Lithium Polymer batteries were a huge revolution for our hobby, even more important than brushless motors. They suddenly made running electric cars a lot more satisfactory, increasing run-times by an average 5 times in most applications. They offer significant benefits over their older NimH counterparts and are much lighter as well (in terms of power density/mass). Off the shelf batteries actually remained pretty much the same weight, just jumped massively in terms of duration as well as power delivery. Really, there is literally no comparison; lipo batteries make their older nickel based counterparts utterly obsolete. I said it briefly in the beginning of the first article on motors and I’ll say it again; if you are not using lipo now, you are doing it wrong.
A beastly MaxAmps 4S 14.8v 5450mAh 120C battery
I won’t go read Wikipedia then pretend I’m an authority on battery chemistry, I’m not. I don’t really know how lipo batteries ‘work’, and at the end of the day, I don’t really need to know. If you want to know, like I said you can just go visit Wikipedia and digest volumes of very dry chemistry and physics type information on the subject of lithium polymer batteries, but it really won’t help you get more out of the batteries or the hobby, that’s what this article is for. 
Dem lipos is just good, m’kay??
So let’s focus on the basic characteristics that it will serve you to know and understand. When you buy a typical lipo battery pack, it has certain attributes and ratings which are described on the packaging and usually on the battery itself. These are, in no particular order:
S – e.g. 2S, 3S, 4S, etc. This is the number of single cells that make up the lipo battery and determine its voltage. First piece of critical information: a lipo cell is nominally 3.7v. There is no limit to the number of cells that can be used in a battery, though it’s rare to see more than 6S in the surface hobby.
V – Voltage. Based on what we just learned, that each cell is 3.7v, we don’t really need to know the voltage if we already have the S. You can calculate the voltage by multiplying the cells (S) by 3.7. So a 2S battery is 2*3.7, 7.4 volts. A 3S battery will always be 11.1v, etc, etc. It’s important to note though, that a fully charged battery can have many more volts in it than the nominal value of 3.7v per cell; lipo cells will often charge to 4.2v per cell. So don’t worry if you see a 3S battery that’s putting out over 12.5 volts, it just means it’s full.
IMPORTANT NOTE: This is really the only caveat of lipo batteries, the only thing that makes them a little bit more finicky than nickel batteries… You mustn’t run them flat! It is critical that we never let any individual cell drop below 2.8 volts. The actual point of no return is much lower, more like 2.4v (at which point a pack may go inert, never charging again), but discharging below 2.8 per cell does no good to the pack at all. This is why we set cut-off values on our ESCs (see the Speed Controllers article earlier in this series as well as more on this in the charging section).
mAh – Milliamp hours. This is the duration of the battery. The higher the number, the longer the battery will last. This number also plays a vital role in determining how many amps, or how much power the battery is capable of delivering safely, more about that in a minute. What the unit actually means is that under a load of X mA the battery will last 1 hour.
C – Current rating (C does not stand for ‘C’urrent rating, that’s just the easiest way to visualise it). This is the most interesting and important unit, as to know what it means, you must understand how to calculate it. It refers directly to how much power or current the battery is capable of providing safely. We’ll talk in detail about calculating the value of C in a minute, first it’s important to mention once again (for those who read the article on ESCs where I touched on this), that this rating can be provided as a ‘sustained’ or ‘continuous’ rating (meaning how much power the battery could deliver all day long without breaking a sweat), and/or a ‘burst’ or ‘peak’ rating describing how much power the battery can deliver in very short bursts of only a few seconds. You will often see both these values written together in this way 30-60C, this means the sustained rating is 30C and the burst rating is 60C.
A 6S Turnigy Nano-tech battery showing 65-130C on the sticker, indicating continuous and burst ratings
Asking a battery to sustain ‘burst’ loads for too long (such as giving it lots of throttle when the wheels are stuck), is asking for trouble. The battery will certainly be damaged and under extreme circumstances could even start smoking! Hopefully all RC users, even the newest, already know not to try to give it lots of throttle when the motor cannot turn, so these situations are rare and nothing to fear (watch out for it when letting non-rc people drive though, they tend to believe that more throttle is the answer to any problem! ;)). The reputation lipo batteries have for catching fire is not really deserved any longer, you really have to abuse them to get them to catch fire, but the chemical recipe for a fire is always there. Exercise due caution, follow the manufacturer’s recommendations for charging and never exceed the sustained ‘C’ rating for more than a second or two.
Almost all lipo batteries these days supply their sustained and burst ratings, but on the off chance that only one value for C is given, it’s important to know if that’s sustained or burst. If it cannot be determined, don’t take the risk and assume that it’s sustained, just move on. When selecting a battery, the number of cells and the duration of the battery determine its physical size and the C rating is usually what determines the battery’s cost, but more C is not necessarily always better; we’ll talk more about that now…
Calculating C rating: How does ‘C’ work, and what does it mean to me..?
So, let’s take a specific battery as an example… A 3S, 11.1v, 5000mAh 30/60C battery. We can use C to work out how many amps it can safely deliver. The calculation is very simple: multiply the duration (remember mAh is duration, 5000mAh or 5A) by the C rating (30C). So, 5A multiplied by 30C = 150A. This battery has a current rating of 150A. Burst ‘C’ is calculated the same way. Note that a longer duration battery does not necessarily have higher delivery potential, if it has lower C rating. Similarly, a higher C battery does not necessarily handle more current than a lower C battery of a higher duration. Both duration (mAh) and C (current handling) must be provided in order to calculate the true current rating of a pack. For example, while a 4000mAh pack at 100C can handle 400A, a 6000mAh pack at 20C can only handle 120A, a huge difference.
So the trick here is to see how much our motor can draw, make sure our ESC can handle that many amps and that our battery can safely deliver that many amps. Both ESC and battery must meet the motor’s standard; it is the motor (combined with other factors such as gearing and weight of the model) that determines how much power your car needs.
Sustained C ratings which result in a total of more than 200A should generally be fine for all uses, if you want to not have to think about it. That means 4000mAh at 50C or 5000mAh at 40C or 6000mAh at 35C. For lightweight models you can get away with lower C batteries, but such low C ratings almost inevitably lead to swelling over time. Going up in C can make a car feel punchier even when way exceeding the requirements. For example, if a 1/8th motor says it can pull 150A max, then you would think that going over 150A battery rating would be pointless, but it isn’t. The more C, the more freely the power flows and indeed the punchier a car will feel. There is of course a ceiling, there’s no need to go crazy and buy 1000A rated batteries, they won’t offer much over a 500A rated battery for significantly less dollar.
Basically, while you do get what you pay for, the cheap 30C batteries are often more than enough for normal use, but spending well does reap additional rewards, you will have to decide which serves your needs better by trying a few.
Charging…
When we’re talking about charging a lipo battery, there are basically 2 main methods; Balance Charge and Fast Charge (aka non-balance charge). Your charger may be capable of other types of charge, such as storage charge, which we’ll also talk about, but these two are the main ones we will use more regularly.
Lipo batteries of 7.4v (2S) or higher, are made up of multiple cells in order to reach the nominal voltage. Since you can easily end up with a voltage mismatch between these cells, we generally favor a ‘balance’ charge, to iron out these mismatches. We’ll talk more about balancing and why it’s important in a minute, for now suffice it to say that this is your go-to charging mode. Many would say there is never any excuse not to balance charge, and while they have a valid point of view, personally I feel that’s a bit over the top, more on why in a minute.
The ‘fast’ or non-balancing charge is for charging batteries more quickly, and naturally, does not balance them. To do this often is risky, as your charger will not address any cell mismatches, but when used on a pack you know to be good and well balanced, can knock a significant length off the charging time without undue risk.
There is much debate concerning what is safe to do, not safe to do, what extends/shortens the lives of your batteries when charging. So, here’s some info to allow you to make up your own minds, with my opinions thrown in for good measure. You should know that I am considered a ‘careless’ lipo user compared to most, but I have few problems and can afford to replace a pack now and then. So why is balance charging so important? It’s all about…
amain.com’s Protek line of chargers. I use an older version of the Prodigy 610 duo for almost everything
Cell balance and Lipo cut-off…
We mentioned earlier that we don’t allow our individual cells to drop below 2.8v minimum in order to prevent battery damage, and we know from the ESC article that our speed controller has the ability to cut-off the power to prevent damage to lipo batteries. The model stops and that’s how we know our batteries need to be changed. We hope that our battery cells will have discharged at very close to the same rate, indicating that our cells are healthy and well-matched; however, this is not always the case. It can happen that one cell is lower than the others, so when the cut-off kicks in, maybe one cell is now at 2.8v, while the other 2 are 3.1v (remember the speed controller only sees the total voltage, not individual cells, refer to the article on ESCs for more on this).
If we were to charge all three cells with the same power (‘fast’ charge or NON-balance charge), obviously, the highest voltage cells would fill up first, then the charger will stop, because it doesn’t want to ‘overfeed’ the cells that are now full (that is a highly simplified analogy, it’s not exactly like that, but it will do). So, we now have some small differences between the cells. At this point they are only a few millivolts out, it’s not really a problem, but the next time we run the car, they will become a little more out of sync, and a little more, and the same goes for each non-balance charge which fails to equalize or ‘balance’ the pack. Eventually it will reach a point where the cell voltages are so different, that one or more cells is being discharged way too low causing permanent damage to that cell and rendering the pack useless or even dangerous.
This picture below is one of my batteries that’s starting to lose it. You can see the cell voltage differences after a balance charge cycle on my charger. This is no reflection on the Zippy packs, which I am a huge fan of for workhorse duties, if a little hit or miss. I have a 6S Zippy pack which has stayed full without swelling for over 6 months and is generally a really well behaved pack, even when caned to bits in the truggy. The battery below may be salvageable after a few balance cycles, though it may turn out to be less of a PITA to just buy a new one.
a poor unhappy of battery of mine, note the voltage difference between the cells
This can be tricky to grasp, so let’s take a real world example…. Johnny has had his Traxxas Stampede VXL for a few weeks, he got a couple of lipo batteries with it, but he doesn’t know much about lipo and the cheap charger he bought isn’t exactly plying him with information, so he thinks he’s good doing a ‘fast charge’ every time. Who wouldn’t? Faster is better! But what’s been happening to his batteries is the following…
One of Johnny’s batteries is a 5000mAh 2S 25C pack. Unknown to Johnny, one of its cells is just a bit weaker than the other. So each time Johnny runs the car, this cell gets a bit lower than the others. Johnny’s VXL speed control is programmed to cut off the power from a 2S battery at 6v (3V per cell), but it isn’t keeping an eye on individual cell levels, all it cares about is the total voltage (6v). Each time Johnny runs, his ESC cuts-off at 6v just like it should.
Yesterday, Johnny ran his truck again, but noticed that he got a little less runtime than usual. He doesn’t think twice about it, goes back inside, and puts the pack on charge only to be told ‘Error’ by the charger. Uh-oh, one of Johnny’s cells has died. But Johnny had his lipo cut-off set correctly, it was working…what went wrong?
Johnny’s problem comes from the weaker cell. Each time Johnny ran the truck, the cells were getting more out of balance, and this imbalance was not being corrected during the charging process because Johnny doesn’t know to balance his packs. The last time he ran, the auto cut-off worked perfectly at 6v, but what nobody could see, was that in actual fact, those 6v were made up of 3.5 in one cell and 2.5 in the other. The cell that dropped to 2.5v is now dying fast or dead and the charger detected that.
Again a highly simplified explanation, but basically, what the charger does when it performs a balance charge, when it sees the pack is starting to get full, it starts to charge each cell individually to even out these mismatches, stopping when a cell reaches peak (usually 4.19 to 4.21 volts) and moving onto the next one (or it may do it in a round-robin way, it depends on the charger). It keeps going in this fashion until it has topped up all cells and the pack is considered once again ‘balanced’, with all cells at as close to the same voltage as possible. If a cell gets badly out of balance, even 2 hours on the balance charge may not be long enough to correct it (most chargers have a time limit to prevent heat build-up in the equipment). When the balance charge seems to go on forever, this is probably good indication that your pack is having cell matching problems. If this happens to a 3S or more pack, and you are handy with electronics, you could consider removing the bad cell and effectively creating a good pack with one less cell than before.
Balance charging is all good if you are in no rush, charging at home the day before a race or a big bash, I would always balance charge. But what if you are at the bash spot and want to get one more run in but you don’t have a full battery left? Personally, this is what I use the fast charge option for. Feel free to put one you just used on the charger on a normal (often called ‘fast’ or NON-Balance) charge, perhaps even at A higher charge rate (more on charge rates below). You won’t hurt the battery to do this occasionally, especially if you know the specific battery is well matched and is a well-behaved pack. I guess I don’t need to say don’t do this with a battery that you know is not balancing well. Even with your good packs, make sure that when you get home you balance charge it (or storage charge it) properly. So my rule of thumb is; in the field, normal/fast (non-balance) charge, and when at home, take the opportunity to balance and/or storage charge, ensuring the health of the batteries.
Charge C rating…
Many batteries will also dictate at how many ‘C’ you can charge them. This rating is always much lower than the discharge rating we talked about earlier, but it is the same unit and calculated the exact same way. Typically, 1C (going back to our example of the 5000mAh battery, that would be 5A) is considered the charge rate which has the best time economy with battery life. Some would say that if you have the time you should charge at .5C for the best battery longevity. Many batteries claim safe charging at very high C rates, but personally, I don’t charge over 2C as a rule, even if they say I can.
Charging at 2C (double the mAh rating, going back to our example, 5A*2=10A) will balance charge a battery in about 40 minutes and fast charge one in less than 30. If you are not at WOT (Wide Open Throttle) all the time, this can often give you the ability to cycle 2 batteries all day if you have a charger than can be plugged into your car or attached to the car battery. This is a huge deal in the electric vs nitro debate, as it means that electric too can now play all day and not just 10 or 15 minutes for each battery brought to the bash spot or track.
Cheaper batteries will limit you to 1 or 2C, but even with the ones that claim 5C and higher, be careful. If you are going to try charging at that rate (most chargers don’t do over 10A anyway, but if you happen to have one that does…) stay close to the battery during charging and regularly check its temperature, take it off if it gets too hot. Charge at high amperages at your own risk, no matter what the label on the battery states, and never forget the fire hazard that aggressive charging can represent. Use a lipo bag if you have one (a fire resistant bag sold at most hobby stores).
This is one of those things that keeps going up as technology gets better, and we are already at a point where we can charge many modern batteries in the same amount of time as it takes to discharge them in a model.
Personally, I charge at 1.2C on balance charges when I’m at home and not pressed for time. The extra 0.2C is simply to knock off the extra time taken to balance at the end, and get my batteries charged in one hour. For example, I charge 5000mAh batteries at 6A.
In the field I can often be seen fast charging at 2C to give me that all important ‘no downtime’ action, but after a day doing that, I will be careful to balance and storage charge them after I get home.
Hitec’s lipo checker, handy little product.
A quick note on Storage Charging to finish off this chapter on charging: Most lipo chargers these days include a program called ‘storage’, and it absolutely should be used for exactly that, storage. When you know that the pack is not going to be used for a few weeks or more, it’s a good idea storage charge it. This is a charging program that will set the cell voltages to their nominal level, usually around 3.7v per cell. Lipos like to be stored at this ‘mid-charge’ point, and will hold this voltage for a surprisingly long time, often many months.
We storage charge for two main reasons…
We already talked about why we don’t let packs get below 2.8v per cell and despite the fact that lipo batteries hold their charge for a very long time, they do slowly discharge, like all batteries which are not used. Of course this allows for the possibility that if they sit for long enough, they will discharge below the minimum safe level and cells will go bad. Storing at nominal level extends their life as long as possible without causing undue stress on the pack.
The other way (storing them completely full), is less of a risk, but still invariably causes damage over time. This is actually the way that I have lost some of my lipos over the years, I tend to be lazy and not discharge them if I don’t use them. This sometimes causes them to swell , damaging the fine layers of material in the cells, again eventually rendering them inert. I have personally gotten away with storing fully charged batteries for very long periods (over 6 months as I mentioned above) without any issues; it seems to depend on the pack. Better packs seem to survive longer, but at the end of the day, there’s no need to risk it, just storage charge them when not in use and refresh that storage charge about once every two to three months during periods of extended disuse.
a serial adapter with deans style plugs, the one above is available from Tower Hobbies
Connection types: Serial vs Parallel
While not specific to lipo batteries, this applies to almost all batteries, the guide would not be complete without some reference to the ability to connect multiple batteries to your model using cable adapters. The two connection possibilities are serial and parallel. These are the two rules to remember about serial and parallel connections:
Serial = increases VOLTAGE (motor speed), not duration.
Parallel = increases DURATION, not voltage (motor speed).
Effectively, when you connect two or more batteries either in serial or in parallel, you are creating a single battery with more of one of its characteristics depending on the connection method.
Irrespective of which connection method you choose, when combining more than one battery, it is not required, but HIGHLY advisable, to use identical packs, ideally even from the same manufacturing run if possible, to avoid major cell differences. While in theory, you could use any two packs of the same characteristics (voltage, C and duration), the cells of one may be weaker or stronger than the other, and remember what we know about lipo cut-off? It can only see the whole battery, it can’t see the individual cells, so that increases your chance of discharging one entire battery too low, while the other one carries on going. This could under extreme circumstances, be a recipe for an on-board fire. Match your packs.
So, parallel first: As stated above, when we connect a battery with another in a parallel connection, what we are actually doing is combining the duration (mAh). Using two 2S 5000mAh 30C batteries in parallel causes the duration to be added together, giving a battery with 10,000mAh. NO OTHER BATTERY UNIT IS AFFECTED. They are still effectively a single TWO cell battery (and that’s how the ESC will see it too), despite the reality being that there are now 4 physical cells, and they are still 30C.
However, remember that C is calculated from duration? So now that the duration has increased, the calculated value of C has also increased. This can be a good way (provided you have space in your model) to get higher current handling alongside double duration. These 30c packs which can provide 150A each now combine to produce a 300A current on demand. The downside to doing this of course is space and weight in your model.
Serial on the other hand, does not affect duration and therefore does not affect current handling (C) either. It only affects VOLTAGE. Using two 2S 5000mAh 30C batteries in series gives you a single 4S battery, still of 5000mAh, still able to discharge at 150A max. All you have done is added the cell count of the new battery to that of the old. Be careful doing this, you can easily create a battery far too strong for your speed controller or motor to handle electrically, and even if they can handle it, doing this increases your maximum motor speed by double, so look out for over-speeding the motor as well.
a body-off pic of my XT8 with ROAR approved 2S packs used in serial to make 4S
Battery sizes and ROAR Approval?
ROAR is the National body for Radio Control car racing in the USA and Canada. Buying a ROAR Approved pack ensures that it will be legal for sanctioned club racing requiring ‘that’ battery specification; it has no bearing on the battery’s quality. You can generally count on ROAR approved 2S racing packs having hard cases, being between 48 and 50mm wide, 24 to 25mm tall and 135 to 140mm long. Many battery trays are specifically designed for this size, which if you are NOT racing can be annoying, as many high cell count and higher duration packs are often larger and have to be shoehorned in. Always try to make sure the battery will fit in the battery tray of the model you want to use it in if you are not buying approved packs. Personally, I have very few approved packs now that I don’t race any more, the large sized packs are more useful for bashing, again, as long as the battery tray is big enough or you own that master of tools, the Dremel!
That concludes the basics of electric power! Hope you guys enjoyed it and found it informative. If you think I missed something or you have a story of how you fell foul to fussy lipo batteries, don’t keep it to yourself, share it below!
Foxy out….