**sticky** Understanding Lithium Polymer Batteries

[Bartman]

I wrote this for the multirotorforums.com site about a year ago and figured some of y'all would find it helpful so I'm reposting it.



In researching a few questions I had on Lithium Polymer batteries recently it occurred to me that it would be nice if we had a comprehensive LiPo battery thread of our own where we could keep the info up to date and provide a central spot for answering battery questions. To that end I wrote the information below using info from the internet and from my own personal experience.



As always, please feel free to point out inaccuracies so that I can make corrections and make the info below as accurate as possible.



Thanks, Bart



[SIZE= 14px]PART 1[/SIZE]



Are you ready to be a LiPo Ninja?



There are a lot of items to be familiar with when it comes to lithium based batteries. We'll establish a few basics and then dive into the more advanced topics. When you're done reading you should be able to size and specify batteries for your multi-rotor helicopter and be competent in handling, charging, and storing your packs.





[SIZE= 14px]FIRST THINGS FIRST, THE THREE C'S[/SIZE]



Before we get started, it's worth pointing out that there are many different types of lithium based batteries but this discussion will focus on Lithium Polymer (also known as LiPo) packs. You may find in your research that LiPo packs are available with a hard case which are generally made for RC cars where the batteries have a better chance of being damaged in normal use. For most of our uses the hard case packs aren't necessary although some FPV or starter quads may benefit from having hard case batteries if they stand a good chance of contacting the ground during take off or landing.





The three basic concepts (the three C's if you will) worth knowing for Lithium Polymer battery packs are

1. sCell count (Number of cells in the pack, represented by the letter S)
2. Capacity (The amount of energy the pack is capable of providing once fully charged, designated by the letters mah)
3. Current (The maximum amount of current or amps the battery can provide at its rated Voltage)

As the heart of our LiPo packs are the individual cells. Take a look at a pack and you'll see there are multiple flat cells stacked together and wired in series.

Each individual cell is rated for a nominal voltage of 3.7 volts. A pack made up of one cell will have a voltage rating of 3.7 volts. 2 cell packs are rated for 7.4 volts, 3 cell packs are rated for 11.1 volts, and so on and so on. Manufacturers designate the number of cells in a pack with the letter S. A one cell pack will have 1S on the label, 2 cell packs will be labeled 2S, and so on and so on. If you can remember that the S stands for sCell count and that the rated voltage is the number of cells times 3.7 volts per cell, then you've taken your first step towards becoming a LiPo Ninja.:victorious:





Another standard designation on LiPo labels is the Capacity of the pack, usually written as a number followed by the letters mah. Mah stands for Milli-amp-hours with milli-amps being a unit of electrical current and hours being time. A 6000 mah battery pack therefore is rated to provide current measuring 6000 milli-amps for one hour. Since there are 1000 milli-amps in an amp you could also say that a 6000 milli-amp-hour pack can provide 6 amps of current for one hour. After that hour the pack will theoretically be fully depleted having discharged all of its capacity. If you discharge this pack at a rate greater than 6 amps then it will last less than an hour. To keep this from getting confusing let's just say that the Capacity number tells you how much electrical power is being stored and when the number gets bigger, the flight times generally increase.



But before you start thinking bigger batteries are always better, keep in mind that much bigger batteries may weigh so much more that they won't enable you to have longer flight times. There's a learning curve involved with determining what will be best for your helicopter equipment but just remember that the mah rating is the Capacity of the pack.





The last of the basic concepts to understand with LiPo batteries is the max Current (or Amps) that the battery can provide. The maximum Current the pack can provide is a function of the pack's capacity. A capital letter C is used on your Lipo's label to designate the max current that the pack is rated for. The maximum current a 20C pack can supply is 20 times the Capacity of the pack. We've already learned that capacity is the mah rating of the pack.



Here's an example, future LiPo Ninja;

Suppose you have a 4S, 5000mah, 20C pack.

The voltage rating of the pack is 4*3.7 volts/cell or 14.8 volts.

The capacity of the pack is 5000 mah (milli-amp-hour or 5 amp-hours).

The max current the pack can provide is 20C, or 20 times 5000mah. 5000 milli amps is another way of saying 5 amps (1000 milli-amps per 1 amp) so the max current is 20*5 or 100 amps.







Picking a battery that is appropriate for your multi-rotor helicopter starts by determining at what voltage you want to run your motors. This allows you to select a pack with a particular cell count (3S, 11.1 volts; 4S, 14.8 volts, 6S, 22.2 volts; etc). Once the cell count is figured out, you can determine the amount of amps the helicopter will draw during normal flight and during periods of maximum throttle which lets you choose an appropriate C rating or maximum current rating. Once these two items (sCell count and max current rating) are in place you can review the range of capacities offered and select a battery with enough capacity (mah rating) to provide sufficient flight times.





Generally speaking, cell count affects battery size in that with more cells the pack will be bigger/heavier. C rating affects pack size/weight to a lesser degree although high C rated cells will be physically bigger than lower rated cells. Because C rating doesn't significantly affect the size of the battery some people say it's good practice to get the maximum C rating that you can afford. I don't follow this line of thinking but some people do. Cell capacity definitely affects pack size/weight due to the fact that as cell capacity goes up so does the physical size/weight of the cells. All things being equal, a 1000mah pack can be half the size of a 2000 mah pack.





With the basics behind us let's move on to other topics worth knowing if you hope to graduate to true LiPo Ninja status.

13023=6707-series parallel battery wiring.jpg

 

[Bartman]

[SIZE= 14px]PART 2[/SIZE]





[SIZE= 14px]UNDERSTANDING DISCHARGE LIMITS[/SIZE]

It was pointed out before that your LiPo cells are nominally rated at 3.7 volts per cell. When they're finished charging they will actually be at 4.2 volts per cell. As the cells discharge during use the voltage being produced will quickly go down from the 4.2 value and remain fairly steady in the range of 3.75 to 3.8 volts per cell. Under a heavy load they will drop as much as .3 to .5 volts but when relieved of that load they will return to and remain at approximately 3.7 volts until almost fully discharged. It is, therefore, this voltage level that represents the rating of the battery.







You may be asking (as a true LiPo Ninja would), how do you determine when a battery is depleted and it's time to land the helicopter? There are a few rules of thumb to determine how much of the battery's capacity can be used safely. One rule of thumb says to not use more than 80% of the battery's capacity. To know if you've gone beyond 80%, it helps to have a charger than can keep track of how much capacity (Total mah) was put back into the pack during the charge cycle. Divide the mah put back into the pack by the rated capacity of the pack (and then multiple by 100) to determine the % that you used and try not to exceed 80%. If you are keeping track of how much time you're flying and how much mah is being returned to the pack with each charge it isn't hard to come up with time guidelines that will keep you from exceeding the 80% discharge rule of thumb.





Another rule of thumb (the one I prefer) is to not let your cells go below 3.3 volts. There are inexpensive and very lightweight battery monitors/alarms that plug into the battery balancing tap/plug and that can be velcroed to the side of the battery. They are generally set to alarm when any one cell in a pack goes below 3.3 volts. The alarms may activate if you quickly increase throttle causing the voltage to sag but if they stop when your reduce throttle then you're generally safe to continue. A continuous beeping alarm during normal hovering flight indicates it's time to end your flight (preferably with a nice landing). Again referring to the information provided by a good computer controlled balancing charger, it's possible to see exactly what the individual cell voltages are in the pack when it's hooked up for charging. This is a good opportunity to decide if it's safe to extend your flight times a little or if they actually need to be shortened.



It's important to know that the cells in your LiPo packs must be kept between a range of 3.3 volts and 4.2 volts. The upper limit is managed by your battery charger in that it should stop the charge cycle when 4.2 volts per cell is reached. It is up to you though to keep from discharging your batteries below the lower 3.3 volt limit. Multi-rotor helicopters are unique in that we don't generally use any voltage protection features within our ESC's to protect the batteries from low voltage damage. If we did, we'd have helicopters falling out of the sky as the ESC's tried to protect the batteries by shutting down the motors. Some flight control systems have methods to alert you of low voltage condition. Consult your user manual or online wiki for more info.





[SIZE= 14px]PROPER CHARGE LIMITS[/SIZE]

There's another detail on the battery label that is important to understand and it too is listed with a capital C. It indicates the limit at which you can charge the battery. Modern LiPo balance chargers will usually auto-detect the voltage of the pack and will choose an appropriate voltage setting for the charge cycle. It's up to you though to select the amount of current that will be applied to the pack during the charge cycle and this will determine how quickly the pack will finish charging. The capital C again stands for capacity (mah of the pack). To understand this let's look at a 4S pack with a capacity of 5000mah and a max charge rating of 2C. THe 2C rating indicates that the pack can be charged at a rate of two times the capacity. Divide the capacity of 5000 mah by 1000 to get 5 and then multiple it by 2C. The maximum current (aka amperage, aka amps) setting you can use on the charger is 10 amps. Some batteries may be rated as high as 15C but keep in mind that charging at maximum rates will likely reduce the life of the battery. A ten minute charge may sound nice but it will likley shorten your battery's life which isn't so nice.





Also regarding chargers and charging, there are a lot of chargers that advertise the ability to charge multiple batteries simultaneously using what is called "parallel charging". This means that groups of packs are connected to each other and to the charger in order to be charged (as far as the charger can tell) as one large battery pack. It should go without saying that if you don't fully understand how to charge one pack at a time then please don't attempt to parallel charge multiple battery packs. A significant danger involved with parallel charging occurs when packs are connected together where one pack has significantly more charge remaining than the other packs. What happens is that the greater voltage in the less-discharged pack will cause current to flow into the more discharged pack with the lower voltage. To put it another way, the higher voltage pack will act like a battery charger on the lower voltage pack! The problem is that this happens very quickly and without any control so it has the potential to start a fire. The rule of thumb for parallel charging is that the individual cells within the packs should all be within .1 volts of each other prior to being connected to the charger. This means all of the batteries being connected need to be at a similar state of discharge.





[SIZE= 14px]BALANCE CHARGING[/SIZE]

Why balance charge anyway you might ask? In the early days of LiPo batteries, it was common to charge the packs like any other battery pack by connecting the charger to the power plug and charging it as if it were one big battery cell. It wasn't that this didn't work, it's just that when one cell within a pack became out of balance with the other cells in the pack, it created the potential for the pack to overheat and catch fire. There are many stories on the internet of people that connected their chargers to their car battery, started a charge cycle and walked away only to find a few minutes later that their battery had caught fire along with their car! Houses, workshops, cars.....they've all been lost to LiPo fires that started during battery charging.







It didn't take too long for the LiPo manufacturing and using community to realize it was important to monitor the individual cells during the charge cycle and to control/charge them as individual cells. When you charge a four cell pack with a balancing charger it is actually running four charge circuits (one for each cell) and speeding up or slowing down the charging cycle for each cell as they lead or lag each other. If one cell takes a charge more quickly and its voltage begins to exceed the voltages of the other cells in the pack, your balancing charger will slow it down so as to keep it at a similar voltage as the other cells. It is "balancing" the voltages of the individual cells as the pack is being charged! Get it? The potential for batteries to catch fire during charging is always there so please only charge your batteries on a non-flammable surface like cement, outdoors if you can, and try not to leave your charger unattended while charging.



If you've come to LiPo batteries after having used NiCad or NiMh packs you may have wondered what that extra set of wires coming off of the battery was. The extra wires are connected to the balancing plug and the wires/plug together are commonly referred to as the balancing "tap". Some chargers will charge only through the balancing tap, others will charge through the main power wires but also through the tap for control of the process. There are charging guides that say it is safe to balance charge every ten or twenty charges but, in my humble opinion, if you have a balance charger, use it every time you charge your batteries.

 

[Bartman]

[SIZE= 14px]INTERNAL RESISTANCE[/SIZE]

Internal resistance is yet another concept that relates to the overall health of your LiPo battery packs. Even a simple length of copper wire has a certain amount of resistance in it. Resistance (measured in Ohms) is the characteristic of an electrical circuit that prevents electricity from flowing. It won't necessarily stop the flow of electrons but it will just make it harder for them to flow. Make it too hard for the electrons to flow and the current passing through the wire will produce heat. Make too much heat and we're back talking about, you guessed it, fire. High resistance in your batteries will make them run hot for a little while and there will be some signs things aren't good before they think about catching fire but if you ignore the warning signs completely a fire may be just around the corner.



Chances are if your battery packs aren't delivering the same power they used to, resistance is building inside the packs and it may be time to considering disposing of them. Other warning signs would include packs that are hotter than usual after use and puffing which is what it's called when your normally flat and tightly packed LiPo battery begins to take on a more inflated and rotund appearance. Slight puffing isn't the end of the world but a LiPo Ninja would take it as a sign to watch things more closely and to monitor/measure the cells' internal resistance on a more regular basis.



Here's a video on how to measure internal resistance







The video demonstrates a procedure for a 3S pack so you'll have to modify the procedure if you're using packs with different cell counts. If you're checking your packs on a regular basis, logging the values on the cells will help you to see when the values begin to increase indicating the pack is aging and isn't as happy as it once was.



Some advanced chargers will be able to display the internal resistance of the cells. Refer to your user manual to find out of your charger has this feature.



[SIZE= 14px]PART 3



STORAGE, INSPECTION, AND DISPOSAL

[/SIZE]



If your LiPo batteries are going to sit on the shelf for a while, they are happiest if the cells are at about 3.7 to 3.8 volts each. If you've just finished flying then chances are your batteries will be close to this but if you've charged your packs and then didn't use them, it's important to have a method handy that will allow you to discharge your packs to this storage level. Some balance chargers have a discharge function but if you're running high capacity (mah) packs they can be slow to complete a discharge cycle. If you rig an alternate method just be sure that it won't discharge the packs too quickly as that can damage your packs making them very unhappy and a true LiPo Ninja can't have unhappy packs.



It's preferable to store your packs in a cool area where they can enjoy a relaxing chilled period of inactivity. Storing batteries in cars, trailers, sheds, etc can expose them to high heat which can degrade the materials in the battery making them not last as long and not hold as much of a charge.



Before putting batteries away after flying be sure to check them for damage as even minor looking dings and dents can cause internal shorting which can lead to a fire. If after putting your stuff away after a day of flying it would be a surprise to find out one of your packs is burning in the basement, no? LiPo fires are hot and intense as well as being difficult to put out. Avoid fires at all costs by understanding your batteries and by maintaining good habits for use, charging, and storage.



Here's a link to a .pdf file that outlines how to properly dispose of LiPo batteries. You may have a recycling facility that accepts LiPo packs. If so, ask to see whatever guildelines they may have for LiPo's. If you decide to throw your old or damaged packs in the trash, follow these directions;



http://konarcflyers.com/Lipo-Disposal.pdf





That's it! If you've followed along and understand the basics as well as the advanced details of LiPo ownership/use then you may already be a LiPo Ninja! Stay safe and please feel free to ask any questions you may have.



Bartman

 

[ProfilePilot]

Awesome info Bartman! LiPo batteries are something that I don't have any experience with. This is just the type of thread I was looking for!!!

 

[BalsaDust]

Lots to take in but thats some great info Bartman. Thanks

 

[mac26r]

Thanks Bartman! This is info I will read and re-read. I didn't understand the charge rate and checked. I have batteries that don't list a charge rate like you wrote about. I've always just charged them at 10% of the mah. But honestly I don't remember why, I suppose someone once told me to do that.

 

[Bunky.F.Knuckle]

Any lipo can be charged at the rated capacity of the pack, even if the the rate is displayed on the battery.



10% is ultra conservative. I charge all my non rated packs at 1C. If memory fails, just remember charge at capacity.

 

[SleepyC]

Good post bart, and yea 1C charge rate is fine. So a 5000 mAh pack can be changed at 5 amps, a 10000 mAh at 10 amps.. no problem!

 

[CatalinaWOW]

All good stuff.  Another thing to think about is that the discharge rating (C) is very differently (and sometimes creatively) applied by various vendors.  Some will give the highest rate that won't damage the battery.  Others give the highest rate that won't drop the output voltage below 3.3 volts, or sometimes 3.0 volts.  You probably won't damage batteries using these various definitions, but your performance can vary noticeably.

 

[Jetpainter]

CatalinaWOW said
All good stuff.  Another thing to think about is that the discharge rating (C) is very differently (and sometimes creatively) applied by various vendors.  Some will give the highest rate that won't damage the battery.  Others give the highest rate that won't drop the output voltage below 3.3 volts, or sometimes 3.0 volts.  You probably won't damage batteries using these various definitions, but your performance can vary noticeably.

I've been wondering about that. On a low C rating battery can the power noticeably drop off even though the battery is still over 50% charge?

I've got a little 31" scratch built pylon racer I just swapped to electric. It makes awesome power for the first minute or so then seems to slow down quite a bit for the next few. After landing I've checked the battery and it's never been under 40%. It's a 20c battery. I've been wanting to try a higher C rating to see if that helps. The battery and ESC stay nice and cool so it's not a heat issue.