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Range and Performance Upgrade

This page summarizes the work I did to improve both the Range and the Performance of my Toyota MR2 EV. The reason for this upgrade was to make up for a goof that happened when originally planning the car. As detailed on my EV First Impressions page and EV Traction Battery page, I designed my car for a 136 volt traction battery, but ended up installing a 102v pack instead. While the lower voltage did not affect the range of the car, it did substantially reduce the performance of the car, to the point where it was only just adequate on the highway.

Four more Trojan T-105 batteries, to be installed in trunk.
Four more Trojan T-105 batteries, to be installed in trunk.
Battery rack to carry four batteries in trunk.
Battery rack to carry four batteries in trunk.
Battery rack installed in trunk.
Battery rack installed in trunk.
Toyota MR2 trunk with batteries.  Still have usable space for cargo.
Toyota MR2 trunk with batteries. Still have usable space for cargo.

Why?

Gee, Brian. With all the planning you obviously did, why are you upgrading the car? Is it not meeting your needs? Well, as I say in various places on this site, I originally planned the car to have an EV Traction Battery consisting of seventeen Trojan T-875 golf cart batteries, for a 136V system. I goofed up in that those batteries were not available with the terminal types I needed, and had a different terminal configuration that would have required me to wait months for the right batteries, and rebuild all my cabling. So I installed a 102v battery pack with seventeen T-105 golf cart batteries for a 102v system. The range was the same (if not slightly better) than the 136v traction battery would have given me, but the top speed and acceleration were a bit lacking at freeway speeds. (Usable, but not what I planned for). So, I chose to upgrade the car.

The Options

My only options to fix the performance problem were to:

  • Either sell off, or wait for my 102V battery pack to wear out, and then order off for the originally planned T-875 batteries with the correct terminals, rebuild all my cabling and install the new battery pack. The car's performance would be substanially improved, the range would be the same as now, the weight would be about the same. This option would be rather expensive, as I doubt I could have gotten more than 50% of the original retail price for the T-105 batteries I already had, plus rebuilding all the cabling would have been another couple of hundred at least.
  • Get ahold of three or four more T-105 batteries, and add a fourth EV Battery Racks location in the trunk of the car. Adding these batteries would improve the top speed, as well as adding substantially to the range of the car. The tradeoff would be more weight, and the loss of some trunk space. I would need to do more fabrication work (build a new EV Battery Racks location) and sacrifice some trunk space.

In both cases, I would need to re-install the boost transformer into my EV Charging System, and re-install my original Intelipower DC-DC converter, that I had pulled from the car since it would not run on 102VDC. No cost here except time, since I already have these components.

What I Did

As it turned out, the decision was basically made for me when I got the opportunity to buy four additional Trojan T-105 batteries that were in new condition, and nearly exactly the same age as my current ones, for about one half of retail price. I bought the batteries in a heartbeat, and this sealed the deal as to which direction I will be going: a 21 Trojan T-105, 126v EV Traction Battery. The car will still be a bit slower than it would have been with the 136v 17-T-875 pack due to higher weight and lower voltage, but it will have about 25% more range and should easily cruise at 65mph if I want to go that fast. By building a sunken battery box, I will only lose about 6" of trunk depth, so it will still be usable for groceries, my tool box, and other smallish cargo.

Process

The basic steps in this upgrade process are:

  • Build a battery rack that will support four Trojan T-105 batteries, about 240lbs of weight.
  • Cut a hole in the trunk and install the battery rack. Seal around the edges, and paint the trunk with truck bed liner, as I did the other compartments that carry batteries.
  • Cut plastic liners for the battery rack, to help keep wind, water and rock hits away from the batteries.
  • Upgrade the Rear suspension to carry the additional weight. I will add 1" spring spacers to lift the rear a bit. The Custom springs I had made previously should be strong enough to handle the extra weight, with the spacers installed.
  • Build the additional cabling to hook in the new batteries, incorporate one more fuse.
  • Come up with some kind of charging ventilation system, like in the front compartment.
  • Re-install the charger boost transformer.
  • Re-install the original intellipower DC-DC converter that I had to replace due to running a 102v pack.

Completion!

This work was complete as of March 16th, 2008. I took a short drive around the neighborhood to make sure everything was working, which it was. I haven't tried out the new top speed yet, but at least in 3rd gear it was still happily accelerating at 50mph, while before the upgrade 50mph was the maximum speed for 3rd gear. I expect it will easily hit 70mph now in 4th, whereas before it topped out around 60. Despite the extra weight, the car now can climb the steep local hills 5 or 6 mph faster as well. Handling still seems alright, despite another 250 lbs of weight in the back.