# Matched IR, in series or parallel



## RIPPERTON (Jan 26, 2010)

All my new Enerland 4350 LiPO's have their Internal resistance written on a spec sheet. Most of them range from 0.009 to 0.012.
should I group them in series or parallel.
Im guessing in series ?


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## EVfun (Mar 14, 2010)

?? I'm a bit confused. You will need to put cells in series to reach the voltage you need. In series the voltage adds up but the amp hours remain the same as each single cell. In parallel the voltage stays the same but the amp hours add up.


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## major (Apr 4, 2008)

RIPPERTON said:


> All my new Enerland 4350 LiPO's have their Internal resistance written on a spec sheet. Most of them range from 0.009 to 0.012.
> should I group them in series or parallel.
> Im guessing in series ?


Hi RIP,

I assume you are talking about how to sort and group cells by the spec sheet internal resistance. My opinion would be to keep the parallel groups as close as possible in the cells' resistance. That way they will current share more equally. 

Series connected cells (or groups of paralleled cells) would not be sensitive to differences in resistance since the current will be equal by virtue of the series connection. So they should charge and discharge equally. 

If you intend to use the battery hard, you might try to keep cells having higher resistance on the outer edge of the pack as they would heat more.

Even though the resistance values look small, there is actually a plus/minus 14% spread. 

Did you purchase individual cells or multicell packs?

Regards,

major


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## Tesseract (Sep 27, 2008)

I think, EVFun, he is asking how the cells should be grouped according to internal resistance. 

Ideally, you want the equivalent resistance of each "cell" in series to be identical. So you parallel cells with various internal resistance figures to result in the same overall resistance as every other bank of cells that are paralleled together to make up a series string (ie - a battery).

The spread of 9 milliohms to 12 milliohms is pretty wide - but very typical of these Chinese cells, regardless of chemistry or manufacturer. Parallel cells so that each group has the same mean internal resistance as all of the cells considered together. Ie - if you have 10 cells at 0.009 ohms, 20 at 0.01 ohms and 10 at 0.012 ohms then the average (mean) resistance of all 40 cells is 0.01025 ohms. Aim to make each paralleled cell group have that resistance and you'll have the best possible balance w/r/t voltage drop under load for each "cell".

An additional consideration is to group by capacity, but that's sorta making things even more complicated. Might want to use a computer to sort that mess out. Not even gonna try it here.


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## major (Apr 4, 2008)

Tesseract said:


> Ideally, you want the equivalent resistance of each "cell" in series to be identical. So you parallel cells with various internal resistance figures to result in the same overall resistance as every other bank of cells that are paralleled together to make up a series string (ie - a battery).


Hi Tess,

You know I hate to disagree with you  And Woodsmith says we should not resort to insults, namecalling and threats. So let's look at a parallel set of cells. Take 4 for example. Use a simple model for the cell of an ideal capacitor and series resistance. Draw the circuit of the 4 cells connected. Use the same C value for each, but different resistance values.

When you put a load on the parallel group, each capacitor will see a different current depending on the individual resistor. The voltage on each capacitor will decrease as charge is removed. Once the load is removed from the parallel group, the voltage on the 4 capacitors are different. So, current will continue to flow within the group of 4 parallel cells until all 4 reach equilibrium. 

This is called charge redistribution. It is undesirable because it is current flow within the system which does no useful work for you. Worst case; it can cause excessive heating. Lithium cells have high Columbic efficiency, so there will be little loss of capacity, just the energy lost in resistive heating.

And I guess I don't see a good reason why series connected cells need to be R matched. There you should match capacity.

Regards,

maj


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## Tesseract (Sep 27, 2008)

major said:


> You know I hate to disagree with you  And Woodsmith says we should not resort to insults, namecalling and threats.


Who are trying to kid, you crusty curmudgeon! You love to disagree with me!?





major said:


> This is called charge redistribution. It is undesirable because it is current flow within the system which does no useful work for you...


Yep, I do agree, but I saw this as the lesser of two evils at really high currents (relative the the cell's capacity) because if a paralleled group consisted of all high internal resistance cells it might trip LVC prematurely. I was keeping in mind what RIPPERTON here would likely be using the cells for - namely, in racing motorcycles. Of course, you have some practical experience in this matter whereas mine is entirely fabricated, so...


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## RIPPERTON (Jan 26, 2010)

pack will be 32s 12p. 52ah 128v 6.6 kWh
4800 peak Amps which is irrelevant because that will never be used
controller can draw 500A cont which is 41Amps per cell
the pack will be ventilated with 1mm air gap between each cell
be used over 20 minute stint to complete 30km at race pace (90kmh av ?)
counting on burning 200Wh/km
theres going to be some very aggressive regen going on too

Just did a cell count out of the 450 cells I bought (need 384 for the pack)
57 cells are 0.009 ohms
234 cells are 0.010 (52%)
71 cells are 0.011
14 cells are 0.012
makes 376. theres a few 0.008's that I will throw in

there were a few freaky ones with 0.004 and up as high as 0.034 which
were sold to a friend for destruction testing.

I also thought to place matched cells in series to minimise disparity during charging and discharging. theoretically all cells in series would charge equally but balance out in parallel


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## RIPPERTON (Jan 26, 2010)

corrections
missed 1 sheet
264 cells are 0.010 = 59%
63 are 0.009
77 are 0.011
making 404 usable cells
I can chuck the 0.012's and make my entire pack out of 9's 10's and 11's


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## tomofreno (Mar 3, 2009)

If the cells are used for racing, so high discharge currents, I wonder how relevant the Rs measurements are, since the effective resistance at higher currents will be very different due to concentration polarization, as described by the Warberg factor, according to the CM video. In that case it seems what would matter is how much the W factors differ between cells, which you would have to determine yourself by discharging them at high rate. The Warberg factor appears to be the "effective" series resistance at high discharge rate, and is much larger than Rs, resulting in much more cell heating, and larger drop in cell voltage.


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## BMI/LiFeTech (Aug 12, 2009)

Daniel, I am surprised if your Enerland cells have an IR as high as 9-12 milliohms. For racing you should get considerably higher discharge power from your LiFeTech X1P cells since they all should be under 3 milliohms. After you ordered your LiFeTech X1P Power cells the factory started labelling the IR of the cells on the the top left corner of each cell tray as part of the QC cell grading process. This makes it much easier to match cells for packs used for racing applications. I would be suurprised if any of your cells have an IR of higher than 3 milliohms since all the the LiFeTech X1P cells are all typically in the range of 2.5-3.0 milliohms.


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## DavidDymaxion (Dec 1, 2008)

Major, I don't have any empirical data for this, so not disagreeing, but I don't understand why that would be the case. It seems end batteries would lack warm neighbors on one side, and would have a longer battery cable to draw away heat. Why would end cells run hotter despite more ability to shed heat?


major said:


> ... If you intend to use the battery hard, you might try to keep cells having higher resistance on the outer edge of the pack as they would heat more. ...


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## major (Apr 4, 2008)

DavidDymaxion said:


> Major, I don't have any empirical data for this, so not disagreeing, but I don't understand why that would be the case. It seems end batteries would lack warm neighbors on one side, and would have a longer battery cable to draw away heat. Why would end cells run hotter despite more ability to shed heat?
> 
> 
> major said:
> ...


Hi David,

Maybe I didn't state that clearly, or you didn't read it clearly, or I don't read your post clearly  Anyway....

One would assume all the cells in the battery discharge at the same current. Therefore cells which have a higher resistance would generate more internal heat than cells with lower R, right? So what I was saying, it is smart to put those high R cells which would get hotter on the edge of the battery pack. Cells on the edge would be able to shed heat better than cells in the middle, right?

I think it is common to see cells in the middle of densely packed batteries get hotter during heavy current discharges. That has been my experience.

And David,

Do you agree about matching internal resistance on the parallel sets? 

Regards,

major

ps...

{On those Kokam cells, for some reason the most positive terminal on the battery gets the hottest. The rest of that most positive cell seemed about the same temperature as the other cells and the cells (edges which could be measured with IR gun) in the middle ran a few degrees hotter than the end cells. But for some reason, that positive terminal ran significantly hotter, on multiple tests, on multiple batteries. Anybody care to guess why?}


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## Tesseract (Sep 27, 2008)

major said:


> And David,
> 
> Do you agree about matching internal resistance on the parallel sets?


Yeah, I'd like to know this too, since I think major is full of crap... 




major said:


> ...But for some reason, that positive terminal ran significantly hotter, on multiple tests, on multiple batteries. Anybody care to guess why?}


The terminal probably fans out internally and so you are seeing increased heating because of current crowding... that's my guess, anyway... Keep in mind I've never even seen one of these cells, so...


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## GerhardRP (Nov 17, 2009)

major said:


> ps...
> 
> {On those Kokam cells, for some reason the most positive terminal on the battery gets the hottest. The rest of that most positive cell seemed about the same temperature as the other cells and the cells (edges which could be measured with IR gun) in the middle ran a few degrees hotter than the end cells. But for some reason, that positive terminal ran significantly hotter, on multiple tests, on multiple batteries. Anybody care to guess why?}


I guess that things that are electrically isolated from each other are also thermally isolated. That means that the higher temperature at the positive terminal indicates that whatever process is responsible for the internal resistance of the battery is active at the positive plate. For the manufacturer, it means if you want to reduce resistance, look at the positive plate structure and reactions.
Gerhard


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## major (Apr 4, 2008)

major said:


> {On those Kokam cells, for some reason the most positive terminal on the battery gets the hottest. The rest of that most positive cell seemed about the same temperature as the other cells and the cells (edges which could be measured with IR gun) in the middle ran a few degrees hotter than the end cells. But for some reason, that positive terminal ran significantly hotter, on multiple tests, on multiple batteries. Anybody care to guess why?}


It is just the positive terminal on cell #14 which is at the positive end of the 14 cell pack. All cells are identical having a positive and negative foil type terminal. Except for the cells on the ends, the foil tabs are compressed together with the adjacent cell tabs with aluminum bars. The same aluminum bars are used on both ends, just compressing a single cell tab with a cable lug screwed to it. And from memory, it is like 20 or 30 degrees hotter than anywhere else on the battery. I have some notes about it in the lab I'll look for. Here is a photo of the battery.


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## DavidDymaxion (Dec 1, 2008)

major said:


> Hi David,
> 
> Maybe I didn't state that clearly, or you didn't read it clearly, or I don't read your post clearly  Anyway... ... I think it is common to see cells in the middle of densely packed batteries get hotter during heavy current discharges. That has been my experience.


Got it, no confusion now. Mr. Literal here read "keep cells having higher resistance on the outer edge of the pack as they would heat more" as cells on the outer edge would heat more than in the middle, and that was desirable for some reason. Here's a clarifying restatement for the Literal Readers like me in the crowd: "keep cells having higher resistance on the outer edge of the pack since they heat more."



major said:


> And David,
> 
> Do you agree about matching internal resistance on the parallel sets?


I have not tested this myself, but my intuition is it is better to match resistances for the paralleled cells. Here's a roundabout way to show it: A local EVer mixed cells of different capacities, I forget the exact capacities but it was in a 2:1 ratio (like 50 and 100 Ahr). He figured the 50 Ahr cells would have twice the resistance, and so would supply 1/2 the current of the 100 Ahr cells. He figured the cells would reach empty at the same rate. He was right, it worked for him. His daily commute almost completely emptied the cells each day, so it wasn't a case of shallow discharges. BTW his drive home was a long uphill with a big elevation gain.

Running with this, suppose he had cells of 2:1 internal resistance but the same capacity. It seems to me one cell would reach empty twice as fast as the other cell, greatly stressing it.


major said:


> {On those Kokam cells, for some reason the most positive terminal on the battery gets the hottest. The rest of that most positive cell seemed about the same temperature as the other cells and the cells (edges which could be measured with IR gun) in the middle ran a few degrees hotter than the end cells. But for some reason, that positive terminal ran significantly hotter, on multiple tests, on multiple batteries. Anybody care to guess why?}


I'd guess it's a thermocouple effect. You have current going from metal A to metal B for the + terminal, but from metal B to metal A for the - terminal -- for different metals generally one way is hotter than the other.


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## major (Apr 4, 2008)

DavidDymaxion said:


> I'd guess it's a thermocouple effect. You have current going from metal A to metal B for the + terminal, but from metal B to metal A for the - terminal -- for different metals generally one way is hotter than the other.


Interesting theory David. The Li cells have one aluminum and one copper terminal. Can't remember which is which, positive negative. But between the end cell terminals and the copper cable lugs I have an aluminum clamp bar. Hopefully I'll get back into testing this month. I think I will make a copper bar clamp to replace the aluminum one on the positive end and see if it makes a difference.

Is there such a thing as a copper aluminum thermocouple 



DavidDymaxion said:


> Got it, no confusion now. Mr. Literal here read....


as? since? I know what I mean when I type it. Sorry, but all you have to do is ask. If I confuse you, chances are others are confused also. I'd rather be questioned than leave something misinterpreted 

major


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## peggus (Feb 18, 2008)

DavidDymaxion said:


> ...I'd guess it's a thermocouple effect. You have current going from metal A to metal B for the + terminal, but from metal B to metal A for the - terminal -- for different metals generally one way is hotter than the other.



That's an interesting theory, I could be wrong but I think the negative terminal is a nickel coated copper foil and the positive is aluminum foil so wouldn't thermocouple effect be most pronounced at the negative terminal? It is easy to test the thermocouple theory though, does the effect reverse when charging?

Other theory, the aluminum to aluminum connection at the positive terminal is higher resistance then copper+nickel to aluminum connection at every other connection point?


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## RIPPERTON (Jan 26, 2010)

Major where did you get the 9 pin plugs
I just bought 4x 8cell log from Hobby King Hong Kong
and they didnt come with any plugs.


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## RIPPERTON (Jan 26, 2010)

> For racing you should get considerably higher discharge power from your LiFeTech X1P cells since they all should be under 3 milliohms


Maybe so Armin but power isnt what I want
I want Power to weight ratio.
I can go 12 parallel with the LiPo's but only 4 parallel with the LifeTechs
and still end up with a 20kg lighter pack.
Plus I wouldnt contemplate for a second to not have air cooled LiPo's
so higher running temp isnt a prob.

The LifeTechs will make a good commuter battery....one day


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## major (Apr 4, 2008)

RIPPERTON said:


> Major where did you get the 9 pin plugs
> I just bought 4x 8cell log from Hobby King Hong Kong
> and they didnt come with any plugs.


Hi RIPPER,

http://www.diyelectriccar.com/forum...etractor-44430p2.html?&highlight=lipo+tractor 

The part numbers are on one of these threads, from DigiKey. If you can't find them, post back and I'll copy them down from the bags.

major


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## major (Apr 4, 2008)

peggus said:


> It is easy to test the thermocouple theory though, does the effect reverse when charging?


These were the 40Ah cells. The hot positive terminal was noticeable above 3C discharge, IIRC. At 8C for a couple minutes, you couldn't touch it  But I can only charge at 16A (0.4C) so I didn't pay any attention to it.


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## RIPPERTON (Jan 26, 2010)

Heres some

http://cgi.ebay.com/8S-JST-XH-Conne...Control_Parts_Accessories&hash=item1e5d95bb3c

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=270470253072&ssPageName=STRK:MEWAX:IT


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## LithiumaniacsEVRacing (Oct 9, 2010)

Your packs come pre-built in series correct? What size wires on the packs? and what is your BMS arrangement.


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## RIPPERTON (Jan 26, 2010)

LithiumaniacsEVRacing said:


> Your packs come pre-built in series correct? What size wires on the packs? and what is your BMS arrangement.


single cells
Im building my own box.
voltage monitoring on discharge only with peizo low voltage alarm.
bulk charging with a single charger to 90% unbalanced
then using RC charger on each cell block (4.2v) to 100% soc.
eventually want 32 individual 4.2v 50A chargers.


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## toddshotrods (Feb 10, 2009)

RIPPERTON said:


> single cells
> Im building my own box.
> voltage monitoring on discharge only with peizo low voltage alarm.
> bulk charging with a single charger to 90% unbalanced
> ...


Nice.  Are you monitoring LV on cell blocks or individual cells - assuming on a block.


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## LithiumaniacsEVRacing (Oct 9, 2010)

Hmmmn! Okay. I am having a custom BMS system built to use the pre-built battery blocks (Turnigy), we are also testing the wires and may have to upgrade to larger, not sure yet. I am building my box in chambers for each pack with a quick disconnect during racing, each chamber will have holes for vacuum. If we have a problem we can just unplug the battery block and replace with a good one.

I have thought of building special battery boxes (like WZ) to hold many different types of pouch packs. That would be a separate project on the side. My future plans might be to build battery boxes that fit certain style Lipo pouch pack for DIY builders.

So, if you are building your own box, you are purchasing the cells separately? 



RIPPERTON said:


> single cells
> Im building my own box.
> voltage monitoring on discharge only with peizo low voltage alarm.
> bulk charging with a single charger to 90% unbalanced
> ...


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## RIPPERTON (Jan 26, 2010)

toddshotrods said:


> Nice.  Are you monitoring LV on cell blocks or individual cells - assuming on a block.


Yep blocks
12 cells in parallel monitored by one volt meter.
they balance out so quickly in parallel.
Now and then you would have to check
for a dud cell that was letting its block down.

The cells Ive bought (4.350Ah) were old stock and they only have another 300 in stock in Korea.
from here on I will be getting the 4.55Ah cell which is exactly the same size.
hope they are cheap too. the 5Ah cell is $15


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## Jimdear2 (Oct 12, 2008)

RIPPERTON said:


> single cells
> Im building my own box.
> voltage monitoring on discharge only with peizo low voltage alarm.
> bulk charging with a single charger to 90% unbalanced
> ...


Ripperton,

A quick question, did you buy the Cell Log 8M that Hobby King Has on their site now for $14.28 and on sale for $13.26? Did yours come without the plug and cable?

The picture on the site show a plug and cable with the unit. I just bought 7 of them. I hope I don't have to put another $35.00 out for something that Hobby King infers comes with the product.

An aside, It appears that we are thinking similar things with our monitoring and charging plans. 

To describe our setup for our tractor. We are going to be using one isolated on board DC to DC converters board for each 3 cell paralleled pack (3 Headway 38120Ps per pack). My strategy is to never charge the battery above about 90%. The little I'll be loosing isn't worth possible damage plus the pack should last longer. The DC to DC I'll be using can be set to about 3.3 to 3.65 volts with a trim resistor. They are very tolerant on the input voltage accepting between 36 to 72 volts and can pass up to 25 amps to the cells. What we plan is one DC to DC board to each buddy pack with input power in parallel to all of the boards. With this set up we can NEVER overcharge a cell. I've seen 5 volt boards with a -20% and +10% trim adjstment. that would get you from 4.0 to 5.5 volts. I don't remember hoy mant amps that board would pass. I looked at bunches of Dc to DC.

Another plus we see is the boards are fairly common and inexpensive. Mine were$ $4.80 each with a volume discount. 

For charging we can use a suitable DC variable voltage/amprage power supply, a dump pack, a battery charger or even a transformer a diode and a cap for a down and dirty. With that range of input and the built in filtering, it leave us with a lot of options. 

For monitoring we will use the Cell Logs low voltage setting to either set off a buzzer/light or even a trip to open the main contactor if a cell set drops too low. During charging we will be able to see if we have a low or slow pack by monitoring the cell logs. Do you know if the cell logs will trip an alarm for both high and low when they reach the set point?

We may even have an option of bulk charging, then balancing with the boards. I have to see if the board has protection, I'm not real good with electronics so research and understanding is slow. Of course with the amprage we can pump through the boards using a 48 volt dump pack or high amprage power supply, why spend $1000.00 on a high end high amp charger.

Be well,
Jim


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## RIPPERTON (Jan 26, 2010)

Jim
I bought the non usb monitor


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## Jimdear2 (Oct 12, 2008)

RIPPERTON said:


> Jim
> I bought the non usb monitor


So did I 

Here is the link, if they are comming without cables it sure looks like a mis-representation to me. 

http://www.hobbyking.com/hobbycity/store/uh_viewItem.asp?idProduct=10952

If mine come with out cables I think I might go to eBay. Want to come as well? PM me if you do. At 5 to 15 bucks a cable OUCH

Jim


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## RIPPERTON (Jan 26, 2010)

I may have too much time on my hands


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## Jimdear2 (Oct 12, 2008)

RIPPERTON said:


> I may have to much time on my hands


Rip,

Nice drawing, what do use. 

Nice placement of old Nick.

I have to get back to that again, but all of my softwear was for win 98. All I have left is the required Adobe Illustrator from my freelance tech illustration line drawing I did before I retired. I loved corel draw and hate adobe.


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## EVfun (Mar 14, 2010)

RIPPERTON said:


> I may have to much time on my hands


Let me correct that for you. 

_I have to much time on my hands._


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## Jozzer (Mar 29, 2009)

Grammar police.
"I have TOO much time on my hands"


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## Woodsmith (Jun 5, 2008)

Alternatively:
"One has too much time on one's hands."

Or:
" Damn! I need to get out more!"


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## GerhardRP (Nov 17, 2009)

Woodsmith said:


> Alternatively:
> "One has too much time on one's hands."
> 
> Or:
> " Damn! I need to get out more!"


I second that, I to have too much time on my two hands.. my 2c


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## Jimdear2 (Oct 12, 2008)

Hey . . . 

All you guys with too much time on your hands.

I'm 67 and getting near the short end of the stick, How about lending/giving me some of the "Too Much Time"?

You all have fun in the New Year, I know I will,
Jim


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## RIPPERTON (Jan 26, 2010)

All of the above

Autodesk Inventor 2008
sometimes doesnt portrait the scale or size of what your are designing
As soon as I put the modelled cell logs in I got a better idea of just how small my battery is.
The cell logs are quite small but take up a lot of the top area of the box


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