Lithium Polymer Guidelines

Lithium Polymer (LiPo) batteries, although generally available as early as 2002, really took off in popularity in 2003, and in 2006 (as of this writing) seem to be the most popular battery for experienced electric R/C pilots. The combination of light weight, good power, and long flight durations have made them the power source of choice for many new planes. Their popularity has mirrored that of cell phones, portable MP3 players, and other high-drain electronic devices which use lower-discharge Lithium Ion (LI-Ion) battery packs.

However, most Lithium Ion packs for consumer electronics have one feature which airplane hobbyists lack: built-in charge-protection circuitry. Some expensive LiPo packs, such as the FMA Direct "Skyvolt" system, boast such protection circuitry, and allow it to be disconnected for flight, but most commodity LiPos and chargers don't. Unfortunately, LiPos have one bad trait: they tend to catch on fire and/or violently explode when abused! In general, LiPo abuse falls into one of three categories:
1. Physical pack damage (e.g. a crash or hangar damage)
2. Pack overcharging or overdischarging
3. Improper heat management in flight or during charging.

I've done a lot of reading regarding LiPo fires over the last year, and this post is my attempt to summarize how to deal with your LiPo batteries in a safe, effective manner. The technology seems to be getting better and safer with each passing year, but there are, regardless, many reports of vehicle and home fires due to unsafe handling of LiPo batteries.

Pilot

  • If you are new to electric power, become familiar with using lower-risk NiCD or NiMH packs prior to trying LiPos.
  • Use the "lip test" to measure the temperature of your LiPo packs after a run. If the packs feel uncomfortably hot against your lips, it's likely you are drawing too much current and should re-evaluate your power setup. A warm pack, however, is normal.
  • Don't carry LiPos in your pockets if you can avoid it. A short could result in very bad burns.
  • Keep your packs in a warm place prior to usage. LiPo power drops off dramatically when the packs are cold. Over-discharging during cold weather, while posing less of a heat risk than during hot weather, can still result in permanent damage to a cell.
  • Be aware of what "normal" flying feels like with your LiPo-powered aircraft. When you feel the power sagging, it is time to bring the bird home. Discharging cells to below 2.7V can cause permanent damage to the pack.
  • Try not to fully discharge your packs every flight. They will last you much longer, and resist cell imbalance.

Charging
Most pack fires occur during charging! Pay special attention to your charging routine and safety procedures with LiPos.

  • Avoid charging LiPo packs unattended. One need not watch them the whole time, but they should be checked, at a minimum, every 15 minutes. It is best to remain in the area, as the odor of a venting pack is distinctive and strong and sure to catch your attention.
  • LiPo packs, unlike NiCD or NiMH, should never get warm during charging if they are electrically sound. If they get warm, they are damaged and should be destroyed.
  • Use only chargers which are designed to charge LiPo batteries. My two favorites are the Great Planes Polycharge 4 ($99 from www.towerhobbies.com), which charges 4 LiPo batteries simultaneously, and the Great Planes Triton ($129 from www.towerhobbies.com), which can also handle my other battery needs (but only for one battery at a time). The Triton has the aforementioned thermal probe, and is particularly useful for the beginner who will be using NiCD, NiMH, and LiPo batteries.
  • Charge LiPo batteries in fire-resistant containers where you can easily see and handle the packs. I use an old crock-pot for this purpose, but small fire-resistant safes, pyrex pans, pots, and ammunition containers work well too.
  • Do not charge LiPo batteries inside an aircraft.
  • Charge your packs at a maximum of 1C. For instance, if you have a 1300mAh pack, the maximum rate at which you should charge the pack is 1.3 amps.
  • If your charger requires you to manually input the voltage, double-check that you are charging at the rated capacity for the LiPo you are using: 3.7V for 1S, 7.4V for 2S, 11.1V for 3S, and 14.8V for 4S.
  • Keep cells in multi-cell packs "balanced". This means keeping the voltage equal for each cell. This avoids overcharge and overdischarge of a pack, and pack balancing should take place at least once a month during flying season, if you fly regularly. Pack balancing can be done in three different ways: 1. Charge the whole pack at 1/10C. For packs without a balance lead, this is often the most practical option. For instance, 1/10C on a 2100mAh pack is 0.21 amps. With 10 hours per pack charge, however, this option is time-consuming.
    2. Connect charge leads to individual cells, and charge to within 0.1V of one another. This option is the "old-school" method of keeping packs balanced.
    3. Use a "balance connector" and either a balancing charger, or a pack-balancing device. For some older packs which lack a balance connector, this may be inconvenient (soldering balance connectors to tabs), but is regarded as the safest, quickest, and most reliable balancing method.
  • If using the 1/10C 10-hour balance charge method, you should use a charger which provides a thermal probe for emergency charge cut-off if a pack gets too warm, and probably charge them out-of-doors if leaving them unattended.

Discharging

  • Figure out how many amps your motor is drawing from your pack in a static test, and evaluate this against the 'C' rating of the pack. For instance, if you have a 2100mAh pack rated at 7C, the maximum sustained discharge is 14.7 amps. If you are close to or over this "C" rating, you risk an in-flight fire. [*}Manufacturers routinely pad their C ratings, particularly off-brand pack manufacturers. Most flyers who get good life from their packs try to size their packs to discharge at half the rated sustained discharge rate.
  • Limit your static tests to less than one minute. LiPos, ESCs, and motors require airflow to remain cool, and long static tests will tend to cause them to overheat. You can figure out your total flight time without testing it statically, using the following formula. Remember that one amp is 1000 milliamps, and you should use a multimeter for accurate amp readings on your motor power draw: (LiPo pack mAh / Full Throttle Amp Draw in mA) * 60 = flight time in minutes
    If you set a kitchen timer for that length of time, you can be certain of never over-discharging your LiPos. For instance, I used a multimeter to check the amp draw of one of my planes, sporting a brushed 480 motor, 2100mAh LiPo, and 5.3x3.3 prop. It drew 10.4 amps:
    (2100mAh / 10,400mA) * 60 = 12.1 minutes
    It's a reliable worst-case estimate. I've found that, if I run my plane full-throttle, at around 13-14 minutes the LiPo power sag occurs and it's time to bring it down.
  • Don't attempt to re-use swollen packs. While it's possible to salvage, for instance, a 3S pack and make it a 2S pack by discarding the swollen cell and keeping the two good cells, it may be wiser to dispose of the whole pack safely.
  • Avoid running your aircraft at full-throttle for long periods until you have become aware of the power profile of your chosen packs and motor.

Upcoming events

  • No upcoming events available