C Ratings and IR Lithium Battery Packs

I understand the  "C" rating on LiPo battery packs (e.g.  15C, 20C, 25C, etc.) refers  to the "discharge  current". How does that translate into differences in performance? For instance if I have 2 different packs for the similar plane, both 11.1v & 1800mAh, but one is rated 15C and one is 25C, what's going to be the noticeable difference in performance?

You are right, "C" rating refers to how much the battery can discharge and how many amps you can pull from it. The amps you can pull from a battery are directly proportional to the capacity of the pack. So for instance, you've two packs, both are lipo 5000mah 7.4 v, but one  is 25C and the other 15C. The 25C lipo pack should be able to deliver about 45amps where the 15C will only give you around 27amps.

So if you have a plane that is drawing around 27amps and use the 15C Gens ace 3s lipo pack you may find your performance down a bit because you are working the battery at its limits of what it can deliver. Any extra load and the battery may actually cut out on you. If you use the 20C pack, it still has amperage in reserve (for lack of a better term) and isn't operating  at its upper most limits. It will be able to more easily dump its current, and will be unlikely to heat up as badly as the 15C pack.

If you are using these batteries in a vehicle that doesn't draw much current you may still see a difference in performance even though you  are not pushing the batteries to their limits. The standard of the battery pack cell and its internal resistance play a huge roll in how easily the  battery will dump its voltage and model helicopters are the most demanding application of all.

In theory, any battery will provide all of its rated capacity in one hour when discharged at 1C. Its voltage during this one hour 1C discharge, will drop from the peak 'open circuit'voltage when freshly charged down to the minimum allowable voltage per cell of that particular battery's composition. The nominal voltage of a cell is typically the average of the peak, charged open circuit voltage and the minimum allowable or minimum useful voltage.

In the case of LiPo's the nominal voltage quoted is 3.7V per cell, which is the average of the 4.2V peak charged voltage and 3.2V, which we take to be the min working voltage.

So a 1000mAh LIPO battery could theoretically deliver 1 amp current at 1C for 1 hour while maintaining an average of 3.7V.

For all practical purposes, and for 8 to 10 minute flight times, leaving enough extra capacity for a go around landing and taxi back, we should really only be drawing an average of around 5C out of any given pack in any given flight or drive. So what good are 20C or 30C packs then?

The C rating is also a good indicator of low internal resistance. The higher the C rating, the lower the internal resistance (IR). Low IR tells us that voltage will remain high during the discharge in comparison to high IR cells and that low IR cells will run cooler, since the higher resistance the cells convert more energy to heat, this is why the better cells have closely matched IR¡¯s. We want the lowest IR cells we can afford.

One Battery vs Two batteries that Equal The Same Mah

Electric car battery packs are made this way with many sub units collected in a array of series and parallel packs to reach both current and voltage desired. I have a battery system for my solar array.  Here  I use batteries sets in parallel to be able to meet the current requirements. 

Take the DJI (sorry) Phantom 2 pack.  It is a Gens ace 3s lipo 5200mah; Internally it has 6 cells 3 series  sets of 2 cells in parallel.  Each cell is 2600mah giving a total capacity of 5200mah. It makes no difference whether this is done by wiring the sets internally or externally as separate batteries. 

While I agree that manufacturers blow up their claims and this can lead to sets not meeting specifications this does not change the basic rules that apply.

I also agree that more connections wire etc increases chances of failure but in the case of one or two batteries I think the chances are extremely low compared to other failure modes. I have never had a battery failure in the air but have had many other failure mode cause a ground intersect.Personally I am not arguing 1 versus 2. I see this as primarily a packaging issue space size COG etc.

My setup

11.1v 2200mah lipo, new and from the same manufacturer (not mismatched) AUW with batteries and camera is 2748 grams and she hovers at about 35-40% throttle (a little over powered I know and should probably be closer to 45-50%)

I get about 13-14 minutes of lazy to medium flying until I it 13.7 volts and bring her in. (it's a stable hexa camera rig, not set up for speed)

One should use batteries that are matched and in good shape. Same chemistry and discharge curves. In this case issue with unequal loading are relatively insignificant. If batteries of different type are used Martin note and not producing a pure additive current capacity would be true. But not with same chemistry / construction. The variation between cells is very low in good batteries.  Mixing a old used battery and new one can cause problems as well. Whether using a single battery with cells in series or batteries in parallel monitoring each battery cell and total pack health is a good idea to prevent in air failure. I do pack charge discharge curve on new packs looking at overall capacity and cell matching.  I do this periodically  ensuring the life of the battery to help ensure I do not have in use failure.  Typically with lipos I see single cell failures which is bad however wired up.

3S or 4S Battery Question

The KV rating indicates an unloaded ( no prop) RPM that the motor will spin at. The actual RPM you get will depend on the prop you use. The ESC will work with a 4S battery, but for any given set up ( load and throttle setting) the 4s will result in a higher average current delivery than the 3S.

The 11.1 v assumption you are making on the the 3S battery is the low end of the charge/ discharge voltage curve, I would use a fully charged battery voltage to think about current draw. If the top end the motor will deal with is 660 watts, at 12.5 v that will be a current draw of 52 to 53 amps, which exceeds the capacity of your ESC ( 45 amp continuous, 50 burst). The ESC seems to be the limiting factor in your power system, so to use the Gens ace 4s battery you may need to consider reducing your prop size and or pitch to maintain the same overall current draw on the system.

Because the ESC deliver current in a time gated fashion ( full current on then off with the interval determined by your thottle setting) you cannot depend on the just assuming you can fly at less than full throttle to keep your current delivery in a safe range for your ESC.

There is the benefit of getting more watts out of your setup. The Afro 20A slim ESC (2S-4S) and 18AWG wire are rated for 20A. Amps is what melts your wires and ESCs. Watts is what makes your motors powerful generally speaking.

For each motor:
A 3S setup will have a theoretical watt limit of 12v x 20A = 240 watts
A 4S setup will have a theoretical watt limit of 15v x 20A = 300 watts

Also the LiPos “contain” more watts per mAh in a 4S lipo vs a 3S lipo:

Lets compare 2200mAh 40C lipos

2200mAh 3S 40C lipo: 2200mAh x 12v = 264 watts – continuous draw: 88A
2200mAh 4S 40C lipo: 2200mAh x 15v = 330 watts – continuous draw: 88A

4S tends to be a bit heavier than Gens ace 3s lipo with the same mAh and C rating though. What makes most sense is to compare the watt to weight ratio.

Another vital factor to be aware of with the Silver series ESC is that they use a % of initial voltage to determine the LVC, as opposed to the fixed voltage that most other manufactures use. As stated in their instructions, never use a partially discharged battery on thier ESC as it may set the LVC at a point that will result in battery damage if flown to the LVC.

The motor will WANT to increase speed by 1000 rpm (per volt), but will lose some to heat and what have you. I wouldn't get too bogged down on that part right now.

Watts = volts X amps

A higher VOLTAGE setup is often used to REDUCE the AMPS required. If you look at the formula above, if you increase the voltage, you reduce the amps.

The esc amp rating has NOTHING to do with the amount of power your setup will draw. It is a MAXIMUM rating for the esc. Think of an electrical fuse. You can flow anything from zero up to the fuse limit with no problems. Exceed the maximum rating and bad things happen. Smaller esc's are usually used to save weight, room, and money.Similar with the C rating of the battery pack. C rating x the battery capacity (in amp hours) = the number of amps the battery CAN provide at its maximum level. You can certainly use a higher C rating battery, but not a LOWER C rating.

Going from 3S to 4S Lipo Battery

4S lipos use 4 cells in serial and therefore reach a nominal voltage of 14.8V. If you increase the battery voltage by 33% (3S to 4S) then you are effectively increasing the motor speed by 33%. To keep all things equal you need to decrease the prop size to match this increased motor speed and prevent the motors from heating up. Once at the same wattage (roughly the same speed and thrust in flight) you would draw 33% less current, and therefore  get about 33% longer flying time. The reasoning behind that is: Watts = Volts x Amps. Thus increase volts by 33%, then drop amps x 33% for same watts.

If you don’t decrease the prop size, the faster spinning prop will generate a lot more speed and a lot more current, increasing the watts very significantly. The resulting effect will be a higher powered (lower hover throttle) quad, with a higher current draw (resulting in shorter flight times). You may also run the risk of overheating your motors or drawing more current than the ESC can handle. This is obviously not desired. Although we do want to increase the power slightly vs. the 3S config.

Also keep in mind that not all equipment can handle the higher voltage of Gens ace 4s. The Tarot Gimbal cannot handle this voltage and therefore needs a voltage regulator. If you are using additional FPV equipment, you also need to check if they can handle the maximum 16.8V that 4S brings.

Material for testing
For testing IRIS with 4S batteries, we need a 4S Lipo, but also smaller propellers. Here is what I’m using for the tests:

1. 7.4v 5000mah lipo battery from Genstattu. This battery is very cheap and delivers roughly as much wattage as a 3S-4000mAh battery. It fits the IRIS battery bay perfectly and already comes with an XT-60 plug. I already own a IMAX B6-AC Charger/Discharger, which is capable of charging these batteries without any issues.
2. APC SlowFly 9×4.7 Pusher and Puller propellers. They are basically the same propeller as the stock propellers that come with IRIS, except that they are 9″ in outer diameter instead of 10″. As they use the same spacer rings for the propeller shaft, you can even leave your current ones on IRIS.
3. Voltage regulator from 4S (16.8V) to 12V. The Tarot Brushless Gimbal is recommended to be operated with 12V and cannot handle voltages higher than 14.8V. We therefore need to place a voltage regular between the 4S battery and the Gimbal. This will reduce the voltage from up to 16.8V on the lipo side to 12V for the Gimbal. As the Gimbal only draws a maximum of 500 mA, the linked voltage regular is a perfect choice. Also the regulator already comes with the right plugs, is very small and lightweight.

How to Calculate Lipo Battery Max Amps

A LiPo battery consists of multipe cells. These cells deliver approx. 3.7 Volts. A 4S LiPo would therefore supply 14.8V.The battery's capacity is defined in mAh. mAh means milli-Ampere per hour. 

A battery pack with a 1000mAh capacity can deliver 1 Ampere (1000mA/1000) for 1 hour. Or 1mA for 1000 hours. The battery capacity, together with the LiPo battery's discharge rate will define its maximum current output (Ampère, A). This fact is very important to bear in mind, when selecting a LiPo battery. The combination of capacity and discharge rate is what you will need. It's important to know that the battery cells don't count up for maximum current draw. The quantity of cells only determine the voltage (V) of your LiPo, when needed for your equipment. 

A battery such as the 4000mAh 20C Gens ace 4s or the other 4400mAh 15C will suffice you. For an average quad, say weighing around 1.5kg, you will draw around 250watts of power on average, say 350watts at peaks. This means about 25-35 amps max. Divide that by the 4 motors and you get about 9 amps per motor at the max. Both of those batteries are capable of delivering that and so you are OK.

And let me also add that the discharge rate is either given in C's (capacities) or in amps. When given a C rating, simply multiply that by the rated capacity (mAh or Ah) to get the maximum rated discharge rate of the battery in mAmps or Amps. Thus, the 5000mah lipo battery 20C battery is suppose to supply a maximum of 80amps and the 4400mAh 15C one 66amps.

Formula: (C-Rating) X  (AH) = Maximum Constant Amp Draw

Now to use this formula you first need to transform your battery size from the common MilliAmpHours to AmpHours.  A MilliAmp is One-Thousandth of an Amp.  So here are some Examples of converting MilliAmpHours to AmpHours:

800mAH = .800AH

1350mAH = 1.350AH

2200mAH = 2.200AH

The converted number will be the “(AH)” in the formula. Now that you've your “(AH)” number, hook it up to the formula with the “C” rating number of your Battery and Calculate.

For example:

(800mAH 10C Lipo Battery):   .800AH  X  10C = 8Amps Max Constant

(800mAH 20C Lipo Battery):   .800AH  X  20C = 16Amps Max Constant

(1350mAH 15C Lipo Battery):  1.350AH  X  15C = 20.25Amps Max Constant

(1350mAH 30C Lipo Battery):   1.350AH  X  30C = 40.50Amps Max Constant

(2200mAH 20C Lipo Battery):   2.200AH  X  20C = 44.00Amps Max Constant

(2200mAH 30C Lipo Battery):   2.200AH  X  30C = 66.00Amps Max Constant

Example calculation of discharge rate:

A 3S 2200mAh 40C LiPo pack can get you this: 2200mAh x 40C = 88.000 mAh discharge rate 88.000 mAh / 1000 = 88 Ampere constant discharge rate.

So, a 2200mAh / 40C LiPo can only handle an Ampère draw of 88A at max. So that's why the combination capacity/discharge rate is of such importance for selecting the right LiPo for you project.

Can I use a 2s lipo instead of 7.2v Nicd

Lipo has some advantages more than a 6 cell NiCad or NiMh, no doubt. You could draw far more current from the pack when it holds a larger voltage, so acceleration and top speed are enhanced. Lipos are about 40% lighter for the same capacity, so you'll be able to drive longer. And you may charge a few of these quite quick, up to 5C, so that signifies your pack will probably be full in about 10 to 15 minutes.

The quantity next to the" C" , discovered within the description of lipos , will be the discharge price. The C number multiplied by the capacity in amps. so a lipo of gens ace 4s 2000mah is 4S(four cells I don't know why they use S but meh...) 20X2000mah =40A max discharge but an 11.1V 3S10C 1000mah will have a discharge price of only 10 amps . If your motor wants 14 amps(random number) you will really get a slower rof using the 11.1V than the 7.4v. So retain in mind the numbers when purchasing lipos.

But, you can find also drawbacks that has some sticking to NiMh for now. Some prefer to preserve an older car fully "vintage" so all gear used to be in the era the car or truck is from. Lipo batteries also are a greater fire hazard than NiMh or NiCad, each while driving and when stored. You'll need unique warning program to prevent discharging them also low, whilst on NiMh you can simply notice the pack is starting to dump, and stop just before troubles arise.

7.4 V is definitely the typical two cell lipo. Every single cell includes a range involving 4.two per cell (so eight.4 completely charged) and three.2v per cell (or six.4v completely discharged.) Massive point here is, lipos are certainly not like nihms, or nicads. You cannot and Have to NOT discharge them under three.2V per cell. For those who do, they swell, and come to be unusable. Looking to recharge an unusable pack may well result within a quite huge fire. Which brings me to the crux of the dilemma. A 7.4v 5000mah lipo battery would probably be fine in your auto, you'd get much better speed and great run instances, On the other hand your esc (electronic speed handle) is not going to tell you when you'r finding close to empty on the battery. Most brushless escs (brushless becoming a distinct kind of electric, google it) may have a 'low voltage reduce off' (google that also). Generally the esc keeps track of the packs voltage, and when it gets to 3.4V per cell, cuts the motor. YOUR esc Is not going to do that, and it's as a result inadvisable to use lipo's within your auto. As in, do not. Visit a nearby hobbyshop and geta greater answer than this, LiPo's could be dangerous, you truly want to do your homework just before you start off using them. 

Besides, not absolutely everyone is prepared to invest revenue into a lipo compatible charger, or requirements the added speed and energy of lipos.

I for example do use lipos on some cars, but I also have a car with brushless system that already screams on NiMh, getting top speeds of over 70 km/h. It's already a handful to drive this way. Imagine putting in a lipo, and getting a 10% or more performance boost in top speed, and even faster acceleration. So I keep using the 2 NiMh packs that came with the car when I bought it second hand, and when these die, perhaps I will get a lipo instead.

Coming back to your auto, I feel a lipo would do well, it would boost acceleration and major speed, and runtimes. And also you currently have the charger, so it is not a huge investment to begin driving with lipos.

How to Increase Quadcopter Flight Time

There is a good chance that there are more efficient batteries available now than the ones you got delivered with your quadcopter. The manufactures are continually improving the batteries. You will need to look for the same type of battery that you got with your drone, but with higher mAh will make the battery run longer.

Batteries that get much more than just warm (105F or so) are being permanently damaged. Unrecoverable damage starts at 115F or so and the higher it goes the more damage there is. Too hot to hold onto is about 130F for most people.

And you are putting the pack at risk of puffing, rupturing, and even bursting into flame. If they are puffed now, you need to be very careful when charging them to make sure they don't over heat. A healthy battery taken down to 80% of it capacity will usually take a little more than 80% on recharge. As a battery ages the recharged capacity will get smaller and smaller and the battery will drop to the 3.7-3.8V under load more quickly. A develops all the aging and used up symptoms more quickly.

If your Gens ace 4s 2200 mAH battery will give you 80% of usable capacity you can use 1760 mAH out of the 2200 that is supposedly there. So your available capacity would be:

1760 mAH = 1.76A
1.76A x 60 = 105.6 Amp/Minutes

At a 32A discharge rage your flight duration would be:

105.6 / 32 = 3.3 minutes (3 min 18 seconds)

So guess what? You've been beating that battery up pretty hard if you flying it for 5 minutes..

Two batteries in parallel will give you the capacity of both batteries and they will both share the load so the C rate would drop in half from the above if it was the same batteries. It would be best if the batteries were the same capacity and even better if they were same brand and bought at the same time.

If you keep an eye on the pack temperatures and amount of capacity that is put back when you recharge it will do a lot for battery life. Keep the recharges down around 80% or so. And checking the pack voltage immediately on landing will tell you a lot. If you land and find the cells at 3.8-3.9V 10-15 seconds later, you were most likely down to 3.7 or less under load.

A battery taken down to 3.7V while under load is 80% discharged and that should be the end of it's day. And it should not be more than warm then either. If you land with a battery at 3.7-3.8V and shut down it will start recovering immediately and may be back up to 3.9V or 4.0V within minutes. But it is still depleted. If you run the motors up the voltage will drop immediately.

Everyone needs a watt meter so see load on their battery and how the battery is doing as far as dealing with the load. If you really want to know what is going on a data logger like the eLogger can be flown to see what is going on. The image is the eLogger data from a flight, the red line is the battery and you can see how the voltage falls as the flight proceeded.

The flight was a stable hover, then three full throttle punch outs, then a few circuits of fast forward flight, then a brief hover and landing. The red line is the pack voltage. The 3S lipo battery 5000mah pack was down to 11.15V at the lowest point and that is your 3.7V quitting time. The voltage recovered a little as he back off on the throttle, hovered, and landed.