Rudy,
Can't actually shed any light, but the FMA 4s charger has the same feature. It nominally seems to be ok--at least I tend to get the same % left after most flights--depending a little on the wind-  more wind=more left in the battery.

My assumption is that the % is based on the voltage and maybe the internal resistance of the battery. The reason I say this is that I watched Peter Mazur charge his 4s3200 pack after a carrier flight. On my packs a ~15V starting voltage is claimed to be 20% capacity, but his voltage indicated another much lower value (9%).

Obviously these automatic chargers do some interesting math to figure out both this quantity and how many amps equals a 1C charge for an arbitrary pack. I am guessing the charge rate is based on how much resistance the charger detects---a high resistance would indicate a smaller pack.

Also note that as time goes on the capacity of these packs decreases--which I am guessing actually means the internal resistance goes up, so this fuel gauge should be giving the % of the current pack capacity, not the  value as sold. It would be interesting to know what they are doing, but I am guessing that these are proprietary calculations. In this knock-off era, telling people how they do it would simple spawn a host of me-too copy catters.

I have the FMA 4s - my observation (and from what I deduce, based on the various "White Papers" that FMA put out) is that the software bases a lot of its decisions on the slope of the voltage/time line.  The initial "fuel remaining" value is based on very little information, therefore and is not really very accurate.  If you sit and watch the charge rate and the "fuel" figures, it may start at 10% but in the first few minutes it will jump to, say 28%, then back down to 20%.

Also, the 1.0C and 1.4C charge rates are similarly figured out - it starts at a charge rate of perhaps 1/2A and if the voltage is going up too slowly, it will ramp up the charge rate.  In fact I have seen mine go from 1/2 A to 3 A, then back down to 1.5 amp then up to 1.75.  All the changes are at fairly regular intervals (one minute, if I remember) suggesting that the software is collecting data and making decisions based on that data.  It amazes me that it does eventually settle on the closest value - of the programmed in rates - and finishes off the charge at that rate.

I would be surprised if the 10s has much different in its software.

Mike A

....

Your post is a reminder to us all that our fuel cells and their chargers are amazing pieces of chemistry/electronics. Like you pointed out, they do not charge in a linear way. I think it is part science, and part magic! 

Regards, 
 

   ... That's why you don't want to let the magic smoke out ...     

  Oh - and one of the FMA white papers says that they have determined that charging regularly at 1.4C (as opposed to the widely circulated 1.0C) will NOT harm the cells - that is why the FMA's have the automatic 1.4C programmed in.  That is what I mostly use and I have had no charging issues nor any bad cells due to charge rate or discharge rate nor any packs with depressed capacity.  Everything that I (or we, being my regular flying buddies) use has been going strong all summer with most packs getting at least two flights per week.  The only cells we have managed to damage in this, our first full year of LiPo use, were all damaged physically - by crashing or by "metallic invasion" of the plastic envelope and shorting out the internal plates.

  Mike @

This isn't quite as exhaustive (or exhausting!) as Dean proposes, but if you look at the orange trace (the power in watts) of one of my Nobler flights, the "average" value of the power during the level laps is about 260 watts (I'm looking at the first 4 laps after takeoff. The average power for that entire run was 284 watts (see the value given in the legend near the bottom of the plot) peaking at 425 Watts (looks like when the plane was released, but this is almost what I am seeing as a peak in the wingovers for example). But you can see the maneuvers are bringing the average up to only 284 watts from the level flight's 260 Watts. By the way, this data was from last May. I tend to be flying a bit slower these days with level flight powers in the 240 watt range.

I guess what I am getting at is that in CL Stunt, we are burning a lot if not most of our battery during the "boring" 2 laps between maneuvers. That's why I have been trying to optimize the power use during that period. I think a good prop is the way to go, since optimizing level laps tends to argue for a slower flight speed (for a given airframe). However our maneuvers depend a lot of the initial momentum going into the maneuver plus whatever help you can get from the prop as the airspeed drops. So you don't want level lap speeds too slow.

I think another point is to have an airframe that likes to penetrate. One reason I chose the Nobler (other than I had one available!) was that the airfoil looks "efficient" to me since it is thinner than most airfoils you see on modern stunters with their behemouth glow engines. These huge fat airfoils of modern stunt planes just look really draggy to me. Of course it doesn't matter with a 91 glow engine! However I am no real expert in aerodynamics. Also keeping the weight down also will minimize the need for large angle of attacks to generate the needed lift in those corner maneuvers, so some of the potential disadvantages of a narrower airfoil can be minimized.

In my new Vector ARC, I have left the leading edge as is, which is definitely pointier than the kit leading edge. We will see!