# Battery Box Design



## ninjanick (Oct 11, 2014)

I'm trying to cram 96 CALB CA 100aH cells into my frame and I need to stack battery boxes on top of each other. I ran some rough thermal calcs and normal driving conditions the temperatures a get a little hot. Using forced air seems to bring temperatures down to a more liveable range, but I'm having issues designing the stacked battery configuration and have air travel from the top to the bottom without severely diminishing flow to the bottom.

Below is a preliminary design. Cell spacing is ~0.1" between each cell, but minimal to the side/wall of the box. There are holes on the bottom of the top box to allow air to flow through, but flow analysis indicates that batteries are the main blockage (duh!). The frame is tight and I have about 3-4" on the top, but pretty much 0 on the remaining 5 sides. Looking for suggestions. Thanks in advance!

https://flic.kr/p/qpNby6


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## ninjanick (Oct 11, 2014)

That was a really old version ... this one is the latest design.

https://flic.kr/p/rGa4ms


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## RIPPERTON (Jan 26, 2010)

Nick is this a 320v system ?
ie how many cells are in series / parallel


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## ninjanick (Oct 11, 2014)

Yes high voltage to max out a Soliton1. So 96S1P pack.


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## dougingraham (Jul 26, 2011)

You should not see significant heating of those cells. If you are running something like a WarP 9 or Warp 11 you will limit the motor voltage to 170 volts which means the battery current will be about 500 to 600 amps at most and at that for only a few seconds at a time. I only have 54 cells of 100AH and so my batteries do see 1000 amps for several seconds during hard acceleration. My battery boxes are insulated and even on hot summer days the temperature of the cells in the battery boxes is never excessive. I think you are worrying about something that is not a problem.


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## dougingraham (Jul 26, 2011)

Additional info. Those cells are rated for 3C continuous. This means 300 amps at 307 volts (nominal) gives 92kw. You would probably need to be traveling down the highway at speeds in excess of 100 mph to maintain that and your batteries would be exhausted in 20 minutes. In a typical car if you are averaging 60mph you are using less than 20kw. This would be less than 65 amps from the battery or about 0.65C. You will have motor heating problems before you have battery heating problems.


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## ninjanick (Oct 11, 2014)

I'll be using a WARP11HV for my setup. I originally calculated 400A continuous current. Now I'm not sure where that number comes from, but I think it came from 150hp. Using 20kw as nominal continuous current definitely reduces the thermal load calcs. Do you data log battery pack temperatures? Since I'll have 10 individual battery locations, I was thinking at minimum log temperature?


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## RIPPERTON (Jan 26, 2010)

What kind of vehicle is this Nick, you said frame so that's a motorcycle but that's to much battery for a bike. Using the gearbox ?
photos ?


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## ninjanick (Oct 11, 2014)

Sorry for lack of up front information, I'm in the beginning phases of an FFR 818 build. 96 cells would definitely be way too much volume for a motorcycle. I'm using the 5MT Subie transmission linked to a WARP11HV.


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## dougingraham (Jul 26, 2011)

ninjanick said:


> I'll be using a WARP11HV for my setup. I originally calculated 400A continuous current. Now I'm not sure where that number comes from, but I think it came from 150hp. Using 20kw as nominal continuous current definitely reduces the thermal load calcs. Do you data log battery pack temperatures? Since I'll have 10 individual battery locations, I was thinking at minimum log temperature?


No I don't log pack temperatures. The batteries don't get hot enough to make this a concern. If you keep the average discharge current below 3C you should be fine. If you are doing 300 amps at 288 volts this is 86.4 kw into the motor (116 HP) and would probably be 98 HP at the wheels. At that power level the 818 could probably do 120mph until the motor overheats.

This will certainly be an exciting EV. You should be able to get a little over 300 HP to the wheels with that setup. Keep in mind that the WarP 11HV is probably only good for around 45 HP continuous. This would be around 100 battery amps which is an hour of operating time. Unless you are on the track this is going to be in the felony speed range.

Good luck!


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## TooQik (May 4, 2013)

I like the FFR 818, have been following it since they had the competition for the body design. I had planned on using one for my first EV build but the lack of a hard top put me off.

Back to the subject at hand, rather than designing air flow from top to bottom, do you have enough space to duct air across the top of each battery box instead, so you're directly cooling the terminals?


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## ninjanick (Oct 11, 2014)

TooQik said:


> I like the FFR 818, have been following it since they had the competition for the body design. I had planned on using one for my first EV build but the lack of a hard top put me off.


There's actually talk regarding an 818C.



TooQik said:


> Back to the subject at hand, rather than designing air flow from top to bottom, do you have enough space to duct air across the top of each battery box instead, so you're directly cooling the terminals?


Would cooling the terminals via forced air actually cool enough to make a difference? I could try and duct air across the terminals as that's easier to solve than ducting air from top to bottom of a stacked configuration.


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## dougingraham (Jul 26, 2011)

I must not have been clear. Unless you are planning on discharging your LiFe batteries in less than 1/2 hour hearing is not an issue. The batteries are rated to be discharged at a 3C rate which is 20 minutes and they do not get hot enough to be a problem at that discharge rate. If you plan to discharge faster than that you are using the wrong kind of batteries.

If you are planning on a less than 30 minute discharge then blowing air over the terminals would probably be as effective as anything except for a water cooling jacket because the terminals are directly connected to the anode and cathode foils.

What is your goal for this EV? 20 minutes on the track followed by many hours of recharge time is probably not what you were hoping for. And yes these batteries can be recharged in 20 minutes but with the pack size you are proposing (30.7kwh) you are talking about a charger that can do 92 kw. The only charging standard that can do this is the Tesla supercharge network and Tesla isn't letting anyone else charge from it. Chademo would take more than 50 minutes. From a dryer outlet it would take at least 4.3 hours to recharge from flat.

Best Wishes!


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## ninjanick (Oct 11, 2014)

dougingraham said:


> I must not have been clear. Unless you are planning on discharging your LiFe batteries in less than 1/2 hour hearing is not an issue. The batteries are rated to be discharged at a 3C rate which is 20 minutes and they do not get hot enough to be a problem at that discharge rate. If you plan to discharge faster than that you are using the wrong kind of batteries.
> 
> If you are planning on a less than 30 minute discharge then blowing air over the terminals would probably be as effective as anything except for a water cooling jacket because the terminals are directly connected to the anode and cathode foils.


You were clear. I was asking more about forced air over the terminals for my own edification.



dougingraham said:


> What is your goal for this EV? 20 minutes on the track followed by many hours of recharge time is probably not what you were hoping for. And yes these batteries can be recharged in 20 minutes but with the pack size you are proposing (30.7kwh) you are talking about a charger that can do 92 kw. The only charging standard that can do this is the Tesla supercharge network and Tesla isn't letting anyone else charge from it. Chademo would take more than 50 minutes. From a dryer outlet it would take at least 4.3 hours to recharge from flat.


I plan to use the Emotorwork ISO 20kw charger which should be able to charge the pack in a few hours. My end goal is a fun commuter and weekend getaway vehicle. My commute is no more than 20 minutes one way in city surroundings (no highway/freeway). I can't imagine discharging the entire pack in less than 20 minutes. I was hoping for at least a spirited 75 mile range.


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## TooQik (May 4, 2013)

ninjanick said:


> There's actually talk regarding an 818C.


Thanks for that.

Unfortunately it's a little too late for my current conversion (ended up choosing a 1976 Toyota Celica RA23) but I'm definitely keen to build one down the track for my next EV. 

Will keep an eye on its progress.


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## dtbaker (Jan 5, 2008)

ninjanick said:


> That was a really old version ... this one is the latest design.



the larger problem with stacking cells on top is that it is a real bitch when you are doing your initial balancing, and or any major manual balancing, if you can't get to each cell with thick enough wires to add/drain current at 10-20amps.

you might wanna get a little more creative with cell placement to have them 1 row high spread under more of the car so that you can get to all cells with removal of a cover.


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## ninjanick (Oct 11, 2014)

Initial balance is most likely going to be separate cells on the bench. I plan to use a Battery Monitoring system by "borrowing" a demo board design from Linear Tech. It has an ISOSPI bus which I will probably interface with an Arduino or possibly a tablet. The demo board will be mounted to the outside of the box and I can use 16-18AWG conductors for a possible balance/measurement point.


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## dtbaker (Jan 5, 2008)

ninjanick said:


> Initial balance is most likely going to be separate cells on the bench. I plan to use a Battery Monitoring system by "borrowing" a demo board design from Linear Tech. It has an ISOSPI bus which I will probably interface with an Arduino or possibly a tablet. The demo board will be mounted to the outside of the box and I can use 16-18AWG conductors for a possible balance/measurement point.



initial top balance on a bench is ok to start.... but in my experience, a final balance after they are all in series and charged with your series charger is a must to get down to +/- .02 at the end-of-charge.

whether you use a BMS, or not, that final balance is critical.


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## ninjanick (Oct 11, 2014)

I was actually going to bottom balance on the bench. The LinerTech demo board has the ability to balance and can be remotely activated through the SPI bus. I was going to attempt this method first.


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## ninjanick (Oct 11, 2014)

Responded with the same question in a really old thread, but figure this one is probably better/more relevant:

I tired to fit 94-96 CALB CA 100aH cells, but couldn't without putting 135lbs of batteries in front of the front axles which I don't like. Battery locations and boxes close if I go with the CA 60aH cells, but it also closes with the CALB CAM cells. I haven't read much on these cells other than to insulate each cell from each other and to chassis.


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## Hollie Maea (Dec 9, 2009)

ninjanick said:


> Responded with the same question in a really old thread, but figure this one is probably better/more relevant:
> 
> I tired to fit 94-96 CALB CA 100aH cells, but couldn't without putting 135lbs of batteries in front of the front axles which I don't like. Battery locations and boxes close if I go with the CA 60aH cells, but it also closes with the CALB CAM cells. I haven't read much on these cells other than to insulate each cell from each other and to chassis.


The CAM series cells would probably be the cheapest and easiest way to arrive at your goals. There aren't a huge number of people who have used them before, so there's a bit more of a guinea pig effect than with the CA cells, but they seem to be decent cells and the volume advantage is significant. The only main problem I have heard is the voltage isolation issue, which is easily remedied.

Probably the biggest unknown you would have to look into is the power capabilities. They are presumably somewhat less than the CA series, but I don't know how much. Hopefully Jack has done some testing on them. I wouldn't imagine that he would be selling them without testing them himself. And the C ratings that people throw around here for the CA series cells are based of his testing.

Obviously, if you go to more exotic options, there are a few different things you can do, including a small but powerful LiPo pack, or a large capacity but compact pack made of 18650 power cells. A123 pouches, if you could find them, would be another way to do a LiFePO4 power pack that is more compact than the CA series cells, but it would be a lot more work than CAM cells, and finding genuine A123s can be tricky.

If you have a big budget, you could consider Enerdels. They are fairly energy dense, very capable in terms of power, and of the highest quality. I think Major might have some for sale.

Looking forward to that build thread...


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## ninjanick (Oct 11, 2014)

I've been tracking the build in another forum. The CALB CA 60aH seems to have a higher max discharge rate of 10C whereas the CALB CAM is rated at 8C. Budget isn't too high so I was trading the CALB cells with Headway cells. Packaging is a challenge, but Headway does offer more flexibility with shape. Complexity increases as does price.


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## Hollie Maea (Dec 9, 2009)

ninjanick said:


> I've been tracking the build in another forum. The CALB CA 60aH seems to have a higher max discharge rate of 10C whereas the CALB CAM is rated at 8C. Budget isn't too high so I was trading the CALB cells with Headway cells. Packaging is a challenge, but Headway does offer more flexibility with shape. Complexity increases as does price.


Well, of course C rating is pretty much meaningless without a corresponding energy density. So even though the CA series has higher C rating, it had such a lower energy density that the power density also ends up being lower.

Doing the math: we will look at volumetric density, since that's your constraint. And we will just look at current, assuming the voltage sag is about the same.

CALB CA 60Ah: (60*10) / (24.8*11.5*4.1) = .51 A/cm^3
CALB CAM 80Ah: (80*8) / (12.7*17.3*4.7) = .62 A/cm^3

Headways have great C ratings, but their energy density is so low that they might not be the slam dunk they appear to be at first glance.


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## ninjanick (Oct 11, 2014)

The CALB CAM 80aH packaging closes if I stack cells on top of each other which is a minor complication (not a huge deal), but it looks like volumetric density is actually better for the CALB CAM 72aH and the 40152 Headway cells.

CALB CAM 72aH: (72*8)/(21.8*3*13.5) = 0.65 A/cm^3
Headway 40152: (15*15)/(4*4*15.2) = 0.92 A/cm^3 (peak discharge 15C)

I like the 40152 cells because I have this odd trapazoid shaped area behind the seats where the gas tank is supposed to go. I anticipate I can fit close to 180 back there and package another 168 near the motor. That leaves 36 for the front which isn't too hard to package. Theoretically could give me a peak discharge of 900A (battery) which calculations indicate I'll only need 720A (battery). 96S4P configuration.

Any advice is much appreciated ... I keep going back and forth, but it looks like I can utilize more available volume with the 40152s which keeps any weight in front of the front axles to a minimum and places most of the weight behind the seats which is fairly center of the vehicle.


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## dougingraham (Jul 26, 2011)

ninjanick said:


> CALB CAM 72aH: (72*8)/(21.8*3*13.5) = 0.65 A/cm^3
> Headway 40152: (15*15)/(4*4*15.2) = 0.92 A/cm^3 (peak discharge 15C)


Headway using your rectangular shape has a volume of 243 cubic cm.
CAM 72 has a volume of 883 cubic cm. But the energy density for the CAM 72 is 0.26 wh per cubic cm while the Headways are 0.19 wh per cubic cm. That is a pretty heavy hit on volumetric energy density.

Looking at my battery comparison spreadsheet Battery Comparison you see that the three important characteristics of a battery are calculated. Those are WH/$ WH/kg and WH/li.

Headway 2.02	100.00	251.30
CAM 72 1.61	121.26	265.09
CA 60 2.16	96.00 164.20

The actual Headway WH/li is less because I compute using actual volume which you cant use because you cant stack them tightly. My chart should show 197.4 instead instead of 251.3. In all cases the higher number is better in that category. For you it sounds like the volumetric number is the most important. The CAM 72 is 1.3 times more dense than the Headways. The CAM 72's from a mass comparison are 1.21 times better than the Headways. The Headways are less expensive than the CAM 72's but the CA 60's are even better on a cost basis. Your mass number is going to be even worse with the Headways because you have to parallel four of them together and you will need to support them as well. You will find this takes up quite a lot of space as well. Mock up a four cell headway 60AH module and compare that to the CAM 72 or CA 60.

Best Wishes!


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## ninjanick (Oct 11, 2014)

If the CALB CAs (real world) can dip into the 10C range, do you think the CALB FI CAMs can do the same? Same overall technology, just different packaging?


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## dougingraham (Jul 26, 2011)

ninjanick said:


> If the CALB CAs (real world) can dip into the 10C range, do you think the CALB FI CAMs can do the same? Same overall technology, just different packaging?


I would not push these cells beyond the manufacturers specs if you expect long life. Can they do it? Probably a few times. Are you going to damage the cells? Maybe.

What you are worried about is not being able to get enough power out of the cells. Battery current is less than or equal to motor current. Take an example of a 340 volt battery driving a 1000 amp motor with a max voltage of 150. Motor power at the peak power point is going to be 150000 watts. The battery if it sagged to 300 volts would still see only 150000 watts but the current would be a maximum of 500 amps. 150000/300 = 500. So in this case because the battery voltage is twice the motor voltage the current the battery sees is half of what the motor sees. If you had 300 volts of 72 AH cells they would in this situation only see a max of 500 amps while the motor sees 1000 amps. And 500 amps is only 6.9C. And what makes it even better is that motor power is a product of voltage and current. And you only see full motor power at the max power point. Below that the current might be 1000 amps which gives you full torque but at low RPM the voltage is low. With a motor like the WarP9 you would only see full power when the motor RPM reaches somewhere between 3000 and 4000 rpm. So until you get to that the battery doesn't even see 500 amps. It is a very brief interval during acceleration that the battery sees max current.

For the most part if you limit the battery current you will only notice it when you get near the peak motor power level. The stall torque will still be at full motor current up to the point where power/battery voltage would exceed the battery current you have configured. (Neglecting controller losses).


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## Hollie Maea (Dec 9, 2009)

ninjanick said:


> Same overall technology, just different packaging?


I'm pretty sure there is more than just different packaging between the CA and the CAM series. I don't think the packaging alone can account for all of the increase in density. It's possible that one method they use is to increase the proportion of active components over current carrying components. That would presumably serve to decrease the power capabilities of the cells.

Perhaps someone knows more about the specifics of the CAM cells? There seems to be not too much information out there about them.


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## ninjanick (Oct 11, 2014)

I'm using a WARP11HV as my motor of choice so at roughly 4000RPM, the motor would draw ~700A (battery). Will I live in that range for a long time? Most likely not, but occasionally during spirited driving. I could definitely set the battery current limit on the Soliton to 600A, but that would reduce output to 165kW. Maybe it's just an adjustment in driving habits? Torque characteristic is obviously different and I feel like running data all the way up to 5500RPM, but it doesn't need to. The only reason I do is because "normal" driving has a gradual increase in RPM through the gears.

I've been looking for some performance data on the CALB CAM 72aH cells, but not many are using these new cells. The only important piece of information I can find is that the housings need to be separated. Either by use of the supplied (at least one user had these) mounting brackets or using an insulative material like G10, FR4, or Polycarbonate in between each cell and box (if conductive).


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## ninjanick (Oct 11, 2014)

Walter Mitch said:


> Helo guys!. bit confused about sensitivity issue of battery box. Would it be a well being issue to have batteries in the battery box with the its wiring.


Are you asking if there will be issues with wiring the batteries inside the box vs assembling them outside and dropping them into the box fully assembled?


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