Monday, June 8, 2009

Porsche Speedster Achieves 100 miles on a single charge

We recently upgraded the 1957 Porsche Model 356 Speedster electric conversion by adding two additional Thundersky LiFePo4 90 Ah batteries in the rear engine compartment.

We placed polyethylene grommets over the terminals of the existing exposed batteries in the rear engine compartment to act as standoffs. We found these on the counter at the local NAPA auto parts store in a large plastic container for $1 per. They are actually grommets for air-brake lines on large semi tractor trailer rigs.

We used these to support 1/4 inch Tivar "shelves" to mount the new batteries on. Tivar is the brand name for high molecular weight polyethyline (HMWP). This has a very high dialectric strength, low friction, pretty good heat tolerance (140C) and is physically quite strong and light. I still think of it as Teflon, although it really isn't.

We used 24 inch nylon zip ties to secure the batteries to the Tivar shelves and the entire rig to the existing battery boxes. Frankly, it looks a bit unprofessional. But it is quite strong, quite safe, and allows us to easily remove them to access the batteries beneath.

A video of the process.

Of course, along the way, we suffered a bit. In installing the batteries, we had to fashion new cables to add one battery to the negative end of the forward battery pack, and one battery to the positive end of the rear battery pack. I dropped one of the cables accidentally, and the terminal end contacted one of the terminals of the batteries underneath.

Despite having all battery switches firmly off, this did connect one end of the pack to the middle of it before the cutoff switch. Even with about 1/3 of the pack voltage employed, contact was but momentary as the heavy 2/0 lug on the end of the cable EXPLODED showering the interior of the car with FLAMING STEEL DROPLETS. I'm always amazed at how easy it is to forget that we are dealing with 20 kW of electrical power here.

The danger is not so much electrocution, although at 120volts and almost unlimited amperage, that is certainly in play. More commonly is the burn hazard. If you got a wristwatch or ring across a couple of terminals or terminals and a wrench, you would get full possession of your arm back AFTER the wristwatch turn bright red, then melted, and burned your hand off, which would fall to the ground. THEN you might get loose. The burn hazard from shorted metal is very real. We have arc welded screwdrivers and wrenches to batteries and they are permanently stuck until they MELT and burn through.

So no rings or watches around the car during maintenance, and we have wrapped the wrenches we commonly use by slipping some milspec shrinktube around the wrench along its length and shrinking it in place with a heat gun.

In any event, we did drive the car just under 100 miles (160km) with the two extra batteries.

We DID DESTROY A BATTERY IN THE PROCESS proving rather emphatically the dangers of over discharge. One of the batteries added was just not quite balanced in with the rest of the pack, although we did add some energy to it when we put it in. After discharging, it ran out FIRST and went to zero, blocking any further current from the rear pack.

As the rear pack had significant energy remaining after the run, I think we have well over 100 miles absolute max range, assuming we properly balance in the replacement battery.

The destroyed battery was at absolute zero volts when we took it out. One cell eventually came back to full voltage, although we have not tested capacity on it in the lab. (To do list). The remaining three cells were irrecoverably destroyed.

I know this sounds kind of bumbling, but it's rather part of the program. Most of the "advice" and wisdom concerning these batteries that we have found on line are just repetitions of what the author saw written in ANOTHER forum somewhere, or wild theorizing based on vague manufacturers notes, translated of course into Chenglish. I call this very popular online game of offering very authoritative advice based on NO direct knowledge TYPING YOURSELF SMART. These guys repeat this stuff endlessly to each other with great energy, virtually none of them ever really testing much to the limits.

And with good reason. The battery destroyed here was about $600 with shipping etc.

I resolved myself to this rather early in the game. We bought 3 GEMS when the batteries first came available and outfitted them with aluminum tool chests on the rear bed. We could then try out the various batteries in small quantities, before attempting a full car project.

We started with Seidens's, then HiPowers, and ultimately Thunderskys. That's how Thundersky's came to be the battery of choice for the Porsche. They seemed better balanced, but to tell the truth, the hardware was just better. The others had very lightweight aluminum screws holding cheap connector bars. The Thundersky's have flexible copper straps of several layers, heat shrinked in the middle, and connected to good quality terminals with good steel standard 8 mm bolts, washers, and lockwashers.

But of course we learned a good bit about discharging and charging and balance problems on the GEMS.

Along the way, in the shop behind the garage we added a 15 volt 100 amp power supply, a 100 volt 10 amp power supply, a 160volt 30 amp charger, a very expensive lab quality multimeter, and oscilloscope, a programmable load, and a Mountain Radio battery test rig that works with a laptop computer to actually drain a battery at a constant current level, graphing the voltage down to whatever cutoff you set, and totalling the amp hours drawn from the battery in the process.

We also built a load with built in ampmeter and voltmeter out of five 5000 watt 1/4 ohm resistors when we REALLY want to put the screws to one.

Bottom line is that I guess you would have to say our mission is not so much to save batteries, but rather to destroy them - artfully. I don't really know another way of definitively finding the limits on these batteries other than to exceed them and see what the results are.

As most hobby converters are in a bit of a financial strain to go to LiFePo4 packs in full sized car quantities anyway, they are deathly afraid of hurting a battery at all. And I well understand this. We are just resigned to the expense of it to gain the knowledge.

And that's pretty much what we are about, learning and fun. In response to the numerous inquiries, we are NOT in the conversion business. No I won't convert a car for you. No, the Speedster and the other cars are not for sale. No, we are not prototyping a new line. I would as soon give myself a root canal with a pocket knife with two light beers for anashesia as I would have anything to do with the automobile manufacturing business.

I'm kind of exploring video as a new kind of magazine format. And we play with electric cars because they are fun, and they involve quite a bit of learning new things, and unlearning old things. And I like all that.

So that's the gig. We will spend what we need to spend to learn the technology. And if there is a "project" in there for me, it is a variation on the Boardwatch Magazine gig, only probably in video, instead of print.

I think electric cars can change the world. And I find good and plenty reason why the existing automotive manufacturers are going to have a difficult time providing a REAL plug-in electric car any time soon. And that leaves the grassroots efforts of the do-it-yourself builders and converters to lead the way - for some time to come I fear.

But my personal interest is to learn, (not by typing furiously until I get smart, but actually by doing) demonstrate, educate, and help the many people who are energetically organizing a "resistance" movement to wrest control of our personal mobility FROM Government Motors and Exxon.

If I have to burn up a couple of Thundersky's in the process, you can't imagine how little that means to me.

In any event, the Speedster now has a declared "safe" range of 85 miles per charge. But yeah, I think it will do 100 in a pinch.

Jack Rickard