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EV Traction Battery

This page discusses the traction battery which collectively refers to all the individual golf cart batteries installed in the car as a whole. The traction battery stores the energy used to power the car. My current traction battery consists of 21 Trojan T-105 golf cart batteries.

Toyota MR2 EV front compartment, just running
Toyota MR2 EV front compartment, just running
Toyota MR2 EV rear compartment, just running
Toyota MR2 EV rear compartment, just running
Toyota MR2 trunk with batteries.  Still have usable space for cargo.
Toyota MR2 trunk with batteries. Still have usable space for cargo.

Traction Battery Statistics

Curent Battery:

  • Technology: Flooded Lead Acid.
  • Battery Model: Trojan T-105 with UT type terminals.
  • Battery Count: 21
  • Pack Voltage: 126v
  • Nominal Capacity (kWH): 28
  • Practical Capacity (kWH, estimate): 16
  • Layout: Ten in engine compartment, seven in front compartment. Four in sunken compartment of trunk, preserving most space for cargo.

Considerations

Technology

How did I decide on this type of battery? There are many battery technologies out there, some old and proven, and some new, exotic, and very promising designs, and a lot of snake oil. Even within the realm of lead acid, there are a multitude of designs. My EV Battery Considerations page attempts to summarize my thought process in deciding to stick with the old standby (flooded lead acid) for the battery in my Toyota MR2 EV.

Form factor

Having decided on flooded lead acid batteries, I still had to choose a form factor (voltage and physical dimensions) that would fit in the car and provide the performance and range I wanted. I had to decide this early on based on my desired range (My EV Performance Analysis told me how much battery weight I would need to carry.) The voltage of the battery would have to be compatible with my EV Electronics, including the controller and motor I chose to use, and choosing these was based on my desired performance (adequate but not exceptional). Once those two factors were decided, I just had to pick the battery that would fit most closely. In my case, I found that 8-volt traction batteries, such as the Trojan T-875, were the best fit. Knowing this, I was able to embark on Chassis Modifications and the construction of the EV Battery Racks to hold the batteries.

Why T-105s?

I designed my car for Trojan T-875 8-volt golf cart batteries. Why did I end up installing 6-volt T-105's instead, which impact the performance (top speed and acceleration) substantially? Basically I goofed up. The type of terminals usually supplied with 8v traction batteries are not appropriate for EV use. (They can't handle the high current loads) and I would have had to special order them. I failed to consider that. I had already built all my cabling to fit the old T-105s I had bought to use as templates for getting the car going. Also, the Terminal positions of the T-875s are different than the T-105s, so I had made all my cabling layout all wrong. Basically, I need to order off (and wait months) for the T-875s with the right kind of terminals, and re-make all my cabling in order to install the right battery pack. After driving the car with a traction battery of seventeen T-105s for a few months, I had the opportunity to buy four more T-105s at a very good price, so I went that route, and did my Range and Performance Upgrade to bring the traction battery up to its current specification.

Management and Maintenance

Some battery types require sophisticated management to control charge and discharge, and monitor temperature and other factors. With flooded lead acid, this is not necessary, making the setup cheaper and simpler. The tradeoff is that you do need to manually check the batteries periodically to ensure they are balanced and that the water level is right. A reasonable interval is to perform battery balancing and watering every 500 miles of driving (10-20 charges) or so. It only takes a few minutes to do this work, so it isn't as onerous as it seems.

While my flooded lead acid traction battery does not require a battery monitoring system, I will be building one that will allow me to monitor voltage in real time on each individual battery within the traction battery. This will allow me to spend less time manually looking for imbalanced batteries, and will allow me to detect weak batteries before they fail. Finally, it will allow me a very accurate picture of the overall operating efficiency of the car. I am in the early stages of designing a system to do this. There are systems available retail that will perform this function as well as others, but I am going to design my own. The current plan is a parasitically powered voltage transducer (powered by the battery that it monitors) on each battery that wirelessly transmits its readings to a central computer which collects statistics while the car is running. These stats will be downloadable to allow for insertion into a spreadsheet, graphing, etc. The system will also have some kind of real-time readout in the car. More to follow.