Tuesday, March 9, 2010

Tango with 32 kWhr pack of Headway LiFePO4 cells goes 108 miles on a charge

Because it was cold and snowing at times, and because this was only the second cycle on the battery, we only used 28 kWhrs out of the 32 kWhrs that the cells are rated for. The Tango used 260 Whrs per mile at an average speed of about 60 mph on highways with about 400 ft of elevation change. The Elithion BMS gave the following status at the end of the trip.

Cell vtg [V]: 3.05 min(# 46) 3.14 avg 3.18 max(# 20)
Total vtg [V]: 314.7
Cell bd temp, all [C]: +23 min(# 14) +29 avg +35 max(# 46)
Cell bd temp, load off [C]: +23 min(# 14) +29 avg +35 max(# 46)
Cell blk res [mOhm]: 25.4 min(# 0) 25.4 avg 25.4 max(# 0)
Total res: 2540 mOhm
Loads on: 0 (Not balancing: avg voltages below min bal vtg)
Cell boards: 100 (# 0 ~ # 99), all reporting
Banks: 12 (# 0 ~ #11), all reporting

This Headway pack has 1,000 cells mounted in parallel groups of 10, and 100 blocks in series. These were then packaged into 12 modules as pictured below.

There is approximately 200 lbs of lead added to the bottom of the battery box to make up for the lighter weight of the lithium cells. Some of this is visible in the photo below.

There are cells available that would allow 50 kWhrs or more to be fitted in the Tango battery box. This would give up to a 200-mile range.



Anonymous said...

I really like this vehicle.He looks like a fish but I do not know what.He has a lot of adventages, but a less-is too low.I greet you from Poland.

RodgerC said...

Why were Headway cells chosen over those from Thundersky, A123?

Was it availability, reliability, cycle life, safety, performance (C rate discharge capability), cost, compatibility with your pack design, etc.?

Kind Regards,
(A wannabe electric car enthusiast)

Tango Electric Cars said...

Headway cells were chosen for all of the above reasons.
1. Availability: Thunder Sky cells are certainly available, however, even though the Tango has probably the safest battery box in the industry, without full FMVSS (Federal Motor Vehicle Safety Standards) certification, A123 won't allow their cells to be used in a vehicle application.
2. Reliability: The Headway cells excel in reliability for high-power applications, as they are designed to handle 20C for bursts and 4C continuous, the Tango specification.
3. Rated cycle life is promising, but unfortunately there is no guarantee that they will deliver 200,000 miles yet. This is the kind of guarantee that we need from cell manufacturers. It's all that matters to the customer. What is the cost per mile? Just the cost of the cells alone, if they last 100,000 miles, that's $.13 per mile. If they last 200,000 miles, that's $.065 a mile, or roughly half the price of gasoline, just for battery replacement. That does not include the labor to assemble them.
4. Safety is excellent with LiFePO4 chemistry. In two different incidents, we've tested the cells to an over-discharged condition. The headway cells simply shorted completely, passing the current through, even with extremely high current draws, and there was no thermal damage at all.
5. C-rate really limits our choice of cells. The Thunder Sky prismatic 100AH design sold by a number of companies, cannot withstand 20C, or 2,000 amps, in the Tango's case, for short bursts. Headway and A123 are capable of these bursts.
6. Cost is higher with Headway than with the Thunder Sky type cells, however not nearly as expensive as A123 or some of the polymer cells that can withstand high currents.
7. Compatibility with pack design: Headway cells require 10 times more assembly than the Thunder Sky design, however, it's about 1/5th of the assembly of A123 cells. 100 to 108 Thunder Sky cells in series make a nice pack for the Tango. It's extremely easy to assemble. I only with that they could meet the power requirement. The Headways require 10 times more cells, 1,000 in all. They have to be screwed into custom-designed plates to make modules of 70 to 90 cells each. Then, 12 modules are connected in series. Each module has 7 or 10 series strings of 10 cells in parallel. With A123 cells, 4,800 cells would have to be welded into modules. That is much more complicated.
We are now in the process of designing a polymer LiFePO4 pack that will carry 62 kWhrs, or about 240 miles of range at 60 mph on level ground. We have various pack designs to meed different customer's needs.