Introduction: Lithium Polymer Etiquette: a Comprehensive Guide to Working With LiPo

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About: A Bay Area native interested in electronics, mechanics, and robotics, and automobiles. Formerly the electronics captain of Team 100 in the FIRST Robotics Competition, I now study Mechanical Engineering at UC …

In this Instructable I will be going over the basics of proper charging, discharging, handling, usage, storage, and care of lithium polymer batteries so you can use them safely and effectively in your future projects.  Now, this is by no means the be-all and end-all of information, and it's always important to consult the instructions for your specific equipment, but I think this Instructable will provide a good basis of knowledge on the subject of these awesome batteries.

Step 1: Terminology

If you're new to lithium polymer/LiPo/LiPoly batteries, there are a lot of terms you will need to know before we get started.  Everything may seem a bit daunting at first, but with some basic understanding, it's all pretty simple, so let's jump in.

When you look at a LiPo's data sheet or casing, you will notice it has a lot of specs.

Cell arrangement - Described using the format xSyP (where x and y are integers), this tells you how the cells in the battery are wired up.  Batteries are made up of cells, whose voltage is determined by cell chemistry and whose capacity is determined by energy density and physical size of the cell.  S stands for series and P stands for parallel.  As you may know, series adds the voltage of the cells and parallel adds the capacity of the cells, so a combination of cells in series and parallel results in a battery.  The battery shown in the second image reads that it has an arrangement of 3S1P, meaning it has 3 cells that are all in series with no parallel wiring.  This may seem confusing because it says "1P," but think of the arrangement as a grid.  By multiplying the 3 and the 1, you get the total number of cells in the battery, which in this case is 3.  If it were a 3S2P battery, there would be 2 sets of 3 series-wired cells in parallel, resulting in 6 cells total.  Often times the parallel arrangement is omitted when discussing batteries, because most packs are 1P (so instead of saying you're using a 3S1P pack, you may as well just say 3S).

Capacity - Usually measured in mAh (milliamp hours), this is determined by the cell arrangement (parallel) and tells you how long you can expect the battery to last on a charge (although it's not quite that simple).  2600mAh as shown on the battery in the picture is equal to 2.6Ah (amp hours), a format you may be more familiar with on larger batteries, like the SLA (sealed lead acid) one in your car, which is probably around 50Ah.  A capacity of 2600mAh means that the battery can discharge at 2.6 amps for one hour (hence "amp hours"), 1.3 amps for 2 hours, etc., before it runs out of "juice."  Because the battery shown has a 1P arrangement, each cell has a capacity of 2600mAh.

Voltage -The voltage of a battery is also determined by the cell arrangement (series), and there are a few common voltage measurements worth noting:
Charged - the voltage of a fully-charged LiPo cell is 4.20V, and charging above this will damage the cell.
Nominal - this can be considered a sort of "half-charged" voltage, as it is 3.70V, in between charged and discharged.  Nominal voltage is what manufacturers use when describing the voltage of their batteries.
Discharged - the voltage of a discharged LiPo cell is 3.00V, and discharging below this will definitely damage the cell.
Because the battery shown has a 3S arrangement, it is marked with its nominal voltage of 11.1V (3.70V*3 cells).  A fully charged 3S pack is 12.60V and a fully discharged 3S pack is 9.00V.

Constant C Rating (Discharge) - The constant C rating (in relation to discharge) tells you how many amps can be safely drawn from the battery constantly.  The "C" in a rating of xC (where x is an integer) actually stands for the capacity of the battery in Ah.  By multiplying the C rating's coefficient by the capacity of the battery in Ah, you can determine the sort of amperage you can draw.  In the case of this battery, with a capacity of 2600mAh (2.6Ah) and a C rating of 55C (that's pretty high, FYI), I can multiply 55*2.6 and get the max constant output of my battery, which is 143A.

Burst C Rating (Discharge) - In addition to the constant C rating, there is also a burst C rating, which is higher.  Most of the time, the "burst" is rated for 10 seconds.  Although it is not marked on the battery itself in the picture, it says in the documentation that this battery's 10 second burst rating is 80C.  So, 80*2.6 is 208A burst. That's a lot!  It's worth noting that your LiPo won't last long when that many amps are being drawn from it.  At 208A, a 2600mAh LiPo will last approximately 45 seconds.

C Rating (Charge) - Determined in the same fashion as the C ratings for discharge, the C rating for charge tells you at what amperage you can safely charge your battery.  This information is generally listed on the back of the battery with all the safety information.  For the battery shown, it happens to be 5C, which means that it can be charged at 13A (2.6*5).  We'll be talking a lot more about charge rates later...

Step 2: The Battery

Now that we have some battery theory behind us, let's take a look at a few LiPo batteries.

All LiPo batteries (should) have 2 sets of wires coming out of them: discharge leads and balance leads (sometimes called balance taps).  The discharge leads are the thicker wires of which there are a positive (red, +, anode) and negative (black, -, cathode), and are used to discharge the LiPo as their name suggests.  The balance leads are used when charging the battery to ensure that all the cells in the battery are charged equally.  There is generally a common ground connection on one side of the balance connector, as well as a positive connection to each cell in the battery.  Therefore, depending on the number of cells the battery has, it will have a balance connector with a different number of pins.

Step 3: The Charger

In order to charge LiPo batteries, you must use a LiPo-compatible charger.  If you try to charge a LiPo with a non-LiPo charger, something WILL catch on fire.  As this isn't a buying guide, I won't go into specific charger models or recommendations, but I will say that 90% of LiPo chargers out there use the exact same UI and have the same basic internals.  Let's compare 2 chargers and talk about specs and differences:

Charger #1: Dynam Supermate DC6
Charger #2: Thunder AC6

Power Input - You need to supply more power to your charger than it outputs due to inefficiency.  My Thunder AC6 can plug directly into the wall because it has an AC adapter built into it, while by Supermate DC6 requires an external PSU (The Thunder AC6 can take power from an external PSU as well but there's not much point for home use).  Both these chargers have 50W max output, which means they need to take more than 50W input... say 60W at least, probably more just to be sure.

Power Output - Like I said, both the Thunder and Supermate feature 50W maximum output.  Remember that wattage is the product of voltage and current, so your maximum current you use to charge your battery depends on your battery's voltage and vice versa.  However, chargers also have a max/min voltage and max/min current output in addition to their wattage limitations.  Both these chargers have a current output range of 0.1-5.0A and a voltage range of 1-6S for LiPo (4.2-25.2V charged).  That means that while you'll be able to charge a 2S battery at 5A (8.4V*5A=42W), you won't be able to use that same current to charge a 3S battery (12.6V*5A=63W).  For a 3S battery, max charge current on a 50W charger will be 3.9A (50W/12.6V=3.968A).

Balancing - Balancing cells is quite possibly the most important part of charging a LiPo battery.  As LiPo batteries are used, their cells may discharge unevenly and become "unbalanced."  To combat this, balancing chargers like these plug into the balancing leads of the LiPo battery as well as the discharge leads, allowing them to individually charge and "balance" the cells within the LiPo battery so that all the cells are the same voltage (4.20V, remember?) by the end of the charge.  Some LiPo chargers don't have balancing capabilities, and when this is the case, it is necessary to buy and use a separate balancer.  As I don't have much experience using standalone balancers, I won't go into detail on them.

Extra Features - Some chargers have extra features like temperature sensing or USB connectivity.  Both of these chargers have a temperature sensor input, which can be useful if you want to stop charging your battery if temperatures exceed a predetermined value (we'll come back to this later).  The Thunder AC6 has USB which works with a Windows application for data logging.  Kind of cool, but not especially necessary most of the time.

Step 4: Other Gear

In addition to batteries and a chargers, there are a few other things you need in order to take care of your LiPos:

Digital Multimeter - Sometimes the voltage readings on chargers aren't totally accurate, so it's good to always be able to fall back on a trusty multimeter to verify.  When I'm using a new LiPo battery, I always check the voltage of each cell with a multimeter after I take it off the charger to be sure it's charged correctly.

Low Voltage Alarm/Cutoff -  These are used in conjunction with your LiPo battery when its being discharged.  A low voltage alarm or cutoff, or LVC as it's more commonly known, plugs into the balance connector on the battery and monitors the voltage of each cell. When any cell below a safe voltage (this threshold depends on the LVC but is generally between 3.3V and 3.0V), the LVC or LVA will either alert you with lights and/or a buzzer, or will cut off power to prevent further discharge.  Electronics meant to run off LiPo batteries will generally have this feature built-in, but if you're using a LiPo with something not designed for it, you'll need to use one of these or something equivalent.

Charging Case/Bag - LiPos should NEVER be charged in an open space for safety reasons.  If something goes wrong within a LiPo, it will quite literally shoot flames out of it, easily setting anything on fire.  Most people charge their LiPos in LiPo bags, which are padded, fireproof sleeves that can vent smoke out but keep flames in.  I, however prefer the ammo box method, mostly because it looks much cooler and properly scares people.  For illustrative purposes of this Instructable, I am charging my LiPos in open air, but I would never do so otherwise, as keeping your LiPos safe while charging is probably the most important thing you can do to.

Step 5: Balance Charging Setup

Charging LiPo batteries, especially balance charging, is a very precise process.  If you get it wrong, something bad will happen, but luckily LiPo chargers do their best to not make fires.

When setting up a charger to balance charge LiPo batteries, you're presented with 2 main parameters: current and voltage.

Charge Current - The current at which you should charge your LiPo battery depends on the battery's capacity and charge C rating.  Regardless of charge C rating, though, most people charge their LiPos at 1C, as that is the safest rate, both from a fire danger and battery longevity standpoint.  Charging your LiPo at a higher rate will make it charge faster, but charging at high rates will also decrease the life of the battery in the long run.

Charge Voltage - This is the nominal voltage of the battery you want to charge.  Often times the charger will state the cell arrangement (such as "3S") next to its nominal voltage for easier recognition.  My chargers check the battery by counting its cells via the balance plug and will not charge if your selected voltage and the battery's voltage don't match, which is a very good safety feature.

Here are a few real-life LiPo balance charging scenarios:

2600mAh 3S LiPo charged at 1C
1C*2.6Ah = 2.6A charge current
3S*3.7V = 11.1V charge voltage
2.6A*12.6V (fully charged voltage) = 32.76W power draw

1800mAh 2S LiPo charged at 1C
1C*1.8Ah = 1.8A charge current
2S*3.7V = 7.4V charge voltage
1.8A*8.4V (fully charged voltage) = 15.12W power draw

5000mAh 2S LiPo charged at 1C
1C*5.0Ah = 5.0A charge current
2S*3.7V = 7.4V charge voltage
5.0A*8.4V (fully charged voltage) = 42.00W power draw

All these charges of their respective batteries are very safe and within the realm of the charger's capability.  Additionally, each of these charges, because they are being performed at a 1C charge rate, theoretically take 1 hour to charge each battery from 3.00V per cell "dead" to 4.20V per cell "full."  In real life, charge time varies depending on the degree of discharge of the battery (most of the time you'll stop using the battery before it hits 3.00V/cell) and the degree of imbalance between the cells (the more imbalanced they are, the longer it takes the charger to balance them).

Just for further illustration, let's take a look at the same batteries, but this time charged at 2C:

2600mAh 3S LiPo charged at 2C
2C*2.6Ah = 5.2A charge current
3S*3.7V = 11.1V charge voltage
5.2A*12.6V (fully charged voltage) = 65.52W power draw

1800mAh 2S LiPo charged at 2C
2C*1.8Ah = 3.6A charge current
2S*3.7V = 7.4V charge voltage
3.6A*8.4V (fully charged voltage) = 30.24W power draw

5000mAh 2S LiPo charged at 2C
2C*5.0Ah = 10.0A charge current
2S*3.7V = 7.4V charge voltage
10.0A*8.4V (fully charged voltage) = 84.00W power draw

We can see that the chargers I own are incapable of charging the 2600mAh 3S battery and the 5000mAh 2S battery at 2C, but there are plenty of other chargers that are.  Charging at 2C means that each charge would theoretically take just 30 minutes.  Never charge your battery at a rate higher than is intended. Even then, I still don't recommend charging any battery above 1C, whether it's rated for it or not.  You can do it if your battery is capable and you're in a time crunch, but repeated charges at higher C rates wear down your battery quicker than charges at lower C rates do.

Step 6: Balance Charging

After you've set up your charger for the LiPo you're going to charge, it's time to plug in all the cables on the battery side of things.  Plug the balance adapter into the charger, and your battery's balance leads into the appropriate slot on the balance adapter (it'll only fit in the one made for its cell count).  Then plug the charger's charge leads into the discharge leads of your battery.  Depending on your charger and its accessories it may plug into your battery in different ways.  In my case, the charger came with a variety of banana plug leads.  I conveniently misplaced the leads that connect to my battery's plug (called a Deans Ultra plug), so I had to use a different plug on the charger's lead and run an adapter between that and my battery...

Once everything's plugged in, go ahead and start balance charging your LiPo.  Like I said, after I tell my charger to start, it checks the battery's cells and asks me to confirm my settings before it starts charging, so yours may do the same.

LiPo chargers follow a 2-part process, using a "constant current" technique first and a "constant voltage" technique second.  During the "constant current" portion of the process, the charger ramps up to its specified amperage output and keeps that amperage constant as cell voltage rises.  When the cells hit a certain threshold, the charger switches over to "constant voltage." During this portion, the charger varies current output to keep all the cells of the battery at the same voltage.  Balancing occurs in this part of the charging process.  As the charger nears completion, current drops off significantly until the battery is fully charged at 4.20V per cell, at which point the charger stops.

While your LiPo is charging, watch out for temperatures (I told you I'd come back to it!).  A properly functioning LiPo shouldn't exceed 90-100 degrees F while charging.  If it does seem to be getting hotter than that (you can feel it with your hand, read it with the temp sensor for the charger, or use an IR thermometer), stop charging immediately.  Within my chargers, I can set the charger to cut power to the battery at a temperature threshold.

Step 7: Storage

If you don't plan on using your LiPo for an extended period of time (a few weeks to a month or more), it's a very good idea to store it properly.  The first step to LiPo storage is to charge/discharge it to proper storage voltage.  LiPos, like all other battery chemistries, do self-discharge, but at a very low rate.  If left discharged, a LiPo can discharge further below its safe voltage range rendering it useless and dangerous the next time you want to charge it.  If left fully charged, the cells in a LiPo will unbalance quickly.  Proper storage voltage for a LiPo is 3.85V per cell.  Most LiPo chargers have a storage function that will either charge or discharge your battery until it hits 3.85V per cell.  

As my chargers have a discharge range of 0.1-1.0A, the max storage charge rate is 1.0A, so I set it as close to 1C as I can (usually the full 1.0A) and set the voltage in accordance to the battery I want to store.

After your LiPo is at a proper 3.85V per cell for storage, you can find a good place for it to stay.  LiPos are best stored in relatively low temperatures (40-45 degrees F), so a refrigerator is an excellent place for them.  It's a good idea to still protect the stored batteries in case of fire, so I recommend placing the LiPos in a LiPo bag and putting the LiPo bag in the fridge.  The fridge is not the only place for LiPos, though.  Anywhere with low humidity and reasonable temperatures will suffice.

Step 8: Discharging

In some instances, you will need to completely discharge your LiPo.  The most likely reason for this is to measure capacity, because charging from 3.0V per cell to 4.2V per cell (or discharging from 4.2V per cell to 3.0V per cell) is the only way to accurately judge capacity.  As I said in the last step, my chargers have a max discharge current of 1.0A, so that's what I use when discharging most of my LiPos unless they're really small (the RC cars I use my LiPos in consistently draw 25-75A, so 1A is not a problem for my batteries).  Think back to the constant C rating for discharge of your battery, and get as close to its max constant discharge current as you can.

For reasons mentioned in the last step, don't leave your LiPos full discharged for long or you risk not being able to charge them again.

Step 9: Usage

If you take care of your LiPo battery, it will take care of you.  Or not burn your house to the ground at the very least.  Here are some guidelines to follow for safe usage of LiPos:

-don't poke it or puncture it. fire will happen

-don't drop it. fire will happen

-don't short it out. fire will happen

-don't overcharge it. fire will happen

-don't let it overheat. fire will happen

-don't throw it in a fire. more fire will happen

All jokes aside, follow the instructions that come with your equipment and you should be fine, but always stay alert.  Avoid walking away from your charger while it's working on your LiPo, because if something goes wrong it's good to be around to make it go right... or at least less wrong.

One of the most important guidelines for a good experience with these batteries is to always work within the electrical capabilities of your LiPo by doing the following:

-constantly monitor the voltage of each cell either manually or, better yet, automatically with an LVC or something similar

-match the components in your project to ensure you never draw too much current from your LiPo


I plan on updating this Instructable throughout the future with added information and answers to people's questions.  In the meantime, please rate and comment on this Instructable.  I hope it provided some helpful insight into the realm of Lithium Polymer!

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