# New Member looking for some battery help



## husker97 (Oct 22, 2014)

hey guys. I am a new member here and I have a project that I need some insight on. I am a university student and I am currently the head of our electrathon team. I'm sure some of you know what eletrathon is, but for the people that don't, it a racing series where students design, build, and race single person electric cars. The races last one hour and the driver with the most laps around the track in one hour wins. We are currently working on building a new car from scratch. Our problem is with the batteries that we have. We currently have two battery packs, 24v at 20ah, and we want to run them in series for 48v at 20ah. We tested them out on our current car but we would draw more amperage than the BMS would allow. The BMS allows up to 60 amp max discharge and we currently pull more that that, close to 100 amps sometimes. we were looking to a new BMS that would allow 100+ amps, i have only found one reasonable BMS that allows a 24v pack to draw 100+ amps. We have a pretty limited budget, so we can's spend a lot of money on a BMS. What are my options? Thanks in advance.

batteries we have
http://www.batteryspace.com/custom-lifepo4-prismatic-battery-24v-20-ah-with-60a-pcm.aspx

Motor kit we have
http://www.electricmotorsport.com/e...rottle-contactor-wire-kit-and-fuse-block.html

Different BMS I found
http://www.greenbikekit.com/8s-pcm-30-50a-lifepo4-bms.html

CAD drawing of new car


New frame next to old car


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## dimitri (May 16, 2008)

pulling 100A from 20AH pack means 5C discharge rate. Not all batteries are meant for 5C rates and you did not specify which battery you have.

Assuming your battery is not top notch quality, which is a fair assumption if you have limited budget, then you need to decide which is more important to you, win the race at a risk of sacrificing your battery life, or preserve the battery life. If former, then bypass the BMS and pull as much as you need from the pack until it dies. Have a fire extinguisher ready for worst case scenario, especially if your pack is LiNMC or LiCo chemistry.

Also, 5C rate means 12 minutes of battery runtime, far cry from an hour allowed for the race. Maybe you need a bigger battery, not just different BMS.


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## dimitri (May 16, 2008)

Sorry, I missed that you linked to the battery, which is made of these cells

As you can see, the specs allow 2C continuous and 10C bursts, so as long as you stay within those specs the cells should perform OK.

Your BMS has MOSFETs, which limit the current. You need to dump this BMS and connect the pack directly to your motor controller via contactor, this way you can pull as much current as controller allows. You can run without BMS if you monitor your cells carefully, or get a simple "monitoring" BMS, which only alarms you of HVC/LVC, but not limiting the current.


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## CKidder (Dec 12, 2009)

Also, 20AH worth of batteries means that you can only average about 20 amps during the hour long race. At that point you'll be out of juice. If you draw upwards of 100A at time then I suspect you aren't going to average 20A and thus will run out early. This might be OK if you can do your laps really, really fast. Unfortunately, acceleration is really your enemy here and you really should be trying to draw less power but over a longer time frame. So, maybe re-evaluate what you're doing. Then dump that BMS. What do you need a BMS for if you have so few cells anyway? Bottom balance those cells and use no BMS. When the car won't move anymore you're done. If you're worried then use a voltage gauge and stop when it starts to crash in voltage.


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## Sunking (Aug 10, 2009)

Well first thing is get rid of the BMS, no need for it in a race car, just extra weight that does not gain you anything in a race.

Second point is your battery math needs some work. Even if you did have batteries that could deliver 5C rate, a 20 AH battery at 100 amps is a whole 11 minutes until dead. you would need a 125 AH battery to last 1 hour and still survive to race another day. To make it through one hour with a 20 AH battery requires you to pull no more than 19 amps on average. 

In racing the winner is usually the one with the largest budget, or no budget at all, and an experienced team of the best people money can buy. .

EDIT NOTE THOUGHTS

If I were in your shoes I would contact all three of the big 3 Golf Cart Manufactures like Textron (EZGO), Ingersol Rand (Club Car), and Yamaha and ask for sponsorship and consulting. I tinker with modifying golf carts which is basically what you are making. For $6K you could easily build a car that goes 40 mph for an hour easily. Not saying they would bank roll the whole project, but until you ask and try you got nothing to loose. 

Give you an example EZGO makes two carts once called the RXV which is a golf cart, and the 2Five which is a LSV. both are the exact same vehicle. Difference is the 2Five has brakes, and a higher speed gear to allow it to go to 30 mph and with a controller programmer will go 36-38 mph. 500 pounds of the pounds of the 1000 pound vehicle weight is lead acid batteries. Replace that with a 100 AH LFP pack shaves 350 pounds and extends the range to about 90 minutes pedal to the medal. Either one will likely blow everything off the track and lap the field several times. 

Did you catch that part about the brakes?. If you have REGEN, get rid of the brakes as you have no use for them and just adds dead weight.


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## husker97 (Oct 22, 2014)

Thanks guys for the replies. One thing I forgot to mention is that we are limited to the weight of batteries we can use. For LiFePO4 batteries, we are limited to 29lbs of batteries. This is supposed to keep the batteries under 1kwh of power. So unfortunately, we cannot run anymore ah unless we decrease the voltage. I think the batteries we have can work because we used these batteries in a race a few weeks ago to try them out. This is when we found out that we needed to pull more that 60 amps in one burst. In about the 20 minutes we raced before we called it quits because of the battery problems, we had used up less than 2 volts. We started the race at 50v and after 20 minutes we were just above 48v. Also at this rate, we were keeping up with the fastest cars in the race. So i think these batteries can work. I know the math doesn't work, but he real world experiment did.

The main problem I have about not running a bms is keeping the batteries within there limits and keeping them balanced right. I know lithium batteries are picky about how they are charged and discharged. So if i take off the bms and replace it with balance modules between each cell, i can pull as many amps as i want? i would just have to keep an eye on the voltage and turn it off before I get too low. 

Would this module keep the cells balanced but also allow for the high discharge?
http://www.batteryspace.com/Smart-L...3.2V-LFP-Cell-with-250mA-Balance-Current.aspx

thanks again guys.


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## major (Apr 4, 2008)

Sunking said:


> Well first thing is get rid of the BMS, no need for it in a race car,...


I think the first thing to do is to read the rules  I have been involved with student competition EV racing in a number a capacities including technical inspection. I would be surprised if there isn't a requirement for BMS. Race organizers really don't want to see baddery incidents on-track for the sake of the show as well as safety.

The same is likely true regarding mechanical brakes


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## husker97 (Oct 22, 2014)

major said:


> I think the first thing to do is to read the rules  I have been involved with student competition EV racing in a number a capacities including technical inspection. I would be surprised if there isn't a requirement for BMS. Race organizers really don't want to see baddery incidents on-track for the sake of the show as well as safety.
> 
> The same is likely true regarding mechanical brakes


There are no rules requiring a bms, but there needs to be mechanical brakes and they test the brakes at inspection before the race.


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## Vanquizor (Nov 17, 2009)

Have you considered using the existing BMS but simply modifying it? I would try simply adding a discharge lead directly off the battery side of your BMS for discharge to bypass the 60A limit, and still run your Key Switch off the original BMS discharge leads. Also do your charging through the original BMS leads.

This should allow you to retain the charge control and balancing as originally implemented, and will cause the controller to dropout due to loosing the KSI when the bms trips low cell voltage protection. This will however cause you to have absolutely no overcurrent protection on discharge, so make sure you have some way of ensuring you don't push them harder than you want to (resettable breaker would be my choice, along with good instrumentation).


NOTE: I make this recommendation with the assumption that you are using some sort of pulse and glide technique to make your 20ah of battery survive 1 hour at the discharge levels you are trying to use, and that you have satisfied yourself the batteries are capable of performing as you intend.


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## bwjunkie (Jul 31, 2013)

Based on your weight requirements, you may need to consider a higher energy density battery type. 

Sounds like you need to drive for 60 minutes on 1kwh of energy, which sounds like a real stretch to me, on that battery type. How much downhill and regen are you talking? Is there any chance the 18650 style battery would help?

-josh


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## Sunking (Aug 10, 2009)

husker97 said:


> There are no rules requiring a bms,


Husker LFP batteries are very forgiving when charging. I know of 3 people, I will be the 4th to convert a golf cart with LFP batteries. None of the 3 I know of, nor I will use a BMS. Trick is to balance them before they go in, then periodically check voltages and either bottom or top balance as necessary which is not that often. We just use a hobby charger for RC models to balance cells as needed. Even just a simple Constant Voltage charger can be used with current limit. Something to think about.

With the battery weight limit, well that opens the door pretty wide especially if you can get a sponsor. One disadvantage to LFP cells is Specific Energy is not that great of around 100 wh/Kg which is a heck of a lot better than FLA. LFP is also cheap and safe. But with a sponsor you might get turned onto some LiCo's or A123 Systems LFP cylindrical cells something in the 200 wh/Kg range. Might even get a connection to some LiPo prototypes for EV's that go up to 250 wh/Kg. If I were you and on your team I would be calling every marketing department of EV's, battery, golf cart, and motor company I could think of begging for help. Not only might you get some hardware but a bit of cash and technical help for inspiring up coming engineers. You might even score an Internship. Worse they can say is say NO THANKS. 

Good luck to you.


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## husker97 (Oct 22, 2014)

Vanquizor said:


> Have you considered using the existing BMS but simply modifying it? I would try simply adding a discharge lead directly off the battery side of your BMS for discharge to bypass the 60A limit, and still run your Key Switch off the original BMS discharge leads. Also do your charging through the original BMS leads.
> 
> This should allow you to retain the charge control and balancing as originally implemented, and will cause the controller to dropout due to loosing the KSI when the bms trips low cell voltage protection. This will however cause you to have absolutely no overcurrent protection on discharge, so make sure you have some way of ensuring you don't push them harder than you want to (resettable breaker would be my choice, along with good instrumentation).
> 
> ...


It has crossed my mind about trying to modify the bms that we have, but I don't know if i want to go that route just yet. I or any other team members don't have that much experience with pbc electronic circuits. I would like to find an alternative if we can. Also our driving style uses a lot of coasting. At one of our track, we can coast around half of it without ever touching the throttle.



bwjunkie said:


> Based on your weight requirements, you may need to consider a higher energy density battery type.
> 
> Sounds like you need to drive for 60 minutes on 1kwh of energy, which sounds like a real stretch to me, on that battery type. How much downhill and regen are you talking? Is there any chance the 18650 style battery would help?
> 
> -josh


If we go with different battery types, the weight requirements change. for li-ions or lipo, its only 15lbs, and nimh is 41lbs. Most cars use lead acid, but we wanted to use something different that could give us an edge. Unfortunately, we are not using regen at the moment, but it has been discussed. we may try and use it later on. Also, with different style batteries such as 18650, we found them to be much more expensive than what we have currently. to get the same output as a prismatic cell from an 18650, it would cost us close to $1500, close to double what we paid for the ones we have now. the power to price ratio is better with the prismatics.



Sunking said:


> Husker LFP batteries are very forgiving when charging. I know of 3 people, I will be the 4th to convert a golf cart with LFP batteries. None of the 3 I know of, nor I will use a BMS. Trick is to balance them before they go in, then periodically check voltages and either bottom or top balance as necessary which is not that often. We just use a hobby charger for RC models to balance cells as needed. Even just a simple Constant Voltage charger can be used with current limit. Something to think about.
> 
> With the battery weight limit, well that opens the door pretty wide especially if you can get a sponsor. One disadvantage to LFP cells is Specific Energy is not that great of around 100 wh/Kg which is a heck of a lot better than FLA. LFP is also cheap and safe. But with a sponsor you might get turned onto some LiCo's or A123 Systems LFP cylindrical cells something in the 200 wh/Kg range. Might even get a connection to some LiPo prototypes for EV's that go up to 250 wh/Kg. If I were you and on your team I would be calling every marketing department of EV's, battery, golf cart, and motor company I could think of begging for help. Not only might you get some hardware but a bit of cash and technical help for inspiring up coming engineers. You might even score an Internship. Worse they can say is say NO THANKS.
> 
> Good luck to you.


So you are saying that if the batteries are balanced before charging/discharging, they will all charge/discharge at the same rate? If I can totally forget the BMS, that would be great. We would just have to check them after every cycle to make sure they are still balanced? 

Regarding the sponsorship, we have not contacted anybody yet, because we actually received a grant provided we use the build as a 1 credit/hour class. We were planning on approaching local business for sponsorships, but no large national companies. I have a feeling they would shoot us down because we only travel within the state, and only have about 3 races a semester, so there wouldn't be much exposure for them. I think local businesses would be more likely to donate than national companies, but it is always an idea.

thanks for all of the replies guys.


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