How to determine flight time for a 2S1000 LiPo?

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Q. I just received a 69" span Extreme Flight Turbo Raven and on your advice went with ProModeler DS205BLHV servos (I considered the DS160CLHV also). It's replacing one I crashed when the ESC died and took the BEC along with it. This is why I find your thoughts on using a 2S LiPo instead of a BEC for powering the receiver and servos (as if it were a 61-powered nitro-model), interesting. But while you have peaked my curiosity, I'm left wondering, about how many flights can I expect from a 2S1000 LiPo pack? I hate the thought of toting the weight of a 1500-2200mAh battery but will if it comes to it.

A. As it happens, I own that model and I'd be fine flying three flights powered by a 2S1000 whether it's equipped with DS160 or DS205 servos, but because you and I fly differently, then maybe instead of 3-flights, you are more comfortable recharging after 2 flights, or maybe after every flight, depends! Let me show you how to figure out what's best for you - 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 as well. Reason is we can kill two birds with one stone because we can calculate safe flight time the same way for both 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, the drive motor of a crawler, or rotor blades. Two different packs - same mission, supply power safely.

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


I am also assuming we're discussing the use of a LiPo for the avionics pack, and while slightly different for A123, LiIon, NiCd, or 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! I'll show you how to account for this as well using safety margins.

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 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 experience.

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 DS205BLHV servos) and get something like this;

The DS205BLHV is a bad ass little servo, isn't it? Anyway, look at the column for 7.4V because you've asked about a 2S LiPo and you'll note three figures with mA . . . current at idle, at no load, and stalled. These are 22mA, 100mA, and 3,400mA respectively (3.4A=3,400mA).

The 22mA correspond to the model sitting there turned on but you're doing nothing to the controls, the servos are just idling. At no load current, the 100mA is literally that, maybe turning the wheels back and forth or the elevator up and down, or going from low to high cyclic but the model itself isn't moving, just the servos with the controls. Then there is stalled current at 3.4A. E.g. put a load on it until it gives way (and measure current consumption). The figures aren't exact because there's some variability in what an elevator weighs or how much drag the grass the wheels are resting on exert, but the figures are fairly close to reality. Close enough to use them.

- Entering knife edge, rudder loads are higher than straight and level


What's missing? How many milliamps of consumption whilst the model is in flight, or whilst driving your truck along a path, or hovering if it's a helicopter? Yup, I hate to say it but we don't know how many milliamps are 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 servo increase and current consumption increase, right? How much deflection, how fast are you going, what's the model weigh, how large are the wheels or ailerons, etc. It varies! Honestly, it would take a supercomputer to model it through discharge - better to determine it empirically, e.g. measure it!

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


Bottom line? The specs can inform you when the servo is doing nothing, when it's moving lightly loaded, and when it's resisting with all its guts but not exactly what your model is consuming at the time it's doing it - sorry! However, you can buy a gizmo to add between a servo and receiver that will record data. But in my experience you don't need to spend the money on one of these. Why not? Simple, because I can help you figure it out based on some experience. I'm about to share a rule of thumb.

Rule of thumb:

During nominal operation, e.g. just cruising down a trail, or flying along fat dumb and happy the current consumption often works out to about 2.5X no-load current (naturally, if you're über aggressive, maybe performing rifle-rolls the number will be higher, perhaps 7-10X - we'll get into more when we discuss safety margin). Anyway, for this particular model, with these servos, then 100mA/servo x 2.5 = 250mA/servo . . . for an hour.

So next we need to account for the number of servos and in this example, it's five servos (two on ailerons, two on elevators, plus one on rudder). More simple math shows us it works out to 1250mAh for an hour (250ma x 1hr x 5 servos). So for 3 flights of 6-minutes each (6-minutes = 1/10th of an hour, or 3/10th for 3 flights) then 1250 x 0.3 = 375mA being consumed (for the three flights). This tells me one important thing . . . a 1000mAH 2S LiPo is probably safe for one flight of 6 minutes. How do I know a 1000mA 2S pack is enough for three flights? I don't, I shared my opinion with you, remember?

Anyway, as an old engineer I share another opinion, which is the Boy Scouts have it right . . . be prepared. So here's what you do next. You fly the model, recharge the pack and see how close the predicted consumption is to 125mA . . . or 375mA for three flights, divided by three, equals 1/3 of 375 for one flight - grok the math?

It's important to figure this out from the get go because what if you had asked me about using a 2S250 LiPo instead of a 2S1000. Yes, you could probably get one flight in safely with a 250mAH pack but things might get dicey if we're wrong about our assumptions regarding current consumption. Remember, everybody flies differently! Anyway, there's more. Let me explain.


Let's diverge 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 - even EDF models (electric ducted fan, e.g. jets - some of the biggest power hogs out there if flown without a care in the world regarding duration). In the case of the Turbo Raven, Extreme Flight suggest a 3300-40006S pack. Doesn't mean you can't fly with an 8S4000 . . . it depends!

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. Not until I have discharged the propulsion pack gently a few times, e.g. with expensive propulsion packs, the goal is to break in the pack (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 model 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 propulsion pack.

For the example 1000mAh pack, it's the same math and leaving 20% means not using more than 800mA out of it. It's the same math regardless of capacity. That's my 80% rule of thumb for a LiPo 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 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 1000mAh 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, 750mA for six flights . . . or pretty darned close to 800mA, the hard limit for a 1000mAH 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 2X. 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.

Note; this business of a 2X 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 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 2S1000 for your model.

Last thing; earlier I mentioned a 2S250mAh pack. If you fly with a 2S1000 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 and recharge after every flight and still have a margin of safety of approximately 2X. Not necessarily recommending this, just saying the numbers don't lie and you'd be making your own decision - not based on an opinion but on your experience. 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 six flights. And I check periodically to get an idea of consumption (works since I fly pretty much the same routine). 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!

Digital capacity meters: https://www.promodeler.com/store/Search.aspx?SearchTerms=tester
Inexpensive 300W charger: https://www.promodeler.com/PDR0098
--
John

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

On Modeling's Different Chemistries 

Care and Feeding of LiPos

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

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