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#### Skywalker

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Hello,

I’m a member of the Formula Student Electric Germany team at the technical university of Hamburg.
In August 2012 we want to race our car against other teams. Our concept is almost finished, except for the battery and charging.

Since your community has been of so much help for us before with finding suitable motors, I’d now like to ask for some advice on batterys. Could you have a look on my rough calculations below and correct me if necessary? I personally have only started to research on batterys last night...

We will use an EMRAX motor for each rear wheel and probably the BAMOCAR-D3 Controller from UniTek.
The Motors are designed for up to 300 Volts. We think that we should use the maximum allowable Voltage for the best performance, since higher voltage means less current and less warming-up at same power. So 300V would be the output voltage of the controller, so the battery voltage should be a little bit above that, is that right? If the max. current for the controller is 250A and the max. dissipation is 2600W, then the voltage drop over the controller will be approx. 10V, so would 310 Volts be the optimum Voltage? Does this calculation make sense?

We would like to use LiPo cells, so for 310 Volts we would need 84 cells in series, 3,7 volts each. We need approx. 8 kwh, so each cell or group of cells connected in parallel would need 100Wh or 27Ah. Considering that we may never draw more than 100kw from the batterys, at 310 Volts the peak current could be 100000/310=322 Amps. So we would need to discharge with up to 12 C.

In the Formula Student rules it says the following:
--------------------------------------------------------------------

“Each accumulator must be monitored by a battery management system whenever
the tractive system is active or the accumulator is connected to a charger.
The BMS must continuously measure the cell voltage of every cell in order to keep
the cells inside the allowed minimum and maximum cell voltage bound stated in the
cell data sheet.
The BMS must continuously measure the temperatures of critical points of the
accumulator to keep the cells below the allowed maximum cell temperature bound
stated in the cell data sheet.
The temperature of at least 35% of the cells has to be monitored by the BMS, if the
used accumulator cells are not intrinsically safe, which has to be proven by
corresponding documentation in the ESF. The monitored cells have to be equally
distributed over the accumulator container(s).
The BMS must be capable of shutting down the tractive system, if critical values are
detected.
FSE recommends to monitor every cell voltage and every cell temperature.”
-------------------------------------------------------------------

Could anyone recommend suitable cells? Should we try to avoid connecting any cells in parallel, not only in order to minimize the amount of voltage and temperature sensors, but also because it is a problem to “balance” them?
Is it common to charge a battery pack with the same circuit as you discharge it, or would it be practical, to separate the pack for charging into smaller units? We will probably have 2-4 smaller packs rather than one big one anyway and we want to charge them outside the car.

Lukas

#### MalcolmB

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Hi Lukas

When choosing your battery pack voltage you might also want to consider the the voltage sag of the pack under load. The voltage that the cells can deliver will decrease under heavy current draw, so your total pack voltage will drop. The amount of voltage sag will depend on the the cell you choose, so to allow for this you will need cell discharge curves from the manufacturer.

Considering the safety requirements and the recommendation for individual monitoring, I would suggest contacting Kokam or EIG battery and asking which cells they would recommend for your application

Splitting the pack up into four sub-packs of around 75V would make it easier and safer to handle. You could also charge each of these sub-packs with their own 75V charger, which may be cheaper and easier to find than a single 300V charger capable of charging the full pack.

Parallel groups of cells balance naturally, so it would not be a problem using smaller cells grouped in parallel to provide your required 27 Ah. But it would of course mean more connections between cells, and possibly the need for more temperature sensors and BMS connections. I believe that Kokam and EIG both supply single cells of 20 Ah and 40 Ah capacity. A123 also produces 20 Ah cells.

I should say that I don't have personal experience of building such a high-performance pack, so these are purely the suggestions of an amateur

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#### idarusskie

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What is the latest information on ultracapacitors mixed with batteries?

#### red187rhino

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You are not going to want to run anywhere near 300V. If the controller has a maximum DC link voltage of 300V you will want to be substantially lower than that assuming you are going to regen. If you arent going to regen then shooting around 275V or so is a reasonable area to be at.

If you are regenerating your nominal voltage is going to be a function of what the internal resistance of your cells will be and what current you will be using them to. Your nominal voltage should be Vmax = Vnom +Ri*I. The internal resistance is very dynamic based on temperature and SOC so you cant use a nominal relaxed RT internal resistance. You will want to take near your worst case and use that. I would imagine you are going to be in the 250V range. Try to use a single string of higher capacity cells in series because it will be cheaper and easier to design around as well as increase cell life and simply BMS cost and wiring. Higher capacity cells are also inherently lower resistance and since the internal resistance is a function of SOC it is good to use higher cells.

I probably wouldnt use LiPo cells if it were me but a lot of people like them.

I would like at the A123 20Ah pouch cells. Gaia makes a few options. EVBtech makes an 32Ah prismatic. There are quite a few options in LiFePO4 that would work well. Especially since you want to use them at those high rates.

#### ecat

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Hi
Just registered, but if this may help, you may want to check LTS Marine, they have developped a ski nautique boat. They certainly have battery knowledge.
I have to agree with the fact that 300 volt is attractive but way to dangerous.
It is costly also, better with packs of lower voltage. You probably heard the way to get cheaper aa batteries.
Emrax is also in my interests, Plettenberg RC planes motors too.
I am designing a hovercraft e / gas hybrid.
If i may suggest, design your gismo with easy interchangeable batteries, that might set you apart from others in the practicality of things for quick charge reset.
cheers and good luck
Mike

#### Skywalker1991

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Hello, sorry for not posting in this thread for so long, but I had lost my login and since we have a dedicated group for the battery in our team, to which I do not belong, I did not think about the batteries so much.

Thank you for all the helpful tips on the batteries. The A123 20Ah Pouch cells look really good, since we anyway decided to switch to LiFePo.
And they are also not that expensive actually. http://shop.lipopower.de/A123-Systems-AMP20M1HD-A-20Ah-Folienzelle, just 50€ each.

We really would like to stick to the tip
Try to use a single string of higher capacity cells in series because it will be cheaper and easier to design around as well as increase cell life and simply BMS cost and wiring.
But since we don't want to go with less than 8kwh, this would mean that we need 121 cells, which is 400V nominal or 435V maximum, although our motor only needs less than 200V RMS. on the phases.

When I asked at the manufacturer of our controllers, UNITEK I was told that this voltage would be too high, since we would not be able to use all the energy if we use such a high voltage. But I don't understand this.
The input voltage of the controller is 400 or 700 Volts depending on the exact model. By PWM they make a sinusoidal at the outputs. I thought that when the voltage is higher, then the "on-pulses" of the PWM are a little bit shorter for the same output and therefore it should draw less current from the battery for the same output, if the battery has a higher voltage. So why can't we use all of the energy?

If it was possible to use this voltage, we would be very interested to do so, because the a123 pouch cells are the most lightweight solution in LiFePo we have found and at the same time very affordable. For example the a123 cylindrical cells with 2.3 Ah would be more than 10kg more, a few thousand euros more and a lot more mess with 1000+ cells...
Or does anyone happen to know a similar pouch cell but with a little bit more capacity, like 24 Ah?

Can anyone explain me if or why my thoughts to justify the high voltage are incorrect? If you need more information I will answer it immediately this time.

Regards,
Lukas

You are not going to want to run anywhere near 300V. If the controller has a maximum DC link voltage of 300V you will want to be substantially lower than that assuming you are going to regen. If you arent going to regen then shooting around 275V or so is a reasonable area to be at.

If you are regenerating your nominal voltage is going to be a function of what the internal resistance of your cells will be and what current you will be using them to. Your nominal voltage should be Vmax = Vnom +Ri*I. The internal resistance is very dynamic based on temperature and SOC so you cant use a nominal relaxed RT internal resistance. You will want to take near your worst case and use that. I would imagine you are going to be in the 250V range. Try to use a single string of higher capacity cells in series because it will be cheaper and easier to design around as well as increase cell life and simply BMS cost and wiring. Higher capacity cells are also inherently lower resistance and since the internal resistance is a function of SOC it is good to use higher cells.

I probably wouldnt use LiPo cells if it were me but a lot of people like them.

I would like at the A123 20Ah pouch cells. Gaia makes a few options. EVBtech makes an 32Ah prismatic. There are quite a few options in LiFePO4 that would work well. Especially since you want to use them at those high rates.

#### ecat

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I don't know if this makes sence, but i throw this at U.
I am not expert at batteries, but i know that you cannot have a bad apple in a battery pack, it will waste the others. But, from what i read, the main drawback of LiFePo batteries is it's speed. What if you have a quick battery pack at the end, the LiFePo pack would recharge on a continuous basis and the end quick ones would answer the peak demand. A bit like we do when we fill scuba divers tanks, we have what we call cascade tanks for quick fillings and the cascades are filled slowly with a little pump over night.
I also saw a Smartstart gizmo from Dometic . www.dometic.com, i don't know if it can have some application in your car. This is for ac powering the surge from air conditionning compressors in busy marinas. Air conditionning units are essential for cars in most parts of the world, heating and defrosting is also a matter in colder climates.
Are U still looking at the Emrax motors ?
Sorry not to be more help for now.
Has there been a concept for a "no battery e-motor" yet ? Not 100% without battery mind you. There is a new "rotary" combustion motor about to be marketed, i dont know if it can run on water i.e. Hydrogen as well ... which doesn't polute very much other than the oils. So much technologies showing up...

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#### ecat

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Hi Lukas,
i had an idea for the hovercar that could serve you or your next project.
Batteries is one thing, but charging and cassetting (no better word for now) are to be considered. I hear alot that ecars are slow (very anoying) to charge. THe new lighter batteries open the idea of cassetting them , so you can have a pack at home, at the office and at authorized centers. Like home depots do with the bbq gas propane tanks. For me , so far, the weight of gel or agms batteries was not only a problem but an advantage for ballasting (for wakesurf) , pumping water (with water pillows) , equilibrate the craft, and some other weight assisting gizmos.
PV cells are expensive but if you concentrate the sun rays with a lens or parabolic mirror, you can reduce the infrastructure cost. The sun will cook them but to cool them you may want to consider an amonia system (or CO2) using that sunlight beamheat. We use propane fridges in fishing cabins up north, no wires there ... the cold from the amonia can cool the PV cell and also air condition the car.
It comes to mind that the coolant can also heat a radiator for heating in winter.
The radiator fan can be a windturbine when the car is parked, going downhill, yes on top of the wheel regenset. I am considering that for my hovercar design , with the props, i am killing manybirds with one stone (forward propulsion, brakes, ventilation, (compost toilet, habitat) shallow water safe, regen from wind, protection from pirates at sea, even a fire extinguisher, i remove the prop and i have a saw, a drill, a grinder/sander, etc, many things not readily available with a water immersed shaft, but a must on a boat at sea.
At the end of the day, all those crafts designed to transport people are only used a fraction of the time, find another use for them. Funny enough my friend has a GM Vibe with a 115v AC outlet, he was going to buy a genset for the few power shortages we have, until i asked him if he can feed his fridge, his cpu, ... with his car ...
Keep us informed of your project, we never know.
Cheers
Mike

#### Skywalker1991

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I don't know if this makes sence, but i throw this at U.
I am not expert at batteries, but i know that you cannot have a bad apple in a battery pack, it will waste the others. But, from what i read, the main drawback of LiFePo batteries is it's speed. What if you have a quick battery pack at the end, the LiFePo pack would recharge on a continuous basis and the end quick ones would answer the peak demand. A bit like we do when we fill scuba divers tanks, we have what we call cascade tanks for quick fillings and the cascades are filled slowly with a little pump over night.
I also saw a Smartstart gizmo from Dometic . www.dometic.com, i don't know if it can have some application in your car. This is for ac powering the surge from air conditionning compressors in busy marinas. Air conditionning units are essential for cars in most parts of the world, heating and defrosting is also a matter in colder climates.
Are U still looking at the Emrax motors ?
Sorry not to be more help for now.
Has there been a concept for a "no battery e-motor" yet ? Not 100% without battery mind you. There is a new "rotary" combustion motor about to be marketed, i dont know if it can run on water i.e. Hydrogen as well ... which doesn't polute very much other than the oils. So much technologies showing up...
Hi, we have decided to use the emrax motors, I also visited the company enstroj in slowenia.

Lukas, have you checked with www.ultravolt.com, or ups inverters ?
As motor controllers we will use the BAMOCARD3 from UniTek, but do you think that ultravolt could help us with chargers or something else?

Can anyone explain if there are problems when using a higher voltage for pwm?

For example if i want to have up to 190V RMS on the three phases of the motor, I would need at least 190*sqrt2=269Volts from the Batteries. But would it be a problem if I used 350V instead?

Regards,

Lukas

#### ecat

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hi Lukas
My design is not for a racing car, but for an hovercraft (hovercar gizmo) . I am working out my designs more as structural and architectural aspect of things. My friend who is now in Madagascar for a year, was installing windmills in Senegal. He is a teacher in college (pre university) in charge of not only teaching electricity, but managing partneships between the working sector and the college. The college can take you to university after 2 years (if you dont flunk anything) or become a professionnal technician in 3 years (after high school), i did a little historia for you to understand the level of that teacher. THere are other advantages at university level that will develop ecolo projects, you can check with the University of Sherbrooke, i forget the exact name i can search it, they built a sailboat with high tech, if i recall correctly, i beleive Gurit supplied knowhow. Check Gurit website at least once.
All this said, my friend always says low voltage is better than high voltage, heat hence fires become a problem and very dangerous for human life. Solomons industries had developped a higher voltage motor for sail boats. Not at those (your) voltages though.
In another forum on electrical engineering, one mentionned ups capable of handling high voltages.
You may want to push the idea of a buffer (fast discharge batterie(s)) between the LiFePo4 and the motor. To bad my neighbor past away 2 years ago, he was very involved with the lithium technologies, his main concern was hydrogen control, he repeated and repeated, beware of that hydrogen. From what i read, LiFePo's advantage is that.
My friend has web connection problems lately, but i will transfer to you infos as i get them.
Have you looked at Plettenberg emotors ? Emrax is looking at increasing the rpm, you probably know that already. Hummm ! high rpm raises some concern for durability (windmills toasting) i would favor some transmission system. (planetary ?)
Dont forget to get your flu shot ...
haha
Cheers
Mike

#### cruzangv

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Formula Student is a testing ground for the next generation of world-class engineers, who gain experience with construction and production, as well as commercial aspects in the automotive engineering.

racing blogs

#### Pawiel

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Hi Lukas

When choosing your battery pack voltage you might also want to consider the the voltage sag of the pack under load. The voltage that the cells can deliver will decrease under heavy current draw, so your total pack voltage will drop. The amount of voltage sag will depend on the the cell you choose, so to allow for this you will need cell discharge curves from the manufacturer.

Considering the safety requirements and the recommendation for individual monitoring, I would suggest contacting Kokam or EIG battery and asking which cells they would recommend for your application

Splitting the pack up into four sub-packs of around 75V would make it easier and safer to handle. You could also charge each of these sub-packs with their own 75V charger, which may be cheaper and easier to find than a single 300V charger capable of charging the full pack.

Parallel groups of cells balance naturally, so it would not be a problem using smaller cells grouped in parallel to provide your required 27 Ah. But it would of course mean more connections between cells, and possibly the need for more temperature sensors and BMS connections. I believe that Kokam and EIG both supply single cells of 20 Ah and 40 Ah capacity. A123 also produces 20 Ah cells.

I should say that I don't have personal experience of building such a high-performance pack, so these are purely the suggestions of an amateur
So, the right way (using the A123 20Ah cells) is splitting at the first step all needed number of cells together in parallel (for example every 4 cells to achieve 80Ah) and then - connect that splitted parallel cells in series (one parallel 4-cell block serial to next one)?
Is this correct?
After that operation - do I need to instantly monitor temperature of each cell to prevent fire or mechanical damage?

#### dougingraham

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So, the right way (using the A123 20Ah cells) is splitting at the first step all needed number of cells together in parallel (for example every 4 cells to achieve 80Ah) and then - connect that splitted parallel cells in series (one parallel 4-cell block serial to next one)?
Is this correct?
After that operation - do I need to instantly monitor temperature of each cell to prevent fire or mechanical damage?
Parallel first then you can treat those parallel blocks as if they were larger capacity cells.

The 20AH A123 cells are actually only about 18.5 AH. I didn't see any of them on EBAY the last time I looked so I am not sure of availability anymore. A123 is gone, called B456 now.

Unless you are pushing these cells to the limits you probably dont need to monitor the temperature at all.

#### Pawiel

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Parallel first then you can treat those parallel blocks as if they were larger capacity cells.

The 20AH A123 cells are actually only about 18.5 AH. I didn't see any of them on EBAY the last time I looked so I am not sure of availability anymore. A123 is gone, called B456 now.

Unless you are pushing these cells to the limits you probably dont need to monitor the temperature at all.
So, for that way serial-parallel battery may be serviced with one BMS? If yes - for safety - I can make a small cell to cell temperature monitoring on Dallas18B20 sensors - it will not be very expensive.

#### dougingraham

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So, for that way serial-parallel battery may be serviced with one BMS? If yes - for safety - I can make a small cell to cell temperature monitoring on Dallas18B20 sensors - it will not be very expensive.
If you insist on having a BMS, paralleling first is the only way to go. It reduces the number of connections by 75% in your example. Unless you are pushing the cells near the limits, measuring the temp is probably not necessary. You can safely dump the A123 cells in 6 minutes (10C, 800 amps for your example) without seeing much temp increase. A 3 minute dump (20C, 1600 amps for your example) would be where I would start wanting to keep a close eye on things. When you parallel the cells the heat coupling between cells is going to be quite good so a single sensor on one of the terminals of each four cell block should be adequate.

Just looked on EBAY and there are two vendors you can choose from. One has no tabs, so probably pulled from something that was manufactured. Those are \$35.5 each including shipping. The other has short tabs but is \$47 each including shipping. Neither are even close to bargains. I don't see these as a viable option anymore.

#### Pawiel

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.....You can safely dump the A123 cells in 6 minutes (10C, 800 amps for your example) without seeing much temp increase. A 3 minute dump (20C, 1600 amps for your example) would be where I would start wanting to keep a close eye on things.
Important and useful information.
Thanks very much.
Maybe I'll buy few cells and check all.

As some people say: practice makes perfect.

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