



OK folks, I just completed the updated spreadsheet. I think I added in all the features that everyone has mentioned. While I was on a role, I decided to get resistor tolerance in the picture (in a very crude way, but it works fine for us). I focused a few hours of my night on putting all the equations involved on paper, so I sure hope I don't have any errors.
Please download and try it out; let me know what you guys think. I'd like to hear everyone's questions and comments so I can polish it to a shine before Junsi links it to post#1. Cheers, Kev [edit: There are actually 4 factors that constrain the D+ process: 1) max power rating for charger cannot be exceeded, 2) charger cannot output negative voltage, 3) current cannot exceed max rating, and 4) resistors cannot exceed max power rating. This spreadsheet accounts for everything, and I added notes so anyone who is interested can learn the math involved in this. Whew!!! I'm going to bed.] {edit edit: I moved the spreadsheet file to my blog so you won't accidentally download an older version. You can find it in my blog. } 



Melbourne, Australia
Joined May 2006
6,407 Posts

Why not optimise for zero discharge in the charger ?
I think there is a confounding factor here which is that you are assuming the goal is to dissipate maximum power inside the charger. IMO the optimal solution dissipates minimal power inside the charger and all the power in the external load. This maximises the possible discharge power while placing minimum stress on the charger electronics. In this optimal mode the charger FETs are essentially fully on and the charger simply acts to control the current and monitor the voltage. The only requirement is to not exceed the maximum discharge current (7A for 106B+/10101B+, 20A for the 208B).
Since 100% of the power is dissipated in the external load, calculating the resistance is as easy as R = V/I where V is the initial pack voltage and I is the desired discharge current. I believe this is the guidance given in the manual. Now it becomes much easier to calculate the minimum external resistance for any voltage pack and target discharge current by assuming a fully charged pack. A resistor larger than the calculated minimum will be safe and simply mean you get less than the target discharge current. A resistor smaller than the minimum will force the charger to start limiting the current and dissipating power internally, which is suboptimal but the charger will always protect itself from overload. 



Quote:
BTW, are you sure the charger will limit the current in D+ mode so it won't fry itself? Speedflyer's experience has me wondering whether there is protection against using too small of a resistance. Even if it did protect itself, it's nice to know exactly what's happening. For example, I have a single resistor setup that will safely discharge 3s from 15A (all I need anyways). Not by chance, but by design. If I optimized for Pc=0 for one current, I could be in trouble at other currents in the desired range. Plus I'd hate to spend money and put time in to an array only to find out my current range is limited by a poor choice of resistors. Anyhow, I'm going to bed. Kev 




I must be honest and say I haven't the first idea what all the calculations and spreadsheets are about but it sounds like you're doing good work so well done!
The 208B is an exciting addition to the range, I wonder when it will hit the shops. What other additions might we see in future? I would like to see a 2012B with the onboard logging, higher power and ability to balance 12 cells. 


Melbourne, Australia
Joined May 2006
6,407 Posts

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I am cycling a few new lipos right now to wake them up. I have the discharge set at 7amps but it is only showing 1.9 amps on the dissplay. When I push start to get the discharge current to flash (edit mode) it shows I have it set for 7 amps. Why wont it go to 7 amps?



So. Cal.
Joined Oct 2004
8,780 Posts

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Excellent work on the resistor calc. sheet Kev! Very exciting stuff. Thanks for your work on this  I'll be ordering some resistors soon.
 David 



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Now that you state that, it makes sense after rethinking it. 1.6 x 12 = 20 watts or so. So the only way to get to 7 amps discharge is to use the discharge + that everyone is talking about? 




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Thanks for pointing out the combo box problem. The problem is now fixed; I lost the data verification for those drop down boxes while I was moving things back and forth across sheets. Cell references for both data verification and conditional formatting become orphaned once copied to a new sheet; good 'ole excel. David, thanks, and you're welcome! Mera 'din, you're right! This is why we're all hankering over this D+ stuff. Regular discharge is very limiting, and D+ is a very exciting feature. With discharge reduce activated, it sure beats the heck out of soldering dedicated relay circuits like I used to do. Kev [edit: BTW, I noticed the spreadsheet was not discarding the irrelevant imaginary roots for the minimum current limit, so in some cases the calcs were undefined. The minimum safe current will now show the correct value instead of "#NUM" for these cases. The sig fig setting for resistance input on the analysis page was too low as well. These problems are fixed as of this edit. ] 




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Thanks BTW. Kev 



Melbourne, Australia
Joined May 2006
6,407 Posts

Getting better Some more suggestions.
* The graph axes have fixed limits that are tied to expected values for 106B+. Ideally the limits would be adjusted when you choose a different charger or perhaps set for automatic scaling (I think this would OK on both graphs work if the plot range only went up to 20A instead of 40A). * Draw horizontal red lines on the graphs showing the maximum allowable value (must be sensitive to the current charger selection) for internal charger power and load current respectively. 



How does this look?
I added all the possible data lines I could think of, and combined the 2 charts on to one. I wouldn't be surprised if someone says it looks more confusing. I can't figure an easy way to make the chart automatically scale the axis in a reasonable way without adding a bunch of extra conditional items to the data table itself (I'm thinking a bunch of "if" functions that set any out of range/invalid data point to 0). I may get around to this if it's a big enough deal for enough ppl, but it would take me a while. Now that I'm in to aesthetics, I'm saving each previous version separate from the updated one, since backward changes are likely. Let me know if there's any lines you'd rather do without. Kev [edit: The resistor comparison page is left the same, since adding similar detail to it's charts would make the white background barely visible.] 



Kev,
I downloaded the .zip above in post 3211, but it doesn't appear to be the updated one like the screenshot above. Is it possible to remove all old versions, and post the updated single graph version or point me towards it? The comments in the excel file are very handy! Thanks, David 



Done! It's posted in my blog, free for everyone to use.
Sorry about the confusion; I was getting out of control with early releases and you're idea of linking to it is great! Thanks BTW, I knew those comments would save me from a bunch of explaining on the threads. Kev 
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