Lander Lab #5: Lithium Polymer Batteries
A column about the Technologies, Strategies, and Use of Ocean LandersLithium-Polymer batteries offer a safe and robust option for portable electrical power. As we collectively have gained experience, application parameters have been refined, defined and improved. Though ubiquitous in everyday life, their acceptance has not quite outrun a troubled past.LiPos have now flown on crewed Space-X spacecraft and are installed on the International Space Station. The global automotive industry is undergoing dramatic change with new battery developments that follow the personal computer and cell phone industries. The Tesla Powerwall, which uses a stack of LiPos, allows homes to get off the grid when the grid goes down. WHOI’s DSV ALVIN, James Cameron’s DSV DEEP SEA CHALLENGER, and Triton Submarine’s TRITON 36,000/2 (DSV Limiting Factor) submersibles are powered by pressure compensated LiPo battery modules. In 2019 and 2020, Japan launched two new diesel-electric submarines, Taigei, or “Great Whale,” and Toryu, or “Fighting Dragon,” equipped with lithium-ion batteries, making the submarines capable of cruising silently submerged for longer.Like different animals at the San Diego Zoo, it is important to recognize LiPo’s are simply different than lead-acid or alkaline batteries. They have their own distinguishing behaviors. I admit to some apprehension when I began studying, then learning-by-doing with LiPos. I started out with LiPo batteries inside of fireproof charging bag, on a concrete floor within a small cinderblock bunker 6-feet away from everything else. I moved to a big clay garden pot with a clay drain pan for a lid. I monitored temperature, and voltage. I read everything I could. I followed up on promising leads. It was DreamWorks’ “How To Train Your Dragon”. I needed to know. Safety remained paramount, as I began to get a handle on the technology a lot of others were already using.I have now used LiPos for several years in a variety of at-sea applications. The more you learn, the more successes you have, and the more possibilities present themselves. I respect their energy density, but also their willingness to do a job.There is a large body of work describing fundamentals of LiPo cells and their application. This article will cite some of them. In brief, always treat LiPo’s with the respect they deserve. It is crucial to stay within the industry recommended operational parameters. The terrible tales of weekend RC enthusiasts can partly be blamed on those end-users straying outside the prescribed boundaries to gain a few more minutes of flight time, or trying to get one more flight in by fast-charging above the recommended max rate. Sometimes it’s a lousy battery they got at a cheap price. Sometimes they don’t have the right devices to protect and maintain the batteries as they should be.Some Basics on Li-ion and LiPo batteriesLithium-ion and Lithium-polymer batteries are rechargeable, secondary cells with similar chemistries. They can be used for the same high-power applications. They are distinguished by different electrolytes and packaging. Lithium-ion and Lithium-Polymer are completely different from primary, non-rechargeable lithium cells. For one, lithium polymers use an intercalated lithium compound for their electrodes, not metallic lithium, and therefore are more stable. They are also not hazardous if exposed to water.Lithium-ion cells use a liquid electrolyte, and are encased in a cylindrical stainless-steel housing. The well-known 18650 cell is an example.Lithium-polymer (LiPo) cells use a gelled electrolyte and are vacuum sealed inside a soft plastic pouch. This is known as a “prismatic cell”. There are several LiPo chemistries, but the most common for prismatic packs is Lithium Cobalt Oxide. The chemistry is specified in the MSDS for that battery. A stack of prismatic cells maybe be bound together using a shrink wrap, for lightweight applications, or inside an injection molded hard plastic casing to provide puncture resistance. Compared to alkaline cells, LiPo’s have lighter weight, higher capacity, higher discharge rate, and are less affected by cold temperatures as may be found deep. Cycle life can be 100’s before battery capacity is reduced to ¾ of its original capacity.A LiPo configuration is specified by the number of prismatic cells, and how they are connected, that is the numbers of cells in series and in parallel. A “6S1P” means there are 6 cells in series, and only 1 stack, with none in parallel. Virtually all prismatic batteries are wired with all cells in series, and so the configuration is simply referred to as “6S”, with the “1P” understood.About voltages: The nominal voltage of a battery is the number of cells x 3.7v/cell, the mid-point voltage of a battery pack. A 6S battery is described as 6 cells x 3.7v/cell = 22.2v, though a full charge is 25.2v.LiPo batteries have four key voltages to know:Max voltage: 4.2V/cell (6S x 4.2v/cell = 25.2vdc)(Managed by the BMS, explained belowStorage voltage (50% of full charge): 3.85V/cell (6S x 3.85v/cell = 23.1vdc)(Charge battery, then discharge with CBA IV Pro to this value. Explained below.)Shipping voltage (30% of full charge): 3.71V/cell (6S x 3.71v/cell = 22.26vdc)(Charge battery, then discharge with CBA IV Pro to this value. Explained below.)Min voltage (Absolute): 2.9V/cell (6S x 2.9v/cell = 17.4vdc)(Managed by the BMS, explained below)Figure 2. State of Charge versus LiPo Pack Voltage (Chart courtesy of Mark Forsyth, RCGroups.com)Using a 10” polyamide sphere, I’ve made a positively buoyant 22v/20Ah (440Wh nom) rechargeable battery. That’s a car battery that floats.LiPo quality varies between consumer and commercial grade batteries. Commercial grade batteries are made by paying attention to quality at every step, including selecting cells that match the characteristics of others in a stack. Consumer grade batteries are made by paying attention to cost. The difference is like buying batteries from Duracell versus Harbor Freight. Two commercial brands I tend to use are Gens Ace Tattu and Turnigy Graphene. One list of the top LiPo manufacturers can be found at