load the effect of internal resistance is to cause the battery terminal voltage to drop, which in turn reduces the maximum current that the battery can provide to the load. To add to this phenomena – as Li-Po batteries age, usable capacity is lost.
Cell Storage – Voltage
A fully charged Li-Po cell will produce a terminal voltage of approximately 4.2 volts. Li-Po’s are different from other battery chemistries as they should never be stored fully charged. In fact, LI-Po batteries should be stored approximately “half full”, or at 50 State of Charge (SoC).
Many of the newer Li-Po battery balance chargers include a “Storage Mode”, which charges the pack to the proper reduced voltage state for storage purposes.
Some commercial chargers charge cells to 3.85Vdc in Storage Mode.
Storing battery packs at the proper voltage level is the simplest thing you can do to lengthen their usable life span (assuming of course proper application).
‘Storage’ should not only be considered as a long term (e.g. “over the winter” situation). Even if, for example, you only fly on weekends, these battery packs are technically in storage all week – week after week – during the entire flying season. Those cumulative hours can add up slowly degrading the battery packs.
Cell Storage – Temperature
Li-Po batteries produce energy via a chemical reaction that occurs inside sealed foil envelopes. The output power is produced by a chemical reaction. The aging/degrading process is also in reality a chemical reaction.
A chemical reaction doubles its speed for every ten degrees increase of ambient temperature.
It is for this reason that Li-Po batteries do not perform as well in cold weather. Cold “slows down” the chemical reaction process – something we need to be aware of when anticipating aircraft performance.
Reducing storage temperature slows the chemical reaction of the aging/degrading process, however there is a limit as to how cold is acceptable. Li-Po batteries should not be for example frozen solid. Laboratory testing has determined that the typical household refrigerator (0 to +5 degrees C) is the perfect storage place.
Li-Po battery packs should be placed in plastic zip top storage bags prior to placing them in the refrigerator for storage. When removed from the refrigerator prior to use, leave the batteries in the zip top storage bag to prevent any atmospheric moisture from condensing on them as they warm. After the batteries have attained room temperature, that may be used as normal.
Storage & Battery Degradation
|Storage Temperature||40% Charge||100% Charge|
|0 °C (32 °F)||2% loss after 1 year||6% loss after 1 year|
|25 °C (77 °F)||4% loss after 1 year||20% loss after 1 year|
|40 °C (104 °F)||15% loss after 1 year||35% loss after 1 year|
|60 °C (140 °F)||25% loss after 1 year||40% loss after 3 months|
Li-Po batteries are intolerant of over discharging, and tend to die if discharged below approximately 2.5 V.
In operation, controller circuitry should prevent the cell voltage from dropping below 3.0 V.
Cell temperature should never exceed 90 °C in order to prevent the internal separator polymers from melting and allowing plate shorting through physical contact.
Batteries with discharge rates of 20C or 25C are commonly available on the commercial market. In battery discharge terminology, each “C” is a discharge current equivalent to the value of the energy capacity of the cell (it is not the abbreviation for the Celsius degree unit).
In the case of a 1,200 mAh rating, 1C is equal to a discharge current of 1,200 mA, or 1.2 amps. A 10C cell can deliver a continuous current to a load of 10 x 1.2 A = 12 A during its discharge cycle.
Safe Disposal of Failed Li-PO Batteries
FOLLOW THE MANUFACTURERS DIRECTIONS FOR SAFE DISPOSAL!
Unlike others, lithium-polymer batteries are environmentally friendly.
It must be remembered that should the outer case of a Li-Po battery be compromised, the lithium inside is highly volatile and will react violently with water.
For safety reasons, it is recommended that Li-Po cells be fully discharged before disposal.
If the battery is physically damaged, discharge is not recommended. Li-Po batteries must also be cool before proceeding with the disposal instructions.
For Li-Po packs rated at 7.4V and 11.1V, connect a 150 ohm resistor with a power rating of 2 watts to the pack’s positive and negative terminals to safely discharge the battery pack.
Connecting the battery pack to an Electronic Speed Controller/motor system, and allowing the motor to run indefinitely until no power remains to further cause the system to function is not an preferred method of discharging the battery pack.
Discharge the battery pack until its voltage reaches 1.0V per cell or lower. For resistive load type discharges, discharge the battery for up to 24 hours.
After discharge, submerse the battery into bucket or tub of salt water. This container should have a lid, but it should not need to be air-tight. Prepare a plastic container (do not use metal) of cold water. And mix in 1/2 cup of salt per gallon of water. Drop the battery into the salt water. Allow the battery to remain in the tub of salt water for at least 2 weeks.
Remove the Li-Po battery from the salt water, wrap it in newspaper or paper towels and place it in the normal trash.
When neutralised these batteries are “reported” to be landfill safe.
Note: Some cells might react to any attempt to discharge them by bloating more. Should this occur, or should any doubt arise as to the stability of the cells, the cells should immediately be placed into a saltwater bath. It may take several days (or weeks) for a fully charged cell to deactivate in saltwater, however it will eventually deactivate.
Li-Po Battery Safety Summary
- Keep Li-Po batteries in a cool environment
- Do not leave Li-Po batteries in direct sunlight. Elevated battery temperatures increase the likelihood of the battery chemicals reacting in a hazardous manner.
- Do not overcharge your battery
- Excessive charging will cause the battery pack to heat up, expand, or explode.
- Handle Li-Po batteries carefully
- Try to prevent holes being punctured into the battery. The chemicals inside the battery may leak out. These chemicals may become volatile in contact with air.
- Never leave Li-Po battery unattended while charging
- Ensure the correct charge settings for the battery are being used.
- The charging area should be away from any flammable material.
- If smoke appears, or there are signs of the battery expanding, or “puffing out”, disconnect the battery charger from the wall socket, eliminating any further flow of energy into the battery.
- Use Li-Po specific chargers only
- EXTREMELY IMPORTANT: Using a NiCd or NiMh charger will create an unwanted reaction within the battery that may result in a fire. Remember to only use Li-Po specific battery chargers!
- Never leave Li-Po batteries plugged to an Electronic Speed Controller (ESC)
- Li-Po batteries are always ageing, even when they’re not connected. The chemicals inside the Li-Po battery are extremely volatile.
- This also puts a heavy reliance on the system failsafes, which are the only thing from stopping a motor powering up if the battery is connected.
- Never arc the positive and negative terminals
- A positive-negative short creates a continuous flow of uncontrolled energy through the battery, with nowhere for the energy to be released.
- Check the battery after each run period
- Check for signs of puffing, for severed or cut wires, for dents and/or damage, and the overall condition of the battery.
- If the battery is starting to puff out, swell, or looks like it has been punctured, do not continue to use it.
- Charging a damaged battery MUST NOT BE ATTEMPTED!
- Chemical reactions of Li-Po battery are not instantaneous
- Be careful if you see a battery starting to smoke without fire. Although it has not yet reached the reaction of a fire, it is highly likely that it will reach the stage of combustion, and burst into flames.
- Remember, if you see smoke, UNPLUG THE CHARGER FROM THE WALL SOCKET AND MOVE THE BATTERY TO A SAFE LOCATION.
- Wait at least 10-15minutes before moving it again just to be safe.
- Never leave Li-Po batteries in your car
- This is for the same reason why you should store your Li-Po Batteries in a cool environment.
- Heat = Reaction. Reaction = Expansion. Expansion = Cause for burning! BE AWARE!
- Do not leave batteries unattended with children
- Children do not know how to handle a battery safely. Be extremely careful and limit access to batteries.
- There are no age restrictions with respect to the use of batteries, however common sense is required.
- The “C” rating – and what it means
- A 3S 11.1 Li-Po battery with 5000mAh have 1C = 5 ampere.
- If the battery is rated for 25C continuous use that means it can output 125 ampere, or almost 1400 watt.