by John Salt
The LiPo Battery Ratings label that comes on most, if not all RC LiPo packs looks very confusing at first glance. I know I didn't have a clue what all those numbers meant when I got my first electric power RC helicopter after years of flying nitro, or if they were even that important to understand. Turns out they are very important!
The 4 main rating numbers you will see most often on RC LiPo battery labels are:
What is Nominal Voltage?
Nominal voltage is usually referred to as the "resting voltage" of the battery cell or battery pack.
Nominal resting voltage is an industry standard (agreed convention) that varies for all battery chemistry types; but for our standard RC LiPo chemistry, the usual nominal voltage is 3.70 volts per cell.
That voltage however is not the fully charged voltage of the cell (which is as high as 4.2V), nor is it the 50% storage voltage (apx. 3.85V), or even the 80% discharged state resting voltage (apx. 3.74V).
Nominal voltage numbers often confuse folks and I get a fair number of related questions. To keep things simple, just think of 3.70 volts as the guide number used to determine the voltage value you see printed on the LiPo battery pack sticker.
Other than the smallest of electric RC models, RC LiPo battery packs will have at least two or more cells hooked up in series to provide higher voltages. For larger RC models that number can be as high as 12 cells and more, pushing 50+ volts.
Here's a list of LiPo RC battery pack "nominal" voltages with cell counts. Again, this is usually the voltage number printed on the battery rating label.
If you're wondering what the 1-14S in parenthesis means; it's the way battery manufacturers indicate how my cells hooked in series (S) the battery pack contains. Fully charged voltage of the packs is the number in green.
I should point out you may run across packs or cells hooked up in parallel to increase the capacity. This is indicated by a number followed by a "P". Example: 2S2P would indicate two, two celled series packs hooked up in parallel to double the capacity.
So, those are the voltages you need to know and each RC model or more specifically, the motor/speed controller combination will indicate what voltage is required for correct operation/RPM.
This number has to be followed to the letter in most cases since a change in voltage equates to a change in motor RPM and will require changing the gearing but more likely the motor to a higher or lower KV rating - not something I want to get into in this write-up (I cover it in my setup & tips eBook). If a model calls for a 3 cell (3S) 11.1 volt battery – let's just say that is what has to be used.
Capacity indicates how much energy the battery pack can hold and is indicated in milliamp hours (mAh), at least on our smaller RC packs. This is just the standard way of saying how much load or drain (measured in milliamps) you can put on your battery for 1 hour at which time the battery will be fully discharged.
For example an RC LiPo battery that has a rated capacity of 1000 mAh would be completely discharged in one hour with a 1000 milliamp load placed on it.
If this same battery had a 500 milliamp load placed on it, it would take 2 hours to drain down. If the load was increased to around 15,000 milliamps (15 Amps); a common current drain in a 3S powered 450 sized RC helicopter while gently hovering - the time to drain the battery would be only about 4 minutes.
Note, as the discharge rate increases, the capacity of a battery actually becomes less than stated due to internal resistance efficiency & other losses; but in our discussion here, we are keeping things simple and "linear" :-)
As you can imagine for an RC model helicopter with that kind of current draw, it would be very advantageous to use a larger capacity battery pack such as a 2000 mAh pack. This larger pack used with a 15 amp draw would double the hover time to about 8 minutes.
In other words, if you want more flight time; increase the capacity of your battery pack.
Unlike voltage, capacity can be changed to give you more or less flight time.
Naturally because of size & weight restrictions, you have to stay within a certain battery capacity range in your model seeing that the more capacity a battery pack has, the larger and heavier it will be.
Think of increasing the RC Lipo battery capacity similar to putting a larger fuel tank in the RC vehicle.
This is the highest charge current the manufacturer states in their LiPo battery ratings that the battery can be safely charged at. It is however not the charge rating for best battery life!
A 5C charge rate as shown on our example LiPo battery ratings label, would mean you can safely charge that battery at 5 times its capacity. With our 5300 mAh pack example above, the calculation is 5 x 5300 mA = 26,500 mA or 26.5 Amps - huge!
Please note however, charging at maximum rates will shorten battery life. Again, this is a safe maximum number, not a best for maximum life number.
Lower charge rates of 1C (or lower) are always best for maximizing LiPo battery life!
This one is probably the single most over rated & misunderstood of all battery ratings.
Discharge rate is simply how fast a battery can be discharged safely.
Remember that ion exchange thing we covered back on the RC LiPo Battery Page? Well the faster the ions can flow from anode to cathode in a battery will indicate the discharge rate. Basically it's the maximum amount of output current the battery can maintain without overheating.
A battery with a discharge rating of 10C would mean you could theoretically safely discharge it at a rate 10 times more than the capacity of the pack, a 20C pack = 20 times more, a 40C pack = 40 times more, and so on.
Using our 1000 mAh battery as an example; if it has a 20C discharge rating, that would mean you could pull a maximum sustained load up to 20,000 milliamps or 20 amps off that battery (20 x 1000 milliamps = 20,000 milliamps or 20 amps).
From a purely theoretical time stand point, this equals 333 mAh of draw per minute so the 1000 mAh pack would be completely exhausted in about 3 minutes if it's exposed to the maximum rated 20C discharge rate the entire time.
Calculation as follows: 20,000 mA divided by 60 minutes = 333 mAh which is then divided into the 1000 mAh capacity of the pack giving us 3.00 minutes).
Many RC LiPo Battery packs will show the continuous C rating and usually a maximum burst C rating as well. A burst rating indicates the battery discharge rate for short bursts (a few seconds maximum) of extended power. An example might be something like "Discharge rate = 40C Continuous / 80C Bursts".
You don't see this often, but the LiPo pack pictured above actually indicates three different discharge C rates, along with the sustained discharge current underneath each one.
Flight Power was the only brand I've ever used that did this. I should also mention, these numbers were completely overrated; realistically being about half of what was claimed. This is how I first found out to never trust the discharge rate printed on the label. They are non-verifiable, and sometimes nonsensical!
The higher the C rating, usually the more expensive and slightly heavier the battery gets. This is where you can save some money, and maybe even a little weight.
Getting an extremely high discharge rated pack when there is no way you could possibly pull the full amount of power is not required but it certainly won't hurt either.
The most important thing is you can't go with too low a discharge C rating or you will damage your battery and possibly your ESC (electronic speed control).
Just like the maximum charge number, the maximum discharge number is what the manufacturer deems is safely possible, but not what will give you the best life.
So how do you know what C rating to get when purchasing your LiPo RC Battery Pack?
The easy answer most will give is to get the largest C rating you can afford. If money is not an object I agree with that almost 100%. The higher the C rating, the cooler the pack should run, and the longer it should therefor last as was covered on the RC LiPo Page in the discharge section. In the long run, a much higher discharge rated pack might actually save you money.
As a very general guideline, 30C to 40C discharge rated packs are the norm for most 150-550 size electric helicopters with general to sport flying in mind. For larger birds, 40C to 60C discharge rated packs are a safe bet (again for normal to sport). Once up to aggressive 3D, that is where the 60C and up discharge rated packs come into play.
If however you get a pack with a C discharge rating at least double of the maximum you intend to pull out of it; with proper care, there's no reason you shouldn't be able to get 300 charge and discharge cycles out of it with average degradation.
As an example, suppose your helicopter uses a motor and ESC both rated at a maximum of 100 Amps, and it uses a 5000 mAh (5 Ah) battery. What C rating should be considered using my double method?
100 Amps / 5 Amps = 20C. Double that = 40C.
If you fly really aggressively however, I would recommend going triple the C rating (60C) or higher.
It of course totally depends on how accurate or how misleading the LiPo manufacturer is with their claimed C rating. Again, use this an estimate guide only.
With largely meaningless discharge ratings, taking a temperature reading of your packs or simply just holding/feeling them after running them is another good way to gauge if you're using a high enough C rating. As we now know, just because a pack says it is rated at 50C doesn't mean it is in real world applications.
C ratings are not verifiable and often overstated by many LiPo manufacturers. On top of that, as packs age, their internal resistance increases which lowers the C rating and makes them run warmer.
The general rule I was told is if you can't comfortably hold a LiPo pack tightly in your hand after using it, it's way too hot! This equates to anything higher than about 60C (140F).
Even that is too warm as far as I'm concerned. Nothing higher than 50C (about 120F) is what I consider safe and I rarely have my packs go much past 40C (100F) unless it's also very hot outside as well. Some of my larger LiPo packs are over 12 years old now with little degradation following this temperature advice; anecdotally, I know this works well.
So, if you find your packs are getting warmer than this, it's a good bet you should consider moving up to a higher discharge rating for your next LiPo pack/s.
Leaving your packs in the car on a hot sunny day can certainly heat them up well past 40C as well. Internal or external heat - both have the same negative effect, hot LiPo's are miserable and they won't last long.
Thought I better mention the Watt Hour (Wh) rating that you will sometimes run across on LiPo battery ratings as shown above.
I really don't see much value in this specific LiPo rating in the world of RC flying; I find Amp hours much more useful & practial, but that is just me. Wh are however easily calculated it if it's not stated on the LiPo pack and you find it a useful rating.
Watt hours are determined by multiplying the nominal voltage x the capacity; so on this GensAce pack, 5100 mAh x 22.2V = 113,220 mWh = 113.22 Wh.
Keep in mind that is the calculated power output over time. In the real world, the number is lower due to internal resistance of the pack and the fact the voltage will decay as the battery drains.
By the way, LiPo internal resistance is another important pack rating that I personally find very useful. It's never listed on the LiPo battery ratings, but it's a very important number to monitor in my opinion.
That pretty much sums up all those RC LiPo battery rating numbers. Out of the 5 covered, the two main ones you must understand are the voltage/cell count and the capacity since they dictate what settings you use on your RC charger to charge the packs as demonstrated in the video below.
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