Batteries

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The enabling technology for electric planes was the advent of good, cheap rechargeable batteries. Nickel Cadmium (NiCAD) and Nickel Metal Hydride (NiMH) came on the scene in the 80's and early 90's respectively, but Lithium Ion batteries, in particular a lightweight no-frills type of cell called Lithium Polymer, or just 'LiPo', have largely supplanted them since they became available starting in the late 90's. Lithium polymer batteries are now the de facto standard for the vast majority of RC builders and pre-built kits, with a small minority sticking to 'nitro' engines (a two-stroke glow-plug internal combustion setup), and a very few beginner/budget models shipping with NiMH.

  • This page covers LIPOs, which are the most relevant option in aerial applications.
For NiMH, see: Nickel metal hydride batteries.
For NiCad, see: Nickel cadmium batteries.
For Fuel cells, see: Fuel cells.
For anything else, see Other batteries.

Contents

[edit] LIPO

Lithium polymer batteries change the typical lithium ion chemistry by placing the lithium-salt electrolyte in a solid polymer composite like polyethylene oxide or polyacrylonitrile rather than in an liquid organic solvent. These can be lower cost, shaped into arbitrary packages rather than cylindrical cells, and have somewhat higher energy density, at a cost of being more vulnerable to punctures, over-charging, and over-discharging, and having a lower cycle life. LIPO cells do not suffer from memory effect.

There are several competing varieties of LiPo chemistry with slightly different cell voltages, safe charge/discharge rates, energy densities, and cycle lifetimes.


[edit] Chemistry

There are three primary material variables in lithium ion battery manufacture:

  • Cathode Material (positive electrode): A variety of materials in Li-Ion cells and Lithium Cobalt & rarely Lithium Manganese in LiPo cells
  • Separator Material: Lithium salt & liquid organic solvent in a Li-Ion cell, lithium salt and polymer in a LiPo.
  • Anode Material (negative electrode): Carbon, Titanate, Silicon, and Germanium

Commentary:

  • Titanate anodes usually signify higher current capacity in Li-Ion & LiPo cells
  • LiFePo4 cathodes usually signify high current capacity and good durability in Li-Ion cells
  • LCO cathode, particularly in polymer cells, is understood to be very touchy, ans is associated with durability/safety problems, but is used despite this for its high energy density
  • The standard choices for RC users are:
  • 1) LCO cathode, carbon anode LIPO cells
  • 2) LCO cathode, titanate anode LIPO cells
  • 3) LiFePo4 cathode, titanate anode Li-Ion cells, such as A123 brand

[edit] Typical Energy Densities

  • Lead Acid: 40Wh/kg
  • NiCd: 63 Wh/kg
  • NiMH: 90 Wh/kg
  • Li-Phosphate: 104 Wh/kg
  • Li-Manganese: 116 Wh/kg
  • NMC: 140 Wh/kg
  • Li-Cobalt: 170 Wh/kg

(ref) - LiPoHobby

[edit] Cells

LiPos are constructed of a number of cells in parallel, and in series, and the array of cells is labelled as, for example, '4S2P' for 2 parallel banks of 4 cells in series (3S and 4S tend to be typical). Each cell has an arbitrary amount of energy it can hold before things break down, and this energy is reached when the battery has been charged up to a maximum safe voltage. This voltage, which is common to all cells of that particular lithium chemistry, is usually in the vicinity of 3.6V, and should be multiplied by the number of cells in series to get the typical working voltage of the whole battery. 'Fully charged' typically is considered to be after the battery has been fully saturated at the working voltage, and the charger has brought it up (in low-current mode) to around 4.2V per cell. Energy divided by voltage is expressed as current * time, which is the normal rating for charge capacity. Typical capacities range from 100mAh for a very small, light model to 5000mAh for a medium-sized multirotor or 2-meter long range FPV setup, up to arbitrarily high levels.

A customary terminology that is unique to RC is the 'C' rating. The C rating is the safe discharge rate in amps divided by the battery capacity in mAh. A LIPO with 5000mAh capacity that is rated at 10C can fully discharge in 1/10th of an hour, at 50 amps. The limiting factor is the internal temperature of the battery. A higher C rating indicates that the internal structure provides a lower 'equivalent series resistance', and generates less heat internally. Usually a higher C rating (higher power density) has a slight tradeoff in lower energy density.


[edit] Charging

Always charge a LIPO with a purpose-built battery charger with a balancer. The standard conservative recommendation is never to exceed 1C charge rate, although it is expected that the higher discharge packs (60-75C) should have higher permissible charge rates than the low end (10-20C).



[edit] Safe Discharge Level

LiPo *cells* cannot be safely "fully discharged" without causing damage, and the further discharged they go, the fewer cycles they last. If discharged to 2.5V once, a typical LIPO will be significantly damaged, perhaps even catching fire the next time it's charged. Typically a smart battery, designed with a controller chip to protect the cells, will give up and refuse to emit any more power after the safe discharge level is reached, and capacity ratings are counted as rising above this minimum working voltage. This is how LIPOs work in things like cellular phones and laptops.

In RC we use dumb LIPO batteries. A dumb battery will keep right on going, and potentially cause problems like failure to charge or even fire the next time it is charged. Most ESCs are programmed with an 'alarm voltage' and a 'cutoff voltage', and typically will stop issuing throttle power after 3.0V or 2.7V is reached. 3.0V is considered the rule of thumb for minimum discharge level for a long battery life, and pushing the battery beyond this point is somewhat unproductive, because around 3.1V the voltage starts to drop precipitously (3.1V -> 3.0V has much less energy than 3.7V -> 3.6V).

The voltage charge/discharge curve is characterized by a rapid rise in overall charge up to a 'typical working voltage', then a flattening of the curve as large amounts of charge are added while only slightly changing the voltage, and then a rapid uptick in voltage as the battery is full and about to be damaged through overcharging.

Special LiPo chargers are required to intelligently manage this process, and they will typically connect through the main charge/discharge cable (usually using a connector known as Dean's Plug), as well as a separate multi-pin cable to detect the voltage of the individual cells. These chargers will detect directly when the cell is fully charged and shut off automatically, but even so, charging is generally recommended to occur in a fire-safe area.

[edit] Fire Safety

If a LIPO encounters an external fire source, or if it's being discharged far faster than its C rating & overheats, or if it's discharged to a damaging level and then charged up, the lithium in the battery can be a potent fuel source for a fire. Special fire-safe bags are designed to protect LIPOs from external ignition sources, and contain a fire from a small battery. It is recommended not to charge a LIPO battery unattended, not to store it in on a car windshield, never to puncture it, to have the charging area as fire-proof as possible, not to begin charging the LIPO if it starts to bulge out of its package, and to always use an appropriate purpose-built LIPO charger with a balancing cable, which is set with an automatic cutoff so that it doesn't overcharge.

In the event that you screw up something else in the aircraft and it causes the LIPO to catch fire, here's what a large LIPO looks like burning:


Here's what a small LIPO looks like being overcharged:


Here's Alishanmao showing what can be done with a partially discharged battery before it will catch fire. The contrast appears to be partly down to the fact that soaking was done first, and partly down to the fact that he didn't try to charge these up to 4.2V after they started puffing up.


Here's a steel fire-safe box, and a demonstration of ignition caused by puncturing a fully charged cell:


[edit] Disposal

A LIPO can't be discharged below 2.5V without damage, but if a battery is already damaged but with significant voltage, simply throwing it in the trash is a fire risk, particularly in the context of garbage trucks and dumpsters with compactors. Most people throw out their batteries when they start to 'puff up' with hydrogen gas. A saltwater bath for a few days is the easiest way to safely (fully) discharge a LIPO battery the final time.


[edit] Links

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