Monday, January 11, 2010

Good News Everyone!

I've received word that the batteries have arrived in the country, Seattle to be specific. The company I ordered them from is going through the shipping container and portioning out everyone's order. I should have the batteries here within a couple weeks.

While I'm talking about batteries, it's occurred to me that I haven't really gone into too much detail about them, the range they should provide, and other such pieces of data. If you're interested in the nitty gritty numbers read on. Other wise... you've been warned.

I'm getting 48 of the Sky Energy SE120AHA batteries. They have a nominal voltage of 3.4 volts and provide 120 Amp/Hours. The batteries will be wired in series. This is just like the batteries in a normal TV remote are set up (positive to negative, positive to negative, etc). What that means is that all of the batteries become one big battery, and the voltage of that one big battery is the total of each added together. So, the math looks like this:

3.4 x 48 = 163.2 Volts

So that whole pack will have a nominal voltage of about 163 volts. Each battery offers 120 Amp/Hours of current, so to figure out the total number of Watts provided, you multiply the Volts by the amps:

163.2 x 120 = 19584 Watt/Hours Or roughly 19.6 kilowatt/hours.

Now, I don't want to draw all the current out of the batteries, because that will shorten the life. If I use only 80% of the power, or less, I'll extend the life of the batteries to upwards of 3000 cycles. So, if I want to leave at least 20% of the power in the battery, that means I'll get to use this much power:

19.6 kWh x .80 = 15.68 kWh

The car will likely use between 225 and 250 Watts/mile. Erring on the conservative side, lets say it will be 250. So then we figure out the range of the car, using only 80% of the current:

15680 / 250 = 62.7

So conservatively, I should have a range of 62 miles. Of course if I'm flooring it when I leave every stop light, that number goes down. If I'm on a freeway going 65 miles an hour, that number will go down. If I travel conservatively on a surface street, traveling no faster than 45, I should be able to extend that a bit. Or in the event I need to tap into that remaining 20% reserve energy for some reason, I could get as much as 16 more miles out the car.

You can see why people that drive EV's cringe at the question "How far can you go on a charge?" (Would you really want to explain that every time?) Or why the manufacturers that are just getting into the market of promoting EV's that they hope to build at some undetermined time in the future are reluctant to give out those numbers. Well that's not true of all manufacturers. Some promise 100 miles when practical experience or simple math like I've done above shows that the range would be more like 50.

To charge the car is simply a matter of how much electricity I can get a hold of and how fast I can dump it into the battery. The charger I have will run off of anything from 110 to 240. It has a dial that will allow me to limit the current (amps) it draws from the socket so that if I'm at a friend's house and they let me charge the car, I don't draw more than the outlet's breaker is rated for. The charger can only draw 20 amps off the service line which is probably the biggest limiting factor I face. But if the battery pack voltage is less than the outlet's, the charger modifies the power by decreasing the voltage to the battery pack, while at the same time increasing the amps it puts out to as much as 30. Such will be the case with the service line I'm running to the garage now.

For the sake of simplicity, lets assume I've drawn the battery pack down by 15 kWh's and the charger opeates at 100% efficiency (more on that in a moment). The charger will essentially be putting out 172.8 volts (the charge voltage for the battery pack) at 30 amps. Here's how it pans out:

172.8 volts x 30 amps = 5184 watts

15000 watt hours / 5184 watts = 2.9 hours

A 120 circuit is a different story. A 120 volt circuit putting out 20 amps equates to 2400 watts no matter how you cut it. The charger will modify that by raising the voltage to 172.8, but that means the amperage output will drop correspondingly to 13.8.

172.8 volts x 13.8 amps = 2400 watts

15000 watt hours / 2400 watts = 6.25 hours

As you can see a 120 volt outlet will take a while longer, but 6.25 hours is still quite acceptable. Now there is some loss when the charger is modifying the power from the wall and pushing it to the battery pack. It won't affect the time on the higher voltage charge, but the 120 volt charge time will likely go up by an hour or so.

So, there you go. Way more data than you ever wanted to know. When ever anyone asks me about range I'll simply refer them to this page. :-)


Gam0ra said...

Holy crap, thats almost as much power as my house generates in a day in the winter.

Tim Catellier said...

I know huh!? People will say to me "You could put a solar panel on the car and drive for free." Hmmm... If I could fit a farm of 20 solar panels on the car, I'd be close. Most people just don't understand the quantity of power involved in either endeavor. Can't blame them really, they haven't had to learn.