LiFePO4 Batteries

[yash101]

Sorry about the confusion . I am aiming for maximum distance that can be travelled. What I meant by 200 ah is the battery holds 200 amps of charge. Otherwise, i did start at hobbyking. They don't have too big batteries and the big ones are overpriced. Also, doing some looking into the manufacturer's website, if I mass order lifepo4 cells (1000s of cells), i can get 10 amp 3.2 volt cells for $1 a piece. That means that Philso mentioned, if i get into a group buy if this mass, we all could get these 10 amp cells dirt cheap! $4 for a 12v equivalent battery of 4 amps is a ridiculous price, especially for one who mostly shops at hobbyking. Also, these tiny cells that I found (10ah a piece) are rated for 2C, more than what is required because they will more-or-less be wired parallel, distributing the load to all ~100 cells. Also, how dangerous are the fires or explosions of these batteries. They are not as bad as lithium LiCoO2 batteries (cell phone battery chemistry, i think). That's why i'm using these. Also, can I permanently wire these batteries in parallel? Won't the cells distribute the charge equally and maintain the same voltage throughout? Also, anyone know about any electric vehicle enthusiast communities?

[AlexH]

Originally Posted by yash101

Sorry about the confusion . I am aiming for maximum distance that can be travelled. What I meant by 200 ah is the battery holds 200 amps of charge. Otherwise, i did start at hobbyking. They don't have too big batteries and the big ones are overpriced. Also, doing some looking into the manufacturer's website, if I mass order lifepo4 cells (1000s of cells), i can get 10 amp 3.2 volt cells for $1 a piece. That means that Philso mentioned, if i get into a group buy if this mass, we all could get these 10 amp cells dirt cheap! $4 for a 12v equivalent battery of 4 amps is a ridiculous price, especially for one who mostly shops at hobbyking. Also, these tiny cells that I found (10ah a piece) are rated for 2C, more than what is required because they will more-or-less be wired parallel, distributing the load to all ~100 cells. Also, how dangerous are the fires or explosions of these batteries. They are not as bad as lithium LiCoO2 batteries (cell phone battery chemistry, i think). That's why i'm using these. Also, can I permanently wire these batteries in parallel? Won't the cells distribute the charge equally and maintain the same voltage throughout? Also, anyone know about any electric vehicle enthusiast communities?

i'd be down for buying some.

http://visforvoltage.org/ is a ev community

[yash101]

right now, i am just requesting a quote. I did contact the company yesterday and I did get a reply. However, the only answer I got would be "the price would be high" because of all the shipping and price + tariffs and other moneyhogs. However, I asked for a quote of just 200 batteries. It'll probably be cheaper if I order in bulk because the expensive price of shipping won't change much if I buy a couple thousand batteries versus 200 of them.

Here is the important excerpt of the reply. I agree the English sucks, but the email was sent from china, so I am quite impressed they know English


We leading Batteries manufacture for years,USA is one of our main market .
we can send to you by TNT without problem. only the cost will a bit high.
Our standard specs:

Model Material voltage capacity life cycles Max. charge voltage cut-off discharge voltage weight approx. measurement(D*H)
HWF42110 LifePO4 3.2V 10Ah up 2000 3.65V 2.3V 330g 42*110mm

Do you assembled the 200cells by yourself ?
And by the way ,may i know some information about your company please?

[yash101]

Copy and Paste from Google Mail Sucks . The table is completely missing, however, if you are really looking into these specs, you may look hard or even look at the manufacturer, Howell's website

[yash101]

Originally Posted by AlexH

i'd be down for buying some.

http://visforvoltage.org/ is a ev community

I'd be ready, however, I do not think anymore that they will be priced as low as a dlooar a piece, more like $5 under ideal circumstances, plus I will need to ship them to, I believe, Pennsylvania? However, I will keep you posted about this, as I think that even $5 is a decent price for 10ah cells

[philso]

Devyash

You should carefully consider the size of the individual batteries and the number that you have to connect in series/parallel to get the energy you want.

The smaller the batteries, the more "strings" you will have and the more connection points you will have. Connection points are the most common failure point in electrical/electronic systems. I have only seen strings of batteries connected in parallel at the two ends of the strings. This allows the use of Battery Management Systems to be used to monitor each cell. I have never seen the intermediate points connected in parallel since it would prevent the monitoring of individual cells.

Since you are using these in a vehicle, you will be concerned with system weight and volume. A pack made up of smaller batteries might weigh more and take more space than one made up of larger batteries. The shell of the battery may constitute a larger percentage of it's weight than in a larger battery. You will have to study the detailed specifications carefully and estimate your overall weight and size. A BMS for small batteries is about the same size and weight as the BMS for a larger battery so a pack made up of small cells will have more weight and volume taken up with the BMS.

If you are using high discharge rates, the smaller batteries will have more surface area per unit of energy storage, making it easier to keep the battery pack within it's specified operating temperature limits.

The flat discharge curve followed by a rapid voltage drop is a good reason to use a BMS to prevent you from damaging the battery pack.

[yash101]

Maybe I could calculate a function to solve for the battery "percentage" and the range of the vehicle. There will be 10 amps of reserve, enough for about 5-10 miles, depending on your driving style. Also, if I add regenerative braking, I shall add more range. Also, since these batteries will be weighing in at about 100 kgs, do any of you know of any high power, commercially available motors that will bring this 200-300 kg car to 60mph? I was looking at this: http://www.hobbyking.com/hobbyking/s...ner_Motor.html
However, it required a very high voltage (52 volts) at 250 amps! that's a lot of juice! The batteries won't provide enough energy for more than maybe an hour of runtime, dropping the range to about 60-100 miles!

[EricH]

I don't know of any motors myself. However, I will again point you to the SAE Clean Snowmobile Challenge, which has a zero-emissions category (AKA electric). They're building for range, power, and lack of emissions--and I bet they know some good motors if you're able to contact one of the teams. There might even be one near you.

[yash101]

Originally Posted by EricH

I don't know of any motors myself. However, I will again point you to the SAE Clean Snowmobile Challenge, which has a zero-emissions category (AKA electric). They're building for range, power, and lack of emissions--and I bet they know some good motors if you're able to contact one of the teams. There might even be one near you.

Living in the middle of Arizona, a desert, I am probably unlikely to find Snowmobile Designers?

[EricH]

Originally Posted by yash101

Living in the middle of Arizona, a desert, I am probably unlikely to find Snowmobile Designers?

You can find them in some unlikely places (though I admit that I don't see any registered teams from anywhere south of Illinois). And remember, there's this wonderful invention called the Internet, which can help you connect with folks who actually have winter. (As a matter of fact, I remember having a couple of folks from Arizona on my Aero team--which worked in the same room as the Snowmobile team--during my final year of college in South Dakota, one of whom may have graduated from your FRC team--I can never keep 1164 and 1165 straight.)


As far as safety, AFAIK, they're pretty comparable as long as you don't do anything utterly stupid. The risk of flammability is a bit higher, but not nearly as bad as LiPOs. If you're shipping them, you have to have someone who's registered to do that, though I don't think that'll be too big of an issue if you're not shipping them. Charging is more of common sense--use the right charger, and make sure it stops when it needs to.

[philso]

Originally Posted by yash101

Living in the middle of Arizona, a desert, I am probably unlikely to find Snowmobile Designers?

There are quite a few people out there who have been building electric cars for a long time. Many of them are members of the EAA (I gave you the link in my first post in this thread). You are best to start by getting in touch with a bunch of them in your area.

[protoserge]

Originally Posted by yash101

Maybe I could calculate a function to solve for the battery "percentage" and the range of the vehicle. There will be 10 amps of reserve, enough for about 5-10 miles, depending on your driving style. Also, if I add regenerative braking, I shall add more range. Also, since these batteries will be weighing in at about 100 kgs, do any of you know of any high power, commercially available motors that will bring this 200-300 kg car to 60mph? I was looking at this: http://www.hobbyking.com/hobbyking/s...ner_Motor.html
However, it required a very high voltage (52 volts) at 250 amps! that's a lot of juice! The batteries won't provide enough energy for more than maybe an hour of runtime, dropping the range to about 60-100 miles!

How are you calculating your numbers? What is your requirement? Before you continue to keep looking at motors and batteries, you have to start with a set of requirements to point your research in the right direction. Until you list out the requirements, we can't really help you solve your problem.

You said the car has to go to 60 mph, but how long does it need to travel at that speed? What is the acceleration required? How many stop-start cycles will be required? What is the vehicle weight constraint? How many wheels will there be?

[philso]

Devyash

You need to study up on the various types of motors available (DC, AC) and the advantages and disadvantages of each type. You then need to study the types of controllers available for each type of motor. Since you are aiming at maximum efficiency, you will have to select the motor and controller very carefully. As Tim above says, you need to have clear specifications for what you are trying to do before you start.

Adding regenerative braking will roughly double the complexity of your controller. It will also roughly double the losses in your controller while transferring power from the battery to the motor, reducing your overall efficiency. Unfortunately, I have yet to find a commercially available AC or DC motor controller for EV applications that has regenerative braking.

[AlexH]

hobbyking brushless motors are grossly over spec'ed. the wattage they give is just stall current times maximum voltage.

[Ether]

Originally Posted by AlexH

hobbyking brushless motors are grossly over spec'ed. the wattage they give is just stall current times maximum voltage.

It's not overspec'd, just deceptive. It's max input wattage instead of max mechanical output power :-(