Re: LiFePO4 Batteries
 

I was looking at the a123 batteries before. They seemed quite repute. However, I didn't know where to get high-capacity cells, and the decent capacity ones were expensivo

Re: LiFePO4 Batteries
 

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 :]

Re: LiFePO4 Batteries
 

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.

Re: LiFePO4 Batteries
 

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!

Re: LiFePO4 Batteries
 

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.

Re: LiFePO4 Batteries
 

thanks, just pm me in chief delphi when you are ready to order. i'm in williamsport pa. i'll gladly pay for shipping.

Re: LiFePO4 Batteries
 

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?

Re: LiFePO4 Batteries
 

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.

Re: LiFePO4 Batteries
 

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

Re: LiFePO4 Batteries
 

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

Re: LiFePO4 Batteries
 

I got two "3000 watt" HK Sk3 motors for an gokart.

Any good ways to find the max mechanical output power?

Re: LiFePO4 Batteries
 

Put it on a dynamometer?

If you know the stall torque and the free speed, and you believe the speed vs torque is linear, then the max output mechanical power in watts is stall torque times free speed divided by 4 (if using SI units).

Re: LiFePO4 Batteries
 

I completely agree. I was planning on using Brushless (AC) motors because they are a tad more efficient and because there are no brushes that wear out over time. The only stress is the bearings, and using something like a ball bearing could last quite a long time.

Re: LiFePO4 Batteries
 

This is the reason why I do not like HK motors. They do not give what they say. Also, I think that their motors are meant for high RPM, not high torque. That would also mean that the bearings have a whole greater load to withstand. Our BAG motors on our shooter keep failing because the motors can get up to 14,000 RPM Also, the motors are ridiculously priced (the ones that seam that they might work)

Anyways, does anyone know where I could buy a car motor? I do not want to use CIMs because they overheat after continuous usage, and they just do not provide the torque required

Re: LiFePO4 Batteries
 

How are your BAG motors reaching 40krpm?