Understanding Lipo Cells

[DPeterson]

Some information pulled from an RC Driver article by Tony Phalen. According to the article Tony has raced RC cars at every level for a quite awhile. He had a lipo burn and took out a bran new Tamyia F104X1. Confused as to why he contacted a good racing friend of his Mr. Ling for some answers.

Mr Ling worked in the product development department of a company that deals with medical devices. During his time there, he was in charge of developing a wallet sized device that was designed to vaporize liquid insulin. For those that don't know, this type of device requires a ton of power in a tiny package. His research led him to the understanding of battery construction, failure analysis and audits of many battery manufacturers. To say that he has some knowledge of lipo batteries is an understatement. After he heard about my lipo woes, he contacted me and we had a chat. We decided that this was probably a good time to clear the air about lipo batteries and this little interview is the result of that chat.

Tony - Ok Ling, what exactly is inside a lipo battery? How similar is it to the older NIMH cells?
Ling - A lipo cell is a classic battery construction. It is composed of an anode, acathode and a seperator. These components are inside the aluminum pouch and it's filled with a liquid electrolyte. Oddly enough, the "Polymer" in modern Lithium Polymer cells refers to the electrolyte-which isn't a solid polymer at all.
The anodes are composed from a copper foil current collector coated with a fine particle graphite slurry. Most cathodes are composed of a LiCoO2 (lithium colbalt dioxide) coated onto an aluminum foil current collector. There are some other mixtures that they throw in there, but for the most part-it's LiCoO2 based. The polymeric seperator does just that, it physically seperates the anodes and cathodes.
Surprisingly, RC Lipos are the exact same chemistry used in your cell phone or laptop. There is a tradeoff between Li-Ion cells: high energy vs. high power. Laptops and cell phones fall into the high energy category and RC Lipos fall under high power. The difference is really just the architecture of the cell.
The high energy type cells generally uses a single anode/cathode pair with a seperator between the two. This sandwich is generally rolled up in a "jellyroll" format to create either a cylindrical cell or a prismatic (rectangular) cell.
The high power type of cells are made of a multitude of anode cathode pairs. This is a stacked cell configuration. A typical Lipo is composed of 20-30 pairs of anode/cathodes. The seperator is either a "Z" shaped accordion or a spiral wrapped between the anode/cathode. Think of an RC Lipo as a whole bunch of smaller cells where all the anodes are all welded together and all the cathodes are welded together. The actual thickness of a one Lithium Polymer cell (typically under 10mm thick) is actually limited by the number of anodes/cathode pairs that are ultrasonically welded together. If there are too many parts of anode/cathodes, the ultrasonic weld will fail.

To Be Continued;

[DPeterson]

Tony - Good to know about the high energy vs. high power. Guess I won't be hooking up a 6500mah 65C rated battery to my iPhone. Speaking of that, what does C rating mean?

Ling - Technically, it means the relative charge and discharge capability. For simplicity, take the capacity in amp hours and multiply it by the C rating. So for a 5 amp hour battery, 1C = 5 amps, 2C = 10 amps etc. The discharge C rating is the maximum continous discharge rating that a cell can handle. There are no industry standards which define when a cell fails a C rating, whether it's the number of cycles, ending temperature, capacity, etc.
In the RC world, the C rating means next to nothing. I can pretty much guarantee you that every RC Lipo would self destruct when discharged at the C rating on the label unless it's a short burst. For a 5000mah battery, a 40C discharge is 200 amps. A complete discharge at 40C would last exactly 1.5 minutes. There is absolutely no RC application that dumps the battery in just 1.5 minutes. A genuine Deans connector starts melting at 80 amps continuous-a fake Deans fails at even less.

To Be Continued;

[DPeterson]

Tony - Well, I always figured some of those numbers were a bit exaggerated. I'm assuming sticking to a quality, brand name battery is probably the best way to get an accurate C-rated pack. Other than that, is there any way to determine a quality pack vs. a cheap pack besides price?

Ling - For the average consumer, it's very hard if not impossible. High quality packs are distinguished by higher safety and abuse limits, higher cycle life, better cell to cell consistency, longer shelf life etc. Higher quality cells have higher standards for incoming material inspection, more in process inspection (high pot testing, x-ray detection of stack etc), and generally a much cleaner production line that limits the amount of impurities that can enter the cell. Once the cell is pouched, pretty much all the cells look alike (high quality vs. low quality) on the outside. The raw cost of a cell from a high quality manufacturer can cost about 4X more than a cell with the same specs from a low end manufacturer. Any high quality pack will cost more to produce, however, all expensive packs are not guaranteed to use high quality cells.

To Be Continued;

[DPeterson]

Tony - So, as far as Lipos go, it can be a 'get-what-you-pay-for' situation. Are there any benefits to hard cases vs. soft cases?

Ling - Absolutely! Remember the copper and aluminum cathode layers I mentioned before? They are seperated by a very thin polymer. A dent (especially along the edges) will cause the sharp edges of the copper and aluminum to damage and start eroding away the thin seperator. This is exasperated by the naturally expansion/contraction of the battery during charge and discharge. Eventually, the eroded hole will lead to an internal short circuit. Best case, the cells short out internally and no longer hold a charge for an extended amount of time. Worst case, the short builds up temperature and leads to thermal runaway-ie.fire. So yes, absoluetly, hard cases are beneficial.

To Be Continued;

[DPeterson]

Tony - Good to know...I'll be placing all of my soft-case packs in a case! Hard-case packs from here on out! Lets talk about charging them. Are there any basic charging guidlines?

Ling - There is not much to it. Almost all chargers follow the standard CC/CV charge profile pretty well. With higher charge rates (like 2C and above), there is almost always a big drop in cylce life associated with it. The exception is Lithium Phosphate (LiFe), which does very well at higher charge rates, but those aren't too common yet...

Tony - So stick with the standard charging guidelines as far as charging amps?

Ling - For Lipos, I'd recommend charging 1.5C or under with a charger designed specifically for lipos.

Tony - A question I'm sure everyone wants an answer to...is it necessary to balance a Lipo pack every time you charge it?

Ling - This is a loaded question. For a high qualkity 2S pack (7.4v), it's highly unlikely that the batteries will ever need balancing. For higher cell counts, 3S and above, it's a good idea to balance charge. For cheap packs, it's always a good idea to balance charge.

Tony - Well, for the sake of safety, is it a problem to balance Lipo packs regardless of their 'supposed' quality?

Ling - Balancing is an insurance policy. It prevents a cell from getting overcharged. Overcharging is a surefire way to get a Lipo cell to erupt into flames. Unlike Nimh or Ni-cad, Lithium Polymer batteries cannot tolerate or absorb any overcharge.

Tony - What does it mean when the battery pack looks puffy or bloated?

Ling - It just means that the electrolyte has started to break down. A lot of things can cause this: over-charge, over-discharge, over-heating, cell contaminants, internal corrosion, poor material purity, etc. It takes very little liquid to form a whole lot of gas. Unfortunetly, puffing is very bad. The anode/cathode layers are vacuum sealed in an aluminum pouch. This vacuum seal helps prevent the anodes/cathodes from shifting around and shorting out on each other. Once puffy, the vacuum is gone and the cell is much more likely to internally short out and as mentioned before, an internal short may lead to fires.

This is a retype of an article in an RC Driver magazine provide by a friend.

Disclaimer - I am not representing any organized racing association.

Be carefull handling those Lipo cells repetitively.

Doug

[JIM MARCUM]

Tony - Good to know...I'll be placing all of my soft-case packs in a case! Hard-case packs from here on out! Lets talk about charging them. Are there any basic charging guidlines?

Ling - There is not much to it. Almost all chargers follow the standard CC/CV charge profile pretty well. With higher charge rates (like 2C and above), there is almost always a big drop in cylce life associated with it. The exception is Lithium Phosphate (LiFe), which does very well at higher charge rates, but those aren't too common yet...

Tony - So stick with the standard charging guidelines as far as charging amps?

Ling - For Lipos, I'd recommend charging 1.5C or under with a charger designed specifically for lipos.

Tony - A question I'm sure everyone wants an answer to...is it necessary to balance a Lipo pack every time you charge it?

Ling - This is a loaded question. For a high qualkity 2S pack (7.4v), it's highly unlikely that the batteries will ever need balancing. For higher cell counts, 3S and above, it's a good idea to balance charge. For cheap packs, it's always a good idea to balance charge.

Tony - Well, for the sake of safety, is it a problem to balance Lipo packs regardless of their 'supposed' quality?

Ling - Balancing is an insurance policy. It prevents a cell from getting overcharged. Overcharging is a surefire way to get a Lipo cell to erupt into flames. Unlike Nimh or Ni-cad, Lithium Polymer batteries cannot tolerate or absorb any overcharge.

Tony - What does it mean when the battery pack looks puffy or bloated?

Ling - It just means that the electrolyte has started to break down. A lot of things can cause this: over-charge, over-discharge, over-heating, cell contaminants, internal corrosion, poor material purity, etc. It takes very little liquid to form a whole lot of gas. Unfortunetly, puffing is very bad. The anode/cathode layers are vacuum sealed in an aluminum pouch. This vacuum seal helps prevent the anodes/cathodes from shifting around and shorting out on each other. Once puffy, the vacuum is gone and the cell is much more likely to internally short out and as mentioned before, an internal short may lead to fires.

This is a retype of an article in an RC Driver magazine provide by a friend.

Disclaimer - I am not representing any organized racing association.

Be carefull handling those Lipo cells repetitively.

Doug

Excellent information Doug. This is the type of scientifically based information we all need, and provides valuable SAFETY lessons to be learned for oldtimers and newbies alike.

#1 - ANY LIOP battery that has a puffy, swolen cell should NOT be charged or used again. Such swolen packs should be disposed of in a safe maner. (exactly what that means is unclear)

#2 - NEVER attempt to "save" a LIPO battery with one dead or low loltage cell buy force charging the bad cell.

#3 - NEVER "top-off" a fully charged LIPO for any reason.

The jury - for some - remains out on how to best control a lipo fire, and prevent secondary fires spread by flaming LIPO fluids to nearby combustables. Based on LIPO manufactures recommendations referenced in other LIPO fire threads, and my personal experience of what has worked well for me, I recommend the following if you are faced with a LIPO fire:

#1 - IMMEDIATELY remove a burning LIPO from inside any home or trailer, and get it outside.

#2 - CONTROL the fire and toxic smoke by submerging the burning LIPO in a fireproof container full of water.

#3 - After the burning LIPO has cooled down & is safe to handle, dispose of the remains in accordance with applicable HAZMAT procedures. Call or visit the nearest firestation for their disposal recommendations.

Be smart. Be safe. JIM