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Old 02-01-2012, 10:02 AM
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Default A123 20Ah pack configuration question

I need some advice from the pros .

More detail on my project can be found here: http://www.diyelectriccar.com/forums...ion-68311.html

I've purchased 500 of the A123s (32kW) and want some advice on how I should configure them. My plan is in two phases.
Phase 1: uses one WarP 9 with the transmission. I'm looking into either a Z2k-HV or EHV. I haven't decided yet. HV saves me some upfront costs.
Phase 2: adds a TWarP 9 for a dual motor setup and removes the transmission. I'll also upgrade the drive train at this time.

I was thinking of having a modular design where I would only use about 60-70% of the cells and then have an extension pack that I could hook up to extend the range to +25 miles.
Here's where it starts to get complicated. I don't mind redoing the pack once I get to phase 2, so a completely different arrangement could be configured for each phase if needed.

To start things off, this is my first proposal:
A modules of 70s4p for 224V so that if pack sags, I'll still have >170V for the motor. I could then combine in parallel an additional module of 70s2p or 70s3p for a total pack of 70s6p or 70s7p given I only have 500 of these cells.

I've heard some say, go for the highest voltage your controller allows. I've heard others say, that connecting modules together in parallel isn't a good idea and that it would be better to have the extension pack in series that adds voltage using the same number in parallel.

What are your opinions on how to configure the pack and would it be different for Phase 1 versus phase 2 ?

Thanks in advance for everyone's input. I'm learning so much about this field everyday. I have a lot of appreciation for the members of this forum.
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  #2  
Old 02-01-2012, 10:25 AM
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Default Re: A123 20Ah pack configuration question

Use all cells or nothing. Having a range extending pack really complicates things, especially if they're not the same SOC. If you parallel them for range extension, the pack with lower SOC will cause an arc from the higher SOC pack. If you do things in series, you have to modify all of your controller parameters. If you use a BMS, it complicates things as well. If you use some sort of SOC meter, it can really complicate things.

Why wouldn't you do the large battery pack up front? Why redo it second phase and mix old with new?
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Old 02-01-2012, 10:26 AM
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Default Re: A123 20Ah pack configuration question

Quote:
Originally Posted by kakheath View Post
I need some advice from the pros .

More detail on my project can be found here: http://www.diyelectriccar.com/forums...ion-68311.html

I've purchased 500 of the A123s (32kW) and want some advice on how I should configure them. My plan is in two phases.
Phase 1: uses one WarP 9 with the transmission. I'm looking into either a Z2k-HV or EHV. I haven't decided yet. HV saves me some upfront costs.
Phase 2: adds a TWarP 9 for a dual motor setup and removes the transmission. I'll also upgrade the drive train at this time.

I was thinking of having a modular design where I would only use about 60-70% of the cells and then have an extension pack that I could hook up to extend the range to +25 miles.
Here's where it starts to get complicated. I don't mind redoing the pack once I get to phase 2, so a completely different arrangement could be configured for each phase if needed.

To start things off, this is my first proposal:
A modules of 70s4p for 224V so that if pack sags, I'll still have >170V for the motor. I could then combine in parallel an additional module of 70s2p or 70s3p for a total pack of 70s6p or 70s7p given I only have 500 of these cells.

I've heard some say, go for the highest voltage your controller allows. I've heard others say, that connecting modules together in parallel isn't a good idea and that it would be better to have the extension pack in series that adds voltage using the same number in parallel.

What are your opinions on how to configure the pack and would it be different for Phase 1 versus phase 2 ?

Thanks in advance for everyone's input. I'm learning so much about this field everyday. I have a lot of appreciation for the members of this forum.
when you go with the dual motor set up are you putting the motors in series or parallel or doing series/parallel switching?

These A123's don't sag as much as prismatics so having a really high voltage pack doesn't make as big a difference.

My logic could be wrong but I would guess that putting the extra range pack in series with the normal pack would allow you to keep things in better balanced. Putting another string in parallel should be fine in theory, but you wouldn't have the parallel cell connections between the two strings, they would be just tied at the +/- extremes.

Another thing to consider (which is a big task for 500 cells) but I'm in the process of measuring the IR and capacity of every one of my cells (my pack will only be 288 of the 20ah A123's) that way I can group them into perfect parallel sets, in my case 3 parallel that total the same value for every single grouping. That way the series groups should all behave exactly the same, mine will be top balanced, but since they are all the same capacity they should behave like they are bottom balanced as well. So far I have been able to group them to within 0.01Ah.
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Old 02-01-2012, 01:59 PM
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Default Re: A123 20Ah pack configuration question

Quote:
Originally Posted by rwaudio View Post
when you go with the dual motor set up are you putting the motors in series or parallel or doing series/parallel switching?

These A123's don't sag as much as prismatics so having a really high voltage pack doesn't make as big a difference.
I've been talking with John Metric from DC Plasma and his calculations show that if you have a high enough pack voltage (like 333V) the series/parallel switching isn't beneficial due to the electromagnetic field emf. So if I string enough together I could avoid all the additional contactors etc.

Quote:
Originally Posted by rwaudio View Post
Another thing to consider (which is a big task for 500 cells) but I'm in the process of measuring the IR and capacity of every one of my cells (my pack will only be 288 of the 20ah A123's) that way I can group them into perfect parallel sets, in my case 3 parallel that total the same value for every single grouping. That way the series groups should all behave exactly the same, mine will be top balanced, but since they are all the same capacity they should behave like they are bottom balanced as well. So far I have been able to group them to within 0.01Ah.
I'm very interested in learning more about this. How long does it take to test the capacity of each cell? What equipment do you use? Perhaps you can guide me on how to do it with a PL6.
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Old 02-01-2012, 07:04 PM
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Default Re: A123 20Ah pack configuration question

Quote:
Originally Posted by kakheath View Post
I've been talking with John Metric from DC Plasma and his calculations show that if you have a high enough pack voltage (like 333V) the series/parallel switching isn't beneficial due to the electromagnetic field emf. So if I string enough together I could avoid all the additional contactors etc.


I'm very interested in learning more about this. How long does it take to test the capacity of each cell? What equipment do you use? Perhaps you can guide me on how to do it with a PL6.
Well since I'm using a PL6 to do it I'm sure I can help. There isn't much to it, I'm using a 4S3P pack of A123's to power the charger so that I can use regenerative discharge (up to 40A, I'm using 38A as a rough 2C discharge) then charge again at 38A (2C). I do have to recharge the pack powering the charger once in awhile but it works very well. I'm using the PC interface cable and the set up works great, everything is controlled from the software on the computer, the cells arrive around 50% SOC so I run "1 cycle" it's an option on the PL6, and basically it charges the cell, then discharges the cell and charges it again. You can choose to end on discharge if you like, if you wanted a bottom balanced pack. If you did the bottom balance method it would take about 15mins to top up the cell and 30 to discharge it, that's 45 mins per cell. I do the charge/discharge/charge so it's around an hour and 15 mins. I have a 2nd PL6 on order so that I can do two at a time (they connect together so it's all controlled through the single PC interface)

Below is the measured capacity of 76 cells.
17.76
17.78
17.82
17.84
17.89
17.95
17.99
18.06
18.08
18.11
18.13
18.15
18.25
18.26
18.27
18.28
18.33
18.33
18.34
18.34
18.35
18.36
18.39
18.40
18.41
18.41
18.44
18.44
18.47
18.49
18.50
18.50
18.51
18.51
18.51
18.52
18.54
18.55
18.55
18.56
18.56
18.56
18.56
18.57
18.59
18.59
18.60
18.61
18.61
18.62
18.62
18.62
18.63
18.64
18.65
18.67
18.69
18.69
18.70
18.70
18.71
18.72
18.73
18.73
18.75
18.77
18.78
18.79
18.81
18.81
18.83
18.84
18.88
18.90
18.92
18.93

Below are the groupings that I created based on this information (72 cells, 4 went back in the pool for grouping the next batch).
All groups total either 55.45Ah or 55.46Ah

A 18.50 18.55 18.41 55.46
B 18.81 18.56 18.08 55.45
C 17.95 18.79 18.72 55.46
D 18.06 18.69 18.70 55.45
E 18.60 18.39 18.47 55.46
F 18.11 18.51 18.83 55.45
G 18.13 18.51 18.81 55.45
H 18.26 18.52 18.67 55.45
I 18.25 18.49 18.71 55.45
J 18.62 18.84 17.99 55.45
K 18.27 18.55 18.63 55.45
L 18.28 18.56 18.61 55.45
M 18.33 18.56 18.56 55.45
N 18.62 18.69 18.15 55.46
O 18.34 18.33 18.78 55.45
P 18.34 18.57 18.54 55.45
Q 18.35 18.59 18.51 55.45
R 18.36 18.59 18.50 55.45
S 18.44 18.62 18.40 55.46
T 18.41 18.61 18.44 55.46
U 17.84 18.73 18.88 55.45
V 17.82 18.73 18.90 55.45
W 17.78 18.75 18.92 55.45
X 17.76 18.77 18.93 55.46

The goal is that all cell groups behave exactly the same.
I didn't know this would actually work until fairly deep into the testing, so I've been topping up all the cells. I may switch to a charge/discharge cycle since the cells should behave basically the same regardless if they are top or bottom balanced. (this would save 30 mins per cell) Or at least allow me to bulk charge the cells in groups once assembled.
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Old 02-01-2012, 07:43 PM
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Default Re: A123 20Ah pack configuration question

just curious rwaudio, are you using the discharge or the charge capacity? Or are they the same? I'm doing similar cycling with CALB 130ah cells with my PL6 and I'm seeing about 2 more ah going in than come out on a cycle.

Granted my cells are a lot larger (and take more like 8 hours/cell to cycle) and 2ah on my cells is more like 0.3ah on yours, but was just wondering which number is considered the capacity of the cell.

I'd also be really curious to see if a single cell was cycled three times, how close are the three capacity values. In other words, how repeatable are the results with the PL6? I'm certainly not going to try and find out since it takes me 8 hours to cycle a single cell.
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Old 02-01-2012, 08:16 PM
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Default Re: A123 20Ah pack configuration question

Quote:
Originally Posted by dladd View Post
just curious rwaudio, are you using the discharge or the charge capacity? Or are they the same? I'm doing similar cycling with CALB 130ah cells with my PL6 and I'm seeing about 2 more ah going in than come out on a cycle.

Granted my cells are a lot larger (and take more like 8 hours/cell to cycle) and 2ah on my cells is more like 0.3ah on yours, but was just wondering which number is considered the capacity of the cell.

I'd also be really curious to see if a single cell was cycled three times, how close are the three capacity values. In other words, how repeatable are the results with the PL6? I'm certainly not going to try and find out since it takes me 8 hours to cycle a single cell.
I'm using the discharge cycle as the capacity (at 2C in my case which is somewhat close to or just above what the car should average), and you are close, I put in about 0.2Ah more than I get out. I did have a higher discrepancy 0.4 to 0.5Ah using the stock banana's and wires at 40A, I have since switched to a better banana and shorter 8awg cables. So a good portion of the loss was in heat, there is still some heat in the cables and connector so I believe actual losses in the cell to be very low. I have done some repetitive testing on a couple of cells and the results were within 0.01Ah (This isn't based on a very big sample though, one cell 3 times, and another cell twice). My testing is in a room with fairly constant temperature (basement with no outside doors that could be opened, and no sources of heat other than the furnace).
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Old 02-01-2012, 08:49 PM
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Default Re: A123 20Ah pack configuration question

Quote:
Originally Posted by rwaudio View Post
Well since I'm using a PL6 to do it I'm sure I can help. There isn't much to it, I'm using a 4S3P pack of A123's to power the charger so that I can use regenerative discharge (up to 40A, I'm using 38A as a rough 2C discharge) then charge again at 38A (2C). I do have to recharge the pack powering the charger once in awhile but it works very well. I'm using the PC interface cable and the set up works great, everything is controlled from the software on the computer, the cells arrive around 50% SOC so I run "1 cycle" it's an option on the PL6, and basically it charges the cell, then discharges the cell and charges it again. You can choose to end on discharge if you like, if you wanted a bottom balanced pack. If you did the bottom balance method it would take about 15mins to top up the cell and 30 to discharge it, that's 45 mins per cell. I do the charge/discharge/charge so it's around an hour and 15 mins. I have a 2nd PL6 on order so that I can do two at a time (they connect together so it's all controlled through the single PC interface)

The goal is that all cell groups behave exactly the same.
I didn't know this would actually work until fairly deep into the testing, so I've been topping up all the cells. I may switch to a charge/discharge cycle since the cells should behave basically the same regardless if they are top or bottom balanced. (this would save 30 mins per cell) Or at least allow me to bulk charge the cells in groups once assembled.
Since 500 would take forever, I definitely would leave them discharged. What voltage are you using for the cut offs? 3.65 & 2.0? Does the PL6 control this automatically or do you have to be present to stop it? How many PL6s can be connected together and controlled by the computer? I figure if I buy two more PL6s and end on discharge, maybe I could be done with them in about 3 weeks if I talked my wife into helping out with the swaps. I may regret that decision later though.
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Old 02-01-2012, 09:48 PM
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Default Re: A123 20Ah pack configuration question

Quote:
Originally Posted by kakheath View Post
Since 500 would take forever, I definitely would leave them discharged. What voltage are you using for the cut offs? 3.65 & 2.0? Does the PL6 control this automatically or do you have to be present to stop it? How many PL6s can be connected together and controlled by the computer? I figure if I buy two more PL6s and end on discharge, maybe I could be done with them in about 3 weeks if I talked my wife into helping out with the swaps. I may regret that decision later though.
I'm charging to 3.65 at 38A that terminates at 0.9A and discharging to 2.25v with a 38A constant current.
The PL6 will do everything automatically. You can actually hook up to 16 PL6's together and control them all from one PC interface.

If you bought a few extra PL6's I'm sure you could sell them here or on the RC market for a reasonable loss that might be better worth your time.

I have a 2nd one coming for that purpose.

It's a good charger for the price, especially based on the software interface and data collection that you can do. I was going to save the log file for each cell as the cells serial number but never got around to bringing home a bar code scanner from work. That might be handy for the obsessive data collector though.
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