Thursday, June 13, 2013

Falling Batteries

Recently, while volunteering for a race event with the Cal Poly Amateur Radio Club, I found myself (with the help of a friend luckily) carrying a 26 amp-hour sealed lead acid battery and mobile radio one mile down a trail. This was much less than ideal and while walking up a somewhat annoyingly tall hill, I came up with the notion that the lead acid battery I was carrying weighed so much (about 20lbs) that it would actually have more gravitational potential energy than chemical energy by the time I reached the top of the hill. In other words, does lead make for such crappy batteries that it's more effective to carry a rock up a hill and use the energy from it falling than from the energy stored in the battery?

This got me thinking, and after a few calculations, it turns out that this definitely isn't true. By a long shot. Even the relatively inefficient SLA would need to be lifted to over 12 kilometers (yes I'm using SI units) before it's gravitational potential energy exceeded the electrical energy it could produce. I tried the same calculation with a number of other batteries to compare them, and with no surprise lithium batteries prove to be about twice as energy dense as SLA's. On top of this, they have a better charge/discharge efficiency. I was, however, a bit surprised to find that high-end lithium 18650 cells as used in laptops and Tesla cars are more than twice as energy dense as the LiFeYPO4 cells in my car.

All in all, this may have been a somewhat useless exercise, but you have to appreciate the humor in measuring the energy density of batteries in kilometers from the ground.

Saturday, June 1, 2013

...And we're back

After a few emails back and forth with EVolve electrics my charger parameters were finally programmed into a shiny new Elcon PFC4000 charger to replace the old, somewhat blown up Zivan NG5. I expected the new charger to be about a wash with the old one in terms of power output and efficiency, but wow the new one has exceeded my expectations. There's no longer any awful 120hz noise on the power grid while charging, and the Elcon charger seems to have a much better power factor, because there's less AC current and I can no longer hear a 60hz hum near my breaker panel while charging. The whole charger barely gets warm to the touch even in mid-charge, whereas the old one was spewing hot air like a hair dryer. On top of that, it's putting out more power and charging the car quicker!

Here's the charger after having just unpacked it. It arrived in about 9 days even after some back and forth emailing to get the parameters straight. I must say I'm very happy with the new charger and it was well worth the upgrade.

Here you can see the charger in the car next to a mess of wiring
that I unzip-tied during installation. 

On top of the efficiency, it's even capable of charging from a 110V outlet, which the old one wasn't, and it can be adjusted to different finishing voltages using a button on the side. It has a sealed DIN connector with two wires that enable or disable the charger, perfect for connecting to my BMS so that I no longer need a 30A relay to switch off the AC to the charger. The only thing I would change is that the fans stay on all the time while the charger is plugged in, even when it is disabled using the DIN connector. The way it is now the fans will still be needlessly going at full speed, even after the car has been charged for hours. This isn't a huge issue though, so I'm not that concerned about it.
The next two weeks I will undoubtedly be busy with school work and finals, but this summer one of my (many) projects will be to make the 12V system in the car a bit more resilient, as it seems the small DC/DC converter and SLA I have aren't able to keep up in the event I have the headlights, stereo, and brake vacuum pump on all at once... which causes the BMS to shut off and the car to disable, so right now it's at the top of my list.