How to determine flight time for a 2S850 LiIon

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As often happens, another day brings another question, and after I've answered the same question (or a minor variation) X-number of times, I say enough, and dig out my notes (or look through emails) and put together an askJOHN to address it. This, so in future I can just point folks to this resource instead of spending an hour on the phone going through it for the umpteenth time. After all, there's only one of me, so I really should be wise about spending my time on new stuff instead of rehashing the same-old, same-old, right?

So here we are, once again, this time related to figuring out how long a battery pack will last before it needs recharging. And best if we can do it without spending more money on a new radio offering telemetry, or buying a telemetry module, neither of which are cheap, right?

Since there's a method that's dead nuts easy, and all you need is a watch, timer (both of which we already have, even for transmitters more than 20 years old), plus your battery charger that displays mA returned to the pack, then here's a solution that's as close to free as it comes!

Q. Hi John, you'll recall I just bought a 69" span Extreme Flight Turbo Raven and because on your advice, I went with ProModeler DS210CLHV servos (I considered the DS160CLHV also but I fly pretty hard). Anyway, it's replacing one I crashed when the ESC burned up and took the BEC along with it. That one had Hitec servos and they were pretty worn so I wanted new ones and based on friends who had used your servos, I decided to give you guys a try. And I find your thoughts on using a 2S LiIon pack instead of a BEC for powering the receiver and servos (as if it were a 61-powered nitro-model), interesting because I've always used the BEC from my ESC but when my speed control burned up, instead of a dead stick landing, I crashed and destroyed the model. But while you have peaked my curiosity, I'm left wondering this; how many flights can I expect from your recommended 2S850 LiIon pack? I hate the thought of toting the weight of a 1500, but I will if I have to. How can I know if an 850 will be enough?

A. As it happens, I own that same Turbo Raven model and I'd be fine flying three flights when powered by a B2S850 whether it's equipped with DS160CLHV or DS210CLHV servos. Thing is, because you and I fly differently, then maybe instead of 3-flights, you'll get 4-flights, or possibly just 2-flights before needing to top the pack up. As for why I can't get closer that this, as I explained on the phone, it's because until we know how you fly, the answer for how many flights you'll get is, we don't know and it depends! I know that's unsatisfactory but it's the best i can do.

So in a subsequent email I wrote up how to determine the answer and before I pressed send, decided to answer for everybody. If you have ever wondered the same thing, then let me show you how easy it is to figure out how many safe flights you can get from your receiver pack using this askJOHN. And it's easy!

Before we begin; I know your question is about the receiver, or avionics, pack, but let's expand on this a bit to include propulsion packs for electric powered models, too. Reason is we can kill two birds with one stone because we can calculate the safe flight time the same way for both types of packs!

Basically, your question involves the same principle for calculating the consumption for any battery pack whether it's the one powering the receiver, servos, gyro, etc. or the one for spinning a prop. Figuring out flight time for two different packs, avionics and propulsion? Yup, same mission, supply power, safely.

- In Russia, Alexey Cheryomukhin prepping to fly off river ice in Moscow . . . brrr!

I am also assuming we're discussing the use of a LiIon for the avionics pack, and while slightly different for a LiFePO4 (alternatively, LFP or A123 - all 3 the same thing), or a NiCd, or even NiMH, the same basic method for figuring out a safe number of flights holds true for all batteries regardless of chemistry.

Also note, and this is important, battery performance decreases with temperature so what works in the summer may be marginal in the winter when it's cold as a witches teat (as in the photo above) because packs deliver less juice when it's cold. This means you may need to repeat the process of figuring out safe duration for both conditions. Heads up!

Last assumption; we're discussing a model new to you 'and' new packs, e.g. you don't know how long the model can safely fly because you don't have a clue about anything propulsion-wise or for the control-packs for the avionics! Figuring this out is easy and I'll show you how using nothing but grade school math and some rules of thumb derived from what the charger tells you when you recharge.

To begin, we need some idea of what to expect in the way of consumption for the avionics. These days, receivers and gyros are negligible and all you really care about is the power consumption of the servos. For ProModeler servos, click the Specs-tab (in this instance DS210CLHV servos) and you'll see something like this;

No question, the DS210CLHV is a bad ass little servo because it offers a lot of torque in a compact package. Moreover, because it transits fast, it's great for 3D maneuvers.

Anyway, look at the column for 7.4V because you've asked about a 2S LiIon and this is about right for the nominal voltage. You'll note three figures with mA . . . current at idle, at no load, and stalled. These are 13mA, 400mA, and 3,400mA respectively (3400mA=3.4A). Unfortunately, none of these represent reality because during flight, the voltage is always varying.

  • The 13mA correspond to the model sitting there, equipment turned on but you're doing nothing to the controls, the servos are just idling.
  • And no-load current, the 400mA figure, is literally that, aggressively moving the sticks with the model on the ground, but unloaded by the flow of air. Neither idle or no-load current-draw figures are worth much, but they're what we have.
  • Then there is stalled current at 3.4A. For example, load the servo until it won't hold any more and gives way (and measure current at that instant).

Again, these are current consumption value that are of little use in the real world. Stalled, for example, only represents the worst possible scenario. Engineers love it for figuring how much can be drawn by a servo when it's working as hard as possible but in the real world this only happens for a few seconds at a time, maybe several times during the flight.

Why can't we give you a better number? Simple, it's because the load always depends on the angle of the control surface, the size of the control surface (area), and the air speed it sees. This, and it also depends on both air temperature and altitude to figure density altitude.

Unfortunately, even with a supercomputer we don't have enough data to predict because every model is different and is flown differently, and at varying temperatures and altitudes. Does it mean we can't predict? Nope. We can do better than that. We can come pretty close by measurement and extrapolation. Here's how.

Background

FYI, if we put an ammeter on each servo and flew the model and recorded current draw, we'd get data for each servo, or if we put one between the battery pack and the receiver, we'd get a current figure for the entire system. We know we can gather this data with telemetry but we're going to show you how to figure it out - close enough - for free.

As a by the way, we know because we've done just that (measured in flight loads) that just cruising along the servos are drawing about 450-550mA (each servo), but if you start to toss it around the current consumption goes to between 1-2A each (multiplied by four servos in a 69" model), but this load is just for 5-10 seconds and then you're experiencing cruising loads again before the next maneuver.

And we also know if you are flying along at top speed and stall the servo, say full deflection of the ailerons at 80mph for a rifle roll, then consumption for two aileron servos can spike over 3A each for a few seconds before dropping to about 1.5A for the duration of the maneuver. Call it for 10-seconds whilst corkscrewing down the length of the field. Then we're back to cruising consumption figures of ~500mA each.

We also know most flights last about 6-minutes of which maybe 4:30 are aerobatics because a fair bit of time is spent on take off and cruising, and setting up for landing. So about 1:30 sees current loads, which are pretty low.

Major take way being, current draw figures are never exact because there's some variability in what load an elevator, rudder, or the aileron servos are experiencing, capiscie? As an engineer, we want to know the worst case scenario, and we tell you in the specs. And knowing four servos can exceed the draw of a single lead, means if I'm depending on a BEC delivering current through the throttle lead via a connector rated at 3.5A, I may be generating head, which is never good. That, and if the ESC goes teats up I'm gonna be out of luck, then I want to be conservative and find a better solution.

For this reason, I prefer to use a battery (or BEC) with two power leads since each Dupont connector is rated at 3.5A and using two means I can deliver 7A to the receiver without creating heat. All ProModeler battery packs come quipped with two leads, and we offer the Castle CC BEC Pro 20 on the website, which has dual power leads . . . so we have you covered. But this is about using battery packs, so let's ignore using a standalone BEC for purposes of this article.

The figures in the above chart are fairly close to reality, but useless in the real world. So what do we do, instead? Simple, we figure out our own consumption based on how we fly, is what!

- Climbing inverted flight, putting on a show - loads are higher than straight and level

Recapping so far; what's missing? Exactly how many milliamps of consumption whilst the model is in flight. Yup, I hate to say it, but we don't know how much current is being drawn at any given time because for any given model, the current draw varies with the size of the control surface, the speed at which the model is flying, how rapidly you're inputting control, flight forces, etc.

Like obviously at the bottom of a loop as the forces of gravity build up, the forces on the elevator servo increase and current consumption increases right along with it, right? How much deflection, how fast are you going, what's the model weigh, how large are the ailerons, are all variables. Heck, even how many loops you perform, varies.

Does this mean we can't figure out the answer? Nope, we'll do something to get a really close figure, we'll determine it empirically, e.g. measure it after the flight. How? Using the charger's function of displaying mA returned to the pack when charging.

- Gorgeous wide open spaces when you have a river to fly off of, eh?

Rule of thumb:

As mentioned earlier, just flying along fat dumb and happy the current consumption often works out to about 1.5X no-load current (naturally, if you're über aggressive, maybe performing rifle-rolls the number will be higher, perhaps 10-15X no-load but only for a few seconds. Turns out we don't need any of this information.

Instead, we just need to keep track of how long we fly (we all settle into a routine of pat maneuvers we like to perform, it's human nature). That (how long we fly) doesn't really vary very much because we're using the same size packs each flight and they begin from a fully charged state means we know how long each flight lasts. So all we need to learn is how many milliamps are returned to the pack and we can suss this out lickity-split! This is called the empirical method.

So here's what you do, fly the model once, and recharge the pack. See what the charger says it returns in mA. If it returned 150mA, then you can fly two flights in perfect safety before recharging. Maybe three or four. Anyway, fly it once again on a fully charged battery to confirm it replaces about the same 150mAh. Bet it does.

Now try doing two flights. Does it replace about 300mAg - two  times 150mA? Means three flights will draw out 450mA from an 850mA pack. Maybe you can pull a fourth flight from the pack, maybe you can't. My thought is, why risk it? Call it three and then top it up with the field charger!

By the way, confirm consumption once again when flying in the winter, and my advice is reduce the number of flights you can draw for the pack by qty 1 because packs deliver less when they're cold. Don't say I didn't tell you.

Let's delve into propulsion packs. New model, new packs and we know nothing whatsoever. Me? I like to break in my propulsion packs and at the same time sneak up on the right amount of flying time for my timer (yes, I'd rather do it the old school way sans telemetry),

So I initially set the timer for 3-minutes and go do my thing. Why 3-minutes? It's because based on my experience 'all' the model types I routinely fly will go three minutes on the recommended LiPo propulsion pack - maybe not an EDF models (electric ducted fan, e.g. jets because they're some of the biggest power hogs out there, and if flown without a care in the world regarding duration you may be in for a surprise. So for EDF, set the time for 2.5 minutes instead of thee).

However, in the case of the Turbo Raven, Extreme Flight suggest a 6S3300-4000 pack. Doesn't mean you can't fly with an 8S4000 . . . it depends! And experience teaches that with a 6S3300 you're likely to be tail heavy and a 6S4000 it will be about right, that and you can probably get 4:30 out of the former and 6-minutes out of the later. So 3 minutes is almost to a certainty going to be fine for first flights while you are still determining safe consumption using the charger and how many milliamps it returns to the pack.

Anyway, it's important to note I don't fly aggressively at all in the first few flights of a new model (and never with new packs) because the point isn't to show off to the peanut gallery, or even satisfy myself and have fun. Nope, that can wait until I have discharged the propulsion pack gently a few times, e.g. broken it in.

Yup, break-in, like with an engine. I kid you not. Believe me, with expensive propulsion packs, the goal is to break-in the pack instead of thrashing it from the get go because they'll last longer - and this is the subject of another article;

So after three minutes I put packs on the charger - both propulsion and control electronics (the term avionics pack is perfectly acceptable if it's within a flying model). Upon re-charging, I just make note how many milliamps were returned to each pack (because I'm using a charger that tells me this). After three or four non-demanding flights, then next I judge if I can push duration to 4-minutes. Then rinse, repeat meaning later I decide if I can go for 5-minutes, 6-minutes, etc.  As for the maneuvers, I don't want prolonged loads - this means no torque rolls or long accelerations out of a hover. Loops, rolls, stall turns - gentleman aerobatics are fine but nothing aggressive.

All this works for me because eventually I am up to my old tricks. My meaning by old tricks is that I am a creature of habit and routine and often fly the same kind of maneuvers (because they're the ones I enjoy). But how do I decide? It's all down to milliamps consumed! You need a charger that tells you and anything worth owning does.

But here's the thing; you are different from me. My advice is to use my experience as a guide but ultimately make your own decision, OK? The point being, the milliamp consumption - for me - is actually pretty consistent from flight to flight because the maneuvers I perform are variations on a theme. What about you? Dunno but I bet a milkshake you're fairly predictable in your routines, also.

Thus, a simple timer set to 5-1/2 minutes works beautifully for many of my models but may not work for you because you fly harder. Fortunately, because I assign the timer to my throttle hold switch (meaning I can't fly until I deactivate throttle hold because until then, the throttle stick is not active), then I always engage a timer before flight.

Doesn't take long to learn what's the right number for minutes of flight before landing. Easy peasy!

- Using the hold switch to arm the ESC and activate the timer is sweet!

Note, I fly a helicopter-specific transmitter because I fly rotary-wing models in addition to fixed-wing and heli-trannies have a switch at the top-right which disengages the throttle from the collective pitch. This is so you may practice autorotation-maneuvers (engine out simulation) with the engine at idle. The idea behind this is it lets you to bail on the maneuver if you've botched it beyond reasonable hopes of recovery and thus, because it's been idling the whole time, you have the engine power available to save the model from a mishap. So for electric models I've found it convenient to assign the timer to the hold-switch because doing it automatically engages it (the timer) when I arm the ESC before beginning the flight . . . . it's one less thing to remember (I'm old and dumb and need all the crutches I can lean on).

Also note; I am in ZERO hurry about any of this. It's a hobby, right? I'm having fun and part of the fun 'for me' is the whole break-in of packs and getting used to the new model with brief flights, yakking with my pals, etc. The point being nobody is paying me to fly. It's not a job. And thus, I am never going to hammer on a brand new propulsion pack like the pilots who get packs for free, or the ones who don't care about the expense (I pay for my packs just like anybody else so 'I' care). The point being, flying models is not my job, nor am I demonstrating how well a model flies, or whatever (not ever). It's about fun. Moreover, if I don't get done that day, I don't care because there's always tomorrow!

Anyway, I continue in this vein until I am familiar with how many minutes I can fly any given propulsion pack in any given model with 'my' style of flight. How? Once again, by using the display on the charger! A charger is my friend because it tells me exactly how many milliamps have been put back into the pack. So for me, maybe a flight is 6-minutes, for you maybe it's 3-1/2 minutes, again, dunno how hard you fly in comparison but the charger display won't lie about it!

All this is predicated on the one rule I 'never break' which is to always stop the discharge (flight) with at least 20% of capacity remaining. Put another way, I'll consume 80% of the capacity. e.g. 3300mAh propulsion pack x 0.80 = 2640mA . . . and this is the number I look to never exceed when recharging a 3300mAh 6S propulsion pack. For a 4000mAh pack, I can draw 3200mAh out of it and retain 20%.

For the example 850mAh control-electronics pack, it's the same math and leaving 20% means not using more than 680mA from it. It's the same math regardless of capacity. That's my 80% rule of thumb for a LiPo and LiIon and in my experience, when I hew to it consistently, packs last me a long, long time. Ignore this and discharge below 20% and troubles begin, e.g. puffing and other things which lead me to discard a pack. E.g. it costs me more money - and I don't like that because I'm cheap!

As long as you remember, consumption from the avionics pack varies by model and servo because different models of servos consume different amounts of current, and different modelers fly differently, etc. so it's different for everybody. However, and quite amazingly, it's relatively consistent from pilot to pilot. This means the power consumed by me for any given model will be similar flight to flight. Same will happen with you. Maybe an altogether different value from mine but for three flights you'll likely find yourself doing variations of the same maneuvers over and over again and your total consumption will be similar flight to flight. Not a guarantee but like I said earlier, I'd bet a milkshake it's true.

Finally, with regard to the control electronics pack (what I also refer to as an avionics pack, e.g. the battery for non-propulsion use), I am no 3D-god so slight differences in pack voltage depending on where it is on the discharge curve mean nothing to me. Higher voltage means more power and speed but on the flat part of the curve at which it's rated, it's very consistent. Good enough for me.

What I am trying to say is this; if you're that type of flier (meaning you look up to Jase and are trying to emulate the maneuvers he performs), then maybe topping up the charge after every flight or two works for you because you enjoy the benefits of a hot pack on each flight (higher on the discharge curve). Me? After 3-flights I top it up because I have selected the servo performance to confirm with operation on the flat part of the discharge curve instead of at the peak. Everybody is different. This is important to bear in mind.

- Camaraderie may be the single most important factor when flying

Safety Factor:

So does this mean the 850mAh won't go four or even five flights? Maybe, it depends. First, I have a sense of how hard I've been flying and if from experience I know after three flights I'm only using 50% of the pack's capacity then I'll be perfectly comfortable flying one or even two more times before recharging. Why? Simple, it's because by then I have experience. E.g. I am pretty sure I won't consume more than 80% of the rated capacity if I use 125mA from the pack after each flight, right? Second, because this works out to 250mA for two flights, 375mA for three, 500mAh for four, 625mA for five flights . . . or pretty darned close to 680mA, the hard limit for a 850mAH pack if you follow the rule of 80%, right?

So if I stop at three flights and recharge, I've left myself a safety factor of 1.5X. Same holds with cold weather when it's my view if I recharged after 3 flights in the summer, if it's a 35° day I'd probably be wise to recharge after two flights because the discharge curves of cold batteries inform me this is a good idea (batteries hate cold weather - some chemistries more than others but they all dislike the cold).

Note; I've been sharing photos from my friend and customer Alexey Cheryomukhin, which he has kindly shared with me of winter flying in Moscow. Honestly, these guys are real experts when it comes to flying in the cold and some of the advice I'm sharing I learned from him. I've also shared photos of Alex Hewson who hails from Christchurch, New Zealand (the south island). Plus another couple of customers, Mark Dobson of Merrimac, MA and Marty Jones of Las Vegas, NV . . . our thanks to one, and all. And special thanks to Alex for sharing a photo of his failed Castle ESC after it burned to a crisp.

Note; this business of a 1.5X safety factor is really nothing but an indication of where my comfort level is set. Remember this about 'me' so when you are digesting and integrating 'my' thoughts into 'your' worldview, e.g. taking whatever I may have to say (like within this article) which is; I am considered by many to be extremely conservative by nature. Recall this when deciding if what 'I' have to say holds water for you, or not.

And by the way, to be fair, I also know pilots whose MO is to hammer the crap out of their packs from the get-go. Of course, I am not privy to how long their packs last so I am only describing what works for me, capice? As for the last part of your question, no, I don't think it's necessary to use a pack of higher capacity than a 2S850 for your model.

Last thing; we offer a 2S250mAh LiPo pack. I'm not fond of LiPos for control avionics packs but this one is so tiny and light I'm less afraid of the chemistry. Anyway, if you fly with a 2S850 and over time develop a certain comfort level with consumption averaging 125mA per flight, then if you're gonzo nuts about weight savings, you can reasonably expect to switch it out to a 2S250 pack and recharge after every flight but have 0X safety margin. Your call and I am 'not' necessarily recommending this, just saying the numbers don't lie and while you'd be making your own decision - not based on my opinion but on your experience, then maybe try it if weight is the be all end all for you. it isn't for me.

Anyway, now do you understand how to use the math to work out what's best for you? Simple, right? That's what I promised when we began.

Another way

An alternative method, and the simpler of the two is to just measure voltage/capacity after each flight - we offer two devices for this on our website (good and better). I use the Futaba BR3000 personally but the cheapo works as well - es machts nichts! Anyway, I stop before the pack is at nominal voltage, e.g. the 'book' says for LiFePO4 stop at 6.6V, and for LiIon at 7.2V but I see no point in pushing things. For what purpose would I risk my model? Just to see how many flights I can get out of a pack? Nope, I don't care enough to push things, personally!

So to recap; I usually recharge after three flights. I like a pack with reserve enough to fly five flights, which the B2S850 does for me in a 60" class model with 160-210oz-in servos. But we offer mini class servos outputting 325oz-in, 355oz-in, and 405oz-in and these draw more current so you need to check based on what you are using servo-wise, capisci? Means this article has good info even if you're not using ProModeler servos because this works for al servos, all brands!

That, and I check periodically to get an idea consumption is still the same, meaning my routine hasn't changed enough I need to reconsider the number of flights (this works for me because I fly pretty much the same routine). And no, I'm not a competition pilot so I don't have an actual 'routine' but my repertoire of tricks is fairly rudimentary and thus, I tend to fly similar routines of maneuvers from flight to flight.

Turns out after three flights (by which time in my experience I'm well and truly into my 'routine'), I've flown enough to let me estimate consumption per flight with surprising accuracy. Yup, after just after three flights! And it's accurately enough I don't bother with telemetry.

Bottom line? I choose to keep things simple and use the gray matter God gifted me with to perform simple calculations within my head (e.g. without resorting to fingers). Note, in addition to determining battery consumption with the hand-held meter, I also cross check with the charger when I recharge (my charger tells me how many milliamps have been returned to a pack (I'm serious, I wouldn't own one without this feature - we offer these also). Here are links!

Finally, for those interested in learning more, these articles on the ProModeler website will open in a separate tab when clicked, and some may be of some interest . . .

. . . these, and others, are filed under the askJOHN menu of the site.

If you have further questions, reach out via email at: info@promodeler.com or just pick up the phone and call us at: 407-302-3361 and we'll try to help.

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