LiPo Batteries

Considerations for battery longevity

Considerations for battery longevity
By David Buxton |
Photos by the author and as noted
As seen in the March 2022 issue of Model Aviation.

How you treat your LiPo batteries will determine how often you buy new ones. Photo courtesy of

How you treat your LiPo batteries will determine how often you buy new ones. Photo courtesy of

A new member has joined your flying club and seems to have a bottomless budget for model airplanes. He has asked you for advice regarding his LiPo batteries. What do you think would be the most important things that you would recommend? Contemplate your answers before you continue to read my list.

  • Use a timer system. Running high-performance batteries until they run out of power will quickly ruin them.
  • Use a modern LiPo battery charger with embedded balancing and other safety features. Always plug in the balancing connector.
  • Some might consider LiPo batteries to be inexpensive, but safety is key. Smoke from a lithium fire is toxic, stinky stuff. A burning LiPo battery is like a flame-throwing blowtorch.
  • Storing fully charged batteries in a closed-up car on a hot afternoon ensures high life-span-reducing stress. One pilot reported that an entire set of batteries had to be replaced. There is also the risk of catching fire when fully charged batteries get too hot.

A few years ago, Model Aviation published a lengthy article explaining why a LiPo battery in a commercial product (e.g., laptop, tablet, cellphone, baby monitor, or alarm clock) is considered to be safe. Extract and give the battery to an RC pilot, and the instant it touches his or her fingers, it becomes dangerous. Why? The battery in its commercial context is connected to a dedicated, conservatively designed charging system. I forget the details of the article, so let me ad lib a list of why our batteries can become dangerous:

  • Incorrect charger settings, such as charging a NiCd battery, followed by a LiPo or charging a large battery, followed by a small battery, and forgetting to change the settings.
  • Using an old charger with separate balancing that is capable of using an incorrect cell count.
  • Not bothering to use cell balancing.
  • A battery drained below 3.0 volts.
  • Batteries can sustain crash damage.

What would be on your list? A budget-conscious modeler might be more concerned about having to replace batteries. He or she is interested in battery life. What should one consider if battery life is super important?

  • Don’t fly those batteries, or at least fly less. Most of the aging stress that we put on our batteries occurs because we put them in airplanes and helicopters and we fly them. But, of course, everyone is going to want to fly, so toss that idea aside. We really would like these pilots to come frequently to fly at our flying field.
  • If you fly airplanes and/or helicopters with 3- to 5-minute flight times, you should switch to flying relaxing airplanes that can fly for 20 to 30 minutes or more. Find an easy chair to sit in, relax, and let the thermals do most of the flying. Also consider that 20C batteries cost less than 80C batteries.
  • If you live in Arizona or Nevada, sell your house and move to a cooler climate. Of course, nobody is going to relocate just to benefit their batteries, but you can move your batteries to a cooler climate. I have a Peltier-cooled picnic box for hot days and can swap the Peltier wiring for warming when it is cold.
  • It takes an hour at 2.2 amps to charge or discharge a 2,200 mAh battery. The C rating for this size battery is 2.2 amps. Charging a battery at its 1C rate is generally recommended. Charging at a 2C or 3C rate is the stress equivalent of charging at a 1C rate twice or three times, respectively. A manufacturer’s statement that you can charge at 3C is a safety statement that has nothing to do with battery life.
  • If your batteries are infrequently used, or get set aside for the winter, proper storage is recommended.
  • Charging your batteries to 4.1 volts instead of 4.2 volts can improve battery life by a factor of approximately 2 or 3. Wow! There is a greater gain from limiting maximum voltage to 4.0 volts. The optimum storage voltage is not much better than at 4.0 volts.

The idea is to use 4.0- or 4.1-volt storage during the flying season if you want to insist on 4.2 volts when it is time to fly. If you don’t feel like charging up the last 0.1 or 0.2 volts, skip the hassle once in a while. It is also strongly recommended that 4.1 volts should be the maximum during hot weather.

  • Some pilots log carefully detailed notes for each battery so that they can be extra cautious about a battery that was discharged below 3.0 volts. Logging each cell’s resistance occasionally can be useful. The center cell is most likely to have an increasingly higher internal resistance. Center cells also tend to lose capacity at a faster rate.
  • Understand that a battery’s C rating is reduced with age as cell resistance increases and capacity fades. Effective C rating is also reduced at lower temperatures.
  • Make sure all of your aircraft systems are operating within their specifications. Use a wattmeter. It is more convenient to use a DC-capable clamp meter. Retest whenever you try a different propeller.

Regardless of your hobby budget, it is important to buy a good battery charger. On three occasions, I have arrived at a flying field and have been asked to diagnose why a battery had puffed up dramatically, belched smoke, and seemed ready to catch fire. Thanks to automatic cell detection, the charger was charging a three-cell battery, thinking it was a four-cell battery (an old-style charger and a Blinky cell balancer). A charger with a balance connector plugged in would have refused to proceed.

Several years ago, I came across an insurance claims list of LiPo accidents. While reading the list, I noticed a high incidence of cell miscount while charging. Make that liability obsolete by getting rid of the old-style chargers.

Using older chargers that can’t be updated requires being extremely disciplined about making sure the charger always ends up with the correct cell count. It is also a good idea to use LiPo sacks, ammunition cans, a carefully cleared location, and observation.

Lower-powered chargers, with embedded balancing, are not that expensive and, for safety reasons, they should be considered. Balancing is important so that at the end of charging, all of the cell voltages are matched. The cells with reduced capacity will otherwise end up with the higher voltages. These weaker cells will age even faster.

Typically, the center cell gets the hottest and loses capacity the fastest. Some battery chargers only engage the balancing process near the end of the charging process. Some will pause the charging process when it’s necessary if balancing has not been accomplished near the end. Staying below 4.2 volts is especially a safety concern in hot weather.

Balance meters are useful for indicating an approximate state of charge using a lookup table that translates voltage into SOC. Photos by Greg Gimlick.

Balance meters are useful for indicating an approximate state of charge using a lookup table that translates voltage into SOC. Photos by Greg Gimlick.

Balance meters are useful for indicating an approximate state of charge using a lookup table that translates voltage into SOC. Photos by Greg Gimlick.

Balance meters are useful for indicating an approximate state of charge using a lookup table that translates voltage into SOC. Photos by Greg Gimlick.

Extreme temperatures are tough on batteries. I think it was approximately the 1960s when electric vehicles were made available on trial. One thing the manufacturers soon learned was that those batteries, in places such as Arizona and Nevada, lasted half as long as those in one of the northern states. Those of you who have studied chemistry probably understand that chemical processes become increasingly energetic with temperature. This is called the Arrhenius effect. A 100° C (180° F) increase above room temperature is two times more energetic, equating to twice the stress and aging when charging, discharging, and storing.

This SOC graph is based on data that the author collected from one of his balance meters.

This SOC graph is based on data that the author collected from one of his balance meters.

If you want to win a Pylon competition, heat up your batteries just before the race, but that comes at a price for battery life. Storage voltage is a topic that nobody talked about until recently. Now it has become a marketing obsession (for some). How important is it? Of course, it is recommended for long-term storage. It’s very important if you store your batteries in a hot car on a scorching summer day in Arizona. It is less important if you leave the batteries in your car in southern Canada.

There is little, if any, value in pursuing a storage-voltage discipline for frequently used batteries. The net benefit might be negative, especially if you discharge your batteries to storage voltage each time you return from the flying field.

The chemical stress on the battery when charging to the full-charge voltage is approximately equal to a month (some say two weeks) of storing the battery at that voltage and temperature. When a battery is on its last flight of the day, land at half time for approximately a 50% charge. The hassle factor is another consideration. I overheard a pilot explaining that he didn’t bring his airplanes because his batteries were not charged and ready to fly. Some pilots have disciplined, checklist-oriented personalities. The hassle factor means nothing to them in regard to asserting a storage-voltage discipline.

Other pilots are much more ad lib, and the hassle of storage voltage stands a good chance of easing batteries into an early retirement from the flying field. My advice is to forget about storage voltage during the flying season, even if these pilots don’t fly very often. For the seriously hassle-averse—don’t bother. Will disciplined storage-voltage procedures extend the life of your batteries from three seasons to four? No. Will reducing peak voltage from 4.2 to 4.1 or 4.15 volts accomplish that benefit? Yes.

Storage temperature is a topic of opposing opinions. Some urge that room temperature is the best, or even the only acceptable way to go. Some recommend 50° to 70° F. Others refer to chemistry’s Arrhenius effect and are enthusiastic about putting their batteries in the refrigerator or even the freezer. (It’s a bad idea to put them in the freezer.)

Apply storage voltage before putting batteries into the refrigerator. Allow the battery to slowly return to room temperature before charging. Flying or charging very cold or very hot batteries applies a lot of stress.

Lithium dendrites can be a serious problem. Unlike NiCd whiskers, which blow like fuses, lithium dendrites are sharp, pointy, and very tough. They can punch their way through membranes and dielectric. It is then somewhat like waiting for the next earthquake. It could happen tomorrow, in a few months, or not in your lifetime, which is most often the case.

A lithium dendrite can short-circuit a cell, which then heats up violently and turns into a blowtorch. I have read about instances in which lithium dendrites grow a) above 4.2 volts; b) below 3 volts; c) below freezing. It’s not like 2.9 volts is bad and 3.0 is good.

How far you go into the zone is increasingly the problem. Dendrites significantly reduce cell capacity, and they are at risk of causing a spectacular fire. Be cautious about dendrite-damaged batteries.

Balance meters are useful for indicating an approximate state of charge (SOC) using a lookup table that translates voltage into SOC. In addition to battery SOC, you can read individual cell voltages to see how well balanced your battery is at the end of a flight. Battery chargers typically feature an SOC estimate; some only display the battery total. The total is useful so that you can cut back on your timer if the end-of-flight voltage is getting low. Understand that your balance meter and your charger can display a significant difference of opinion.

At the flying field, I found that a battery tested on different balance meters and chargers indicated different SOC percentages, ranging between 30% and 45%. I figured that different lookup tables were involved. Perhaps one manufacturer was selling mostly super-high-C batteries and another was marketing inexpensive, low-C batteries.

I used a three-channel DC power supply to simulate a three-cell LiPo battery. I discovered that all of the balance meters used the same lookup table (with minor differences at the low end). What I concluded was that the cheapest meters used inferior low-accuracy voltage metering. This makes a big difference where the SOC slope is relatively flat. None of these balance meters are expensive, so pay slightly more for one with a more accurate and stable voltage-measuring circuit.

Also consider that different model batteries exhibit somewhat different SOC behavior. My Power Lab 8 and 6 battery chargers have the built-in ability to cycle a battery and calculate an SOC table. You can use a different SOC table for each of your LiPo battery types.

Referring again to the graph, the slope is relatively flat in a broad range near the center, where storage stress is the lowest. As the slope gets steeper, the stress levels increase. Beyond 4.2 volts (100%), the stress level dramatically rises. Continue to pump in a charging current (trickle charge) and your battery will puff up, eventually vent toxic, smelly smoke, and will be at risk of catching fire.

Life Cycle Experiments

Dr. Jeff Dahn’s Li-Ion research group at Canada’s Dalhousie University was busy with battery-life experiments several years ago. Each investigation started with the following baseline for a typical, generic battery delivering approximately 500 cycles.

  • Cycle between 20% and 4.2 volts
  • 68° room temperature
  • 1C charge rate; 5C discharge rate
  • End of life at 80% capacity

Experimenting with maximum charge voltage produced the following results:

  • 4.1 volts: 2,000 cycles
  • 4.2 volts: 500 cycles
  • 4.3 volts: 100 cycles
  • 4.4 volts: 5 cycles

The recommended sweet spot between performance and life cycle was 4.15 volts.

Similar performance could be experimentally concluded at the low end. End your flights conservatively and your batteries will last longer. Fly a high-performance battery until you notice the motor slowing down and your energetic battery will soon fade away. Use a timer to signal when to land. New pilots experiment with low-voltage alarms, either audio or by telemetry. They soon discover that this approach is unreliable.

According to a statement found on the Battery University website, starting at 4.2 volts, "every reduction in peak charge voltage of 0.10 volts per cell is said to double the cycle life." That is a somewhat different finding, but it’s for Li-Ion batteries. The website also states that 3.92 volts per cell is the optimum long-life voltage and that anything below that offers little gain in life-cycle performance. Urging a more optimum storage voltage is primarily a theoretical argument. You can save the theoretically optimum storage voltage for seriously long-term storage.

Closing Summary

Things you can do that will rapidly age your batteries are having voltages below 3 volts, above 4.2 volts, high temperatures, or very low temperatures. Reducing peak voltage below 4.2 volts will significantly improve battery life.

Charger-embedded cell balancing helps to extend the life of the weakest cell(s) and is very important for safety. Charging at a rate of less than 1C will help improve battery life.

For a fun-fly event, you can break from your standard battery routines and charge your batteries all the way to 4.2 volts at a 2C or 3C rate. Why not? You’ll have time to have lots of fun at the fun-fly!

SOURCES: Spektrum RC

By David Buxton |


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nothing was said about using a battery for the first couple charges. does it effect battery life how one discharges [Flys or use throttle control] for initial couple charges. Is it important to go easy for first couple charges? say stay around half throttle.

This makes me very happy that I sprung for a Spektrum smart charger and smart batteries when I re-entered the hobby last year.

What causes this condition and are they still usable. ?

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