How much left in my lipo?

[Yorkman]

Flew the Mossie today (she's sorted-hurrah!) for eight minutes on a 2s 1800 lipo.

Just checked the voltage of said lipo-7.83 volts.

According to Timbo's charts, that means it still has more than 80% charge....or, I've used less than 20% of the charge-theoretically making 40 minutes possible? Or am I mistaken?

[ken anderson.]

hello yorkman - i would say you are wrong by a long way-----or the rest of us having been missing out on 30 min's extra flying with our lipo pack's.....timbo will no doubt be along shortly to 'enlighten' you...........

 

 ken anderson......... ne 1...

 

ps........the way i would tell what was what (so to speak)...is put your lipo on charge and see what the charger put's in...i would think something in the region of 1000m/amp's to bring it back to capacity....i also think you'll have been near to the cut off point for your battery..which could have spelt   for your mossie......

Edited By ken anderson. on 22/02/2010 10:11:40

[Clive Matthews]

Part of the problem is that your voltage reading will have been off load. As Ken has said, the only way to tell exactly is to measure how many milliamps the charger has to put back in. This assumes that your battery is working to the stated capacity of 1800 mah.

[Yorkman]

Don't believe it myself-in fact did wonder if I checked the correct battery! Will do some further flying and testing-oh, btw, in its previous incarnation with bigger motors and stratospheric (as opposed to merely ballistic) performance, was flying for 10 minutes no probs...

[Yorkman]

Posted by Clive Matthews on 22/02/2010 11:09:37:
Part of the problem is that your voltage reading will have been off load.

 

As I said-according to Timbo's chart-which I'm pretty sure is 'offload' voltage.

Unfortunately the weather's turned, so no flying at the moment-if I get a window I will investigate further.

[Clive Matthews]

Apologies for missing that.

[Peter Beeney]

    Yorkman,
             It seem to me your flight figures might make an interesting analysis.  I don’t have all the details, so I have to make some assumptions here, consequently this may be way out of kilter.

    You have a 1.8Ah, 7.4V battery. This then gives you: 1.8 × 7.4 = 13.32Wh or 800Wm.

    You flew for eight minutes so you consumed, on average, 100 watts / minute. That’s assuming that the battery was exhausted at the end of the flight.

    Another assumption, your model weighs 2lb. So that’s a power to weight ratio of 50 watts to the pound. You mentioned you have ballistic performance so I guess you might have been flying flat out all the way.

    The figures for your first model, if you used the same pack, would be 80 watts / minute, or 40 watts per pound.

    I don’t think I’m convinced that any twin of reasonable size could fly for eight minutes on that battery and still have that amount of capacity remaining. A broad indicator of the state-of-charge, (SoC), of a lithium cell is it’s open circuit voltage, so a 2s pack that’s discharged will have an o/c voltage of about 6 volts and may not recover greatly from this. (Without charging). In fact, this can lead to problems, with some chargers sometimes unable to read the correct number of cell/charge voltage.
    If you checked the wrong battery, then that’s one thing, but if not then I think I’d be inclined to take a quick ganders at the volt meter. 

    From another angle, if you had only used 20% of that battery in eight minutes the figures would surely be…  
           
           800Wm × 20% = 160Wm ÷ 8 = 20w/m ÷ 7.4V = 2.7A ÷ 2 motors = 1.35A.

    Each motor would be running on 1.35 amps.  Very economical! But not a lot of poke, perhaps!

    I hope these figures are somewhere near the button and it does make some sense. At least to you, ‘cos I’m not sure about me………        PB      

[Nick Farrow]

Hi,

 

I would look at it this way (probably incorrectly)

 

Assuming 100w per pound and a mossy (GWS formosa foamy) weighs 1 pound

 

With a 7.4v battery you drew 100/7.4 = 13.51 amps

IF you did this for an hour you would need a 13.51Ah battery, but you did this 8 minutes or 0.1333 of an hour, so used 13.51x0.1333 = ~1.8Ah!

 

I dont think you have much left in the tank!

 

Regards

 

nick

[Yorkman]

Hi Nick and Peter

got my head spinning with all those figures.

First, to put things straight-8.4v from a 2s lipo, surely?

Model is a GAD Mosquito, 32" span, 14.5 or 15 oz flying weight depending on 1800 or 2250 lipo.

Just been out this morning and got two flights in on the two packs-8 minutes straight on the 1800 pack, now 'open circuit' voltage of 7.7 volts.

Had 2 four minute sorties on the 2250 pack (landed to adjust ail expo)-voltage remaining 7.82.

I'm assuming my voltmeter is somewhere near correct, I have no reason to doubt it.

Oh, I must confess I am pretty gentle on them whilst flying-I don't drill around the sky on full chat the whole time, but tend to 'cruise' more, kind of medium throttle/speed flyby followed by pull up, roll, cut throttle again, cruise round for the next pass...sorta thing.

[Nick Farrow]

Hi Yorkman,

 

You had a better morning than me - I'm at work

 

I dont know of Timbo's chart, but perhaps the other way to look at it could be (just my take)

 

My esc limits my lipos to 3v which is perhaps a good measure of 0% (useful) capacity

My charger puts the cells back upto 4.2, so thats 100%

 

I dont believe lipos have the same substantial rise in open circuit voltage that dry/nicads have, so it perhaps a reasonable indication of charge. However I have seen with my watt meter that full load will show a drop in voltage.

 

If the lipo has a linear plot of voltage to charge then you could project forward with

full voltage - empty voltage give range, so 4.2-3 = 1.2v range from 0 ->100 % charge.

Therefore 0.012V = 1% of charge.

 

So your first figures of 7.83 (2 cells), give 3.91-3 = 0.91v representing the charge left, or 0.91/0.012 or ~75%.

 

I guess you could confirm these figures with a quick bench test noting the voltage every 2 minutes of a given load and plot a graph and see if it looks a reasoanble metric, as you have more time than me!

 

nick

[Yorkman]

Nick

Timbo's chart is in the first topic in this forum-Looking after your LiPo cells

where I believe he has done exactly as you suggest, and plotted battery voltage as a representation of % charge remaining-your arithmetical figure of 75% being slightly worse that his, but probably safer to use (I'm certainly not trying to get every second out of each charge!)-but still giving me a theoretical 30 minute flight time!

[Peter Beeney]

    Hello Yorkman,
                                 It would certainly seem as though my economical gibe was in fact perhaps nearer to the truth than I thought. So that’s me shot down again!
    The 7.4V come from the way the system calculates the capacity of the cell. The cell voltage ranges from 4.2V, fully charged, down to 3V, fully discharged. If you used either of these figures for any calculations you would get a very misleading figure so you use the centre point, 3.7V. Originally it was 3.6V, I feel this was probably a more accurate assessment, but then someone came along with a ‘mine is better than yours‘ argument, which gave the advertiser a few more watts. Or he was probably able to advertise a few more thousand milli-amps, so beloved of aeromodellers. So now 3.7V is pretty universal.
    To check your meter, try a fully charged pack. Your charger is unlikely to be far off the beam, so that should give you a good indication.  

     To avoid too many figures then, let’s just assume that your 7.7 volts reading is half discharged. Then it should be good for 16 minutes. Then juggling about with what I said before, it seems to me you are getting really good performance on 50 watts/lb. Does the ‘twin factor’ apply here? And perhaps smaller models perform better generally, I’ve not had any experience of these, but they certainly seem to sometimes have some very small batteries! If 7.7V is less than half discharged, it gets even better.
    As Clive said, one way of finding the level in the tank is to measure what you put back in. Or more accurately, continue to discharge the pack down to 6V. Recharging is slightly misleading, you always have to put a little bit more in than you get out. There’s never something for nothing. For any calculations you do, you are assuming your pack is a 1800mAh lump, you have to measure it to be absolutely sure.
    Looks as though this is an Extremely Long Range, (ELR), Mossie………

    Looks good, any more developments would be interesting.     PB      

[Yorkman]

Peter

first pack just came off the balance charger with 8.27v showing-so perhaps I'm not starting with as full a pack as I thought...

I don't generally use the 'fast' charger that gives figures for time'amps etc, so not able to check that.

What I used to do with nicads, was run the model at full chat and time until the power went, and use that as my timer setting for the first few flights, after landing seeing how much was 'left in the tank' by running once again to BEC switching, and adjusting flight times accordingly.

I don't do that with lipos as I'm frit of discharging too far-read too many horror stories! Hence I started with 8 minutes, and checked volyage levels.

I wasn't for one moment serious about being able to fly for 40 minutes, but will extend my flight times to 10 minutes next time, and see what we have then. When it stops raining....

PS-was thinking of doing her in PR colours-some marks of which I believe did carry extra bomb bay fuel tanks....

[Peter Beeney]

    Hello Yorkman
            It certainly looks good from here.    To get a proper handle on those voltages you really need to check them out with a sub-standard meter. That’s a meter that’s been calibrated against a standard instrument. Unfortunately, that can get a bit expensive, so your average Pound-Puller DMM is probably sub, sub… … sub standard. Having said that, they can be quite accurate, voltage monitoring is very easy in today’s electronic world. They can also be diabolically dangerous.
    One way of getting a real fix on your flying time could be like this. I’m assuming that your fast charger is lipo charge and discharge enabled. If so, then charge the pack as you normally would and then discharge at 0.2C, the five hour rate, or 360mA. Or thereabouts. Then you know exactly what the capacity is. Let’s say it’s exactly 1800mAhs’  Recharge and fly for 10 minutes, then discharge the remainder of the pack at around 3C, say 5 - 6A, that’s what you sort of seem to be consuming now. So, now we’ve got some figures. Let’s say the discharge showed 600 mAh remaining. Then your flight used 1800 - 600 = 1200mAh. Divide 1200 by 10, = 120 milli-amps per minute. 600 divided by 120, = 5, you could have had a further 5 minutes flying time, down to the alarm bell. 
    Forget the model, it’s beginning to looks as though you need to make sure the low volt alarm on the tranny can still cut the mustard!
    But, in one hit, we can see that here you would definitely know you would be able to have a good ten minute flight and still have enough gas to comfortably go round again, twice or more if needs be.

    One of my hero’s is AVM Sir Wilfrid Freeman, who together with Geoffrey de Havilland, was responsible for producing the Mosquito in the face of a great deal of opposition. In the early days it was known as Freeman’s Folly! But it soon became the Wooden Wonder!
    There are many good Mosquito books, like Martin Bowman’s ‘Mosquito Menacing the Reich’. The ballistic performance is appropriate too, for eighteen months after it’s inception I believe it was the fastest plane in the sky.

    Happy Flying.         PB 
    

[Yorkman]

well, went for a couple of sorties this morning-9 minutes on the 2250, 7.82volts remaining, and 10 minutes on the 1850-7.69 left. According to Timbo's chart, that's 80% and 60%, so-if I ever get the damn thing to fly properly, which is another story-20 minute flight times look on the cards.

I will keep boring you all as I discover more!