Stephen,
Wow... This is the difference when dealing with an individual/small company! Response speed!
Can't believe you've already updated your website with a full set of graphs... smashing!
That's just what I was looking for. I have four of these packs, so it's nice to know that I can reuse them in emergencies....
I notice, you said that the voltages involved with the rear light are less than those with the front light, but I see on your data that the "penalties" for running a 2S pack are the same as for the front light, i.e. 3S1P ≈ 2S2P.
Still, it's really nice to see that the run times with s 2S2P are entirely practical and run for longer than the Magicshine 816 (1.8h at - rarely used- full power) which is what I'm currently running with.
On a different note, how difficult is the reprogramming, say, were I to swap between an 3S and a 2S pack in order to set protection switch off voltage?
I know that the maxflex has some user programmable aspects, but accessing all the function menus though a one button interface sounds about as much fun as punching out programme cards with my bare fingers
Also, could you explain to us a little more about the cell balancing aspect of the new build? That sounds like a really high-end feature that no $300 dollar light currently boasts. That would really put your new build "in the spotlight" so to speak...
As always, keep us posted. I'm really enjoying hearing your progress.
Craig.
Thanks Craig.
Let me see if I can clarify on few of those points.
First off on the 11.1V vs. 7.4V issue. Yes, there is a small decrease in controller efficiency as the input voltage drops. This, in and of itself however, is not a large contributor to the total power consumption, but since it's all manifested in the controller board itself, then from the controller board's perspective, it can make a big difference. From the studies that have been done on the Maxflex, the general rule of thumb is to avoid exceeding 2 watts in the controller unless you can provide a method of heat-sinking for the power inductor. In a package like this, it's virtually impossible to do that, so the only recourse is to just hold the power down.
The "penalty" aspect of dropping the input voltage comes from the fact that this is a constant power device. As you drop the input voltage, more and more current is required to maintain the same power level. And as current goes up, so must the battery capacity to maintain the same run time. It just so happens to work out that the 2800mAH 11.1V and 4400mAH 7.4V are nearly equal with regard to power capacity. In fact, just look at these numbers. They could be considered to be "power" capacity. We're just not used to seeing a battery described this way.
2.8*11.1 = 31.08 Amp-Hour-Volts
4.4*7.4 = 32.56 Amp-Hour-Volts
Now we need to penalize the 7.4V pack by about 5% (the difference in the controller efficiency) and we have:
32.56 * 0.95 = 30.93
Which is practically identical to the 11.1V pack.
Concerning programming... I must say, when I first starting working with the Maxflex, it was a bit daunting to get my head around all the options. However, once you distill it down to just the few functions that are required for this particular application, it really is pretty easy. I plan on making some tutorial videos just to help folks get past the learning curve, so they can quickly be able to perform the operation exactly as you describe. Say for example you find that your 11.1V pack is dead and you want to use your 7.4V pack. If you don't re-program the cutoff limits, then as soon as you try to use the 7.4V, the light will think it's dealing with a seriously depleted battery and go into shutdown.
So, you'd then break out your cheat sheet with just a few lines of instructions telling you which menu to select. You'd then enter the new cutoff voltage by tapping out the number for each digit XY.Z. So for a 6V cutoff, for example, you'd be entering 06.0 in a process like this:
no-click = 0, followed by a PRESS to enter,
6 clicks, followed by a PRESS to enter,
no-click = 0, followed by a PRESS to enter.
In this case, a video is worth a thousand words. If it was something you did often, then you'd easily be able to change between each voltage standard in a matter of minutes.
Now, on the flip side, if you were programmed for 7.4V operation and needed to use a known good 11.1V pack for a while, you could do it straight away without the need to reprogram.
Concerning balancing... I know that some packs say that they perform this function, but I'd certainly not assume that all packs do. If you look at any of the lithium polymer packs typically used in RC-airplane applications, they are literally bare cells. They have no hard metal case, and they have no on-board circuit protection (except for the speed controller), so when you charge a battery like this, you really NEED to use a balancing charger. This means that each cell will be monitored during the charging process and the charger's "brain" will compare each and make adjustments (discharging some if necessary) to insure that each cell is at the same voltage. This prevents any individual cell from becoming over charged or over discharged over time, which is not only a safety factor, but will also prolong the life of the pack. All of the 11.1V packs that I've spec'ed out, have to pass a quality test before they get to me. And I run each one through a full charge and load test before letting them go.
As an aside...
I posted a few more
user-pics over on the new web site today if you're interested.
Thanks for all the great questions!