Electrathon America Forum

ProEV · Senior Member

Who else is running experimental class this year?

ProEV · Senior Member

Here are the bare cells as shipped from Kokam:

Here is the battery pack assembled with BMS attached, ready to be installed:

Zahrack · Member

Just to clarify, I am not racing anything yet, but I am VERY interested in the use of Lithium type batteries.

I do have some questions though. Your set up looks like it could be very promising. However, after doing my own research, I found some issues that may or may not be problematic.

The main issue I have is amp draw. How does your car fair in that area? I am guessing that you just crank the volts up to speed up, but from what I know motors are regulated by the voltage but are free to pull the current they need to operate. Does your regulator (The chip built onto the battery.) give the required voltage for the speed and ensures a safe current draw? What is the discharge "C' rating on you batteries? Can you safely pull more than 16 amps out without damaging your batteries? Another thing. I know that there is a limit to how many batteries you can put into series. Does your series push that limit or are you "safe" inside its ability to pass voltage through without overheating the connections and internal parts?

I know I just put a lot out there. As I said. I am VERY interested in Lithium type batteries for use on electrathon vehicles. I appreciate your time and consideration,

Zach

P.S. I think I may have stumbled on some better battery power to weight ratio's then what you have here. I am willing to share. Cheers!

mhodgertt · EA President

The problem with using the experimental batteries is that unless more cars use them you are racing against yourself. In the NW there are only about 3 cars that are using them all of the others are in the 'Standard Class' or lead acid. Since the Lithium etc. take off a large amount of weight for the vehicle to create the same or even more power, it would not be fair for them to race straight across with the lead acid. We have had a few individuals build a car, race it and do well, but then find out that they were not in the top spots of the cars that raced because they were the only 'Experimental Class' vehicle. This has caused some hurt egos and bad feeling so I want everyone to understand this issue.

I hope in the future that more teams/cars start using the Lithium, etc. batteries and they become more common and have straight up competition.

ProEV · Senior Member

Hi Zach,

You are asking some good questions but it seems you have some misconceptions.

Let's start with the 'chip built onto the battery'. That is the Battery Management System, BMS for short. It's job is to make sure the batteries stay within their optimum limits. It does not control the motor in any way. It could be set up to warn the motor controller of a problem with the battery pack.

While different BMS work in different ways, this BMS monitors individual cell voltages and temperatures. If any cell goes below a specified voltage, a warning signal is giving. If any cell goes above a certain voltage (such as during charging) a warning signal is sent to the charger and a resistor connects in parallel with the individual cell to use up the excess power.

Many advanced batteries require being kept within closer limits than lead acid batteries. If you overcharge or over-discharge a lithium polymer cell, it will be damaged. The BMS allows charging of all the cells fully without over-charging any cell.

Here is a link to info on the Manzanita Micro BMS : http://www.manzanitamicro.com/productspage=shop.browse&category_id=36

A lithium cell is 'empty' at 2.8 volts. It is fully charged at 4.2 volts. We use 3.7 volts as the 'nominal' voltage for calculation. At the start of the race, the pack voltage is 66 volts. By the end of the race, the pack voltage will have dropped to a resting voltage of 44.8.

"The main issue I have is amp draw.....What is the discharge "C' rating on you batteries? Can you safely pull more than 16 amps out without damaging your batteries?"

Another very good question. Batteries have to be judged by many parameters. Different batteries for different jobs.

These Kokam 'High Powered' (as opposed to the Kokam 'High Energy') cells are rated to be discharged at 8C continuous and 15C pulse. Even though the car will average 16 amp current over the one hour race, the batteries have the ability to supply 240 amps (15*16) which is more than our motor controller can utilize. Here is a link to the Kokam specs: http://www.kokam.com/new/kokam_en/sub01/sub01_01.html

" I know that there is a limit to how many batteries you can put into series."

I do not know of any theoretical limit to how many cells can be put in series. We put 180 Cyclon single cell lead acid batteries in series for the Electric Imp (385 volts)(www.ProEV.com/WHistPgs/Hist0012.htm). We now run with 95 Kokam lithium polymer in series (close to 400 volts at full charge). The standard 24 volt electrathon pack is made up of 12 (2.1 volt) lead acid cells in series. Most of the connections are inside the two batteries but that is almost as many cells as the 16 in the Kokam electrathon pack.

There is a whole other interesting discussion about the advantages and disadvantages of higher voltage but I am running out of typing time biggrin .

Zahrack · Member

So, yea. It has been a little while since then. I have done many more hours of research and have found some pretty crazy-good Lithium-polymer batteries.

I see where the BMS comes in handy. Now people just need to start making more Electrathons with "special" batteries in the lithium class. I may see you out there someday. When I do I'll say hi. =)

Zach

dfarning · Veteran Member

ProEv,

What are you looking at for total cost of your LiPo drive system?

Currently, I am looking at standard gel cell batteries, a re-manufactured Etek motor, with controller. If the price difference is not insane, I would be willing to consider bumping up to LiPo.

ProEV · Senior Member

Hi David,

Kokam batteries are amazing batteries but they are expensive. Our racing team has a relationship with Kokam as they are one of our sponsors on the Electric Imp.

there are some good deals out there on lithium batteries. They require different care than lead acid so you need to factor in chargers and either a BMS or ways of protecting the individual cells from getting over discharged.

i suggest you look at the possibility of using cells from the Leaf electric car. They can often be purchased from here for $119 a module. A electrathon pack would require two modules. They do not list any in stock but they respond quickly to requests. http://hybridautocenter.com/HAC4/index.php?option=com_hikashop&ctrl=product&task=show&cid=3&name=nissan-leaf-battery-module-model-2012-new&Itemid=605

The modules consist of four cells connected two in series and two in parrallel. This means the two modules would create a 14.8 nominal voltage pack with a 60 amp hour rating. This is 912 Watt hours. The total battery weight would be 14.09 lbs.

A 14.8 nominal pack would not be very useful so the module would need to be disassembled and re-wired as 8 cells in series. This would make the pack 29.6 volts nominal and 30 amp hour. Here is a link to a series of videos on disassembling Leaf modules ( the Internet is a wonderful place ). http://evmc2.wordpress.com/2014/03/01/nissan-leaf-battery-disassembly-with-a-machete/

One thing to investigate:

The cells sem to be rated various ways. I think the nominal voltage which we use to calculate capacity is 3.7 volts for this chemistry. They seem to talk about an average voltage of 3.8 volts which might be marketing to make the cells seem more attractive. They also say the cells are rated at 30 amp hours but later on the page the cell's amp hours are listed as 33.1 amp hours. The only way this is an issue is if we use 3.8 volts as nominal and the 33.1 amp hours fiquire, the capacity calculates out to 8*3.8*33.1= 1006.24 Watt hours. Just over the limit.

At $240, this is about the price of a lead acid pack. It should have much better cycle life than lead acid. You will need to fiquire out a proper charging and balancing system. Also lithium are more sensitive to over discharge than lead acid, so a system for shutting down the pack before any individual cell gets to low needs to be set up.

Cliff

dfarning · Veteran Member

Thank for the update.

For the last couple of weeks I have been doing some research on on various battery technologies that might work for electrathon vehicles. This morning I started writing up my notes on the thread at electrathonamerica.activeboard.com/t59025673/my-journey-towards-a-lithium-ion-battery-pack-for-an-electra/ in the hope that the information might be helpful to someone else:) I'll include information on the leaf modules in that thread.

To give away the theme of the next couple of posts on that thread I am going to describe designing and testing a pack made up of recycled 18650s. I don't know if I will actually use 18650s, but the exercise has been fun and educational.

David

dfarning · Veteran Member

Ahh yes,

I found the leaf modules in my notes.

On the plus side they were inexpensive and already assembled.

On the minus they were rated at .3C, giving them a full discharge time of about 3.5 hours... So they didn't make the first cut of having an average discharge rate of 1C with the ability to handle frequent surges of 1.5C.

As a side note, did you by chance happen to run a data logger on your BMS last season? I am trying to create a accurate discharge profile for testing.

David

ProEV · Senior Member

Hi David,

I think you are confusing the way the Leaf battery amp hours were rated with the power capabilities of the cells.

Battery manufactures like to use slow discharge rates because batteries can deliver more of the energy stored in them and thus show a higher amp hour rating. This is why lead acid batteries advertise their 20 hour rate.

The Leaf batteries are rated at 1/3C which is a discharge over three hours. But if you look at the original link I sent, you will find they are claiming a maximum continuous discharge rate of 4C. They also give a peak permissible discharge of 9C.

If the modules are re-wired in series, they can be discharged at 120 amps (4*30) and will deliver short bursts of 270 amps (9*30).

Keep in mind that available capacity will be less at the higher rates of discharge so information about what percentage of capacity is available at the 1C is what we really need.

dfarning · Veteran Member

Sorry, I am an old curmudgeon:( I tend to skim the marketing material and skip straight to the product specifications.

I agree the C-rating is a poor term for the purposes of this discussion.

In my limited experience of running a hand full a batteries through sequence of discharges I have been finding that below a certain rate (somewhere near the manufacture's C-Rate) batteries tend to be able to discharge a large percent of their rated capacity. If one increases the discharge rate past that point, there is an inflection in the curve indicating that the battery is unable to discharge as much total energy. Thus, discharge faster has a significant effect on effective capacity.

I will freely admit that my hypothesis might very well be due to confusion. A couple of days ago I ordered a Cellpro PowerLab 6. It has some really graphing software. If I get some useful data on the effect of discharged rate on effective capacity I'll post it here.

Update: Since I am new to this subject and forum, it might be easy to take me for a troll.... So, just ordered a Leaf module to run it though testing :)

-- Edited by dfarning on Wednesday 19th of November 2014 02:23:26 AM

ProEV · Senior Member

David,

It sounds like you are talking about Peurket's effect http://www.39pw.us/car/peukertEffect.html

(and the way batteries are marketed smile

). It is always wise to get the information for the discharge rates you plan to use the batteries at.

These modules are designed for use in an electric car which should see discharge rates in excess of what we see in electrathon racing. There seem to be a number of independent tests of the modules on the Internet. A lot of people also seem to be experimenting with not charging over 4.1 volts per cell and not discharging under 3.0 volts to try to avoid using a BMS and to get greater cycle life.

There are some 1C, 2C and 3C discharge curves from the manufacturer here: http://www.eco-aesc-lb.com/en/product/liion_ev/

Someone roughly calculated (from the graphs) 89% of full capacity at 3C, 94% at 2C and 97% at 1C. http://www.elmoto.net/showthread.php?3668-Nissan-Leaf-battery-testing/page7

If this is correct, then this 14 lbs pack should give around 884 Watts, cost about the same as a pair of red tops and survive longer.

It sounds great so I will be interested to hear what your testing shows.