Battery Expert Weighs in on Tesla’s Powerwall
Shawn Wasserman posted on June 24, 2015 |
From safety to practicality, does the Powerwall make sense?

The announcement of the Powerwall, a rechargeable battery for home use made by Tesla, has been met with a lot of praise and scepticism from many engineers.

David Petersen, vice president of Vecture, a leading design and manufacturing company for battery management systems, has an interesting prospective on the new technology. Given what he knows — and doesn’t know — about the Powerwall, he has some questions with respect to the safety and economics of the product.

How Safe Is the Tesla Powerwall?

Japan Airlines Boeing 787 Li-battery fire of 2013. Courtesy of National Transportation Safety Board.

Japan Airlines Boeing 787 Li-battery fire of 2013. Courtesy of National Transportation Safety Board.

When asked about the limitations of lithium battery technology, Petersen went straight to the elephant in the room: safety.

“Safety is of concern and can be a major limitation of the program, should a battery issue develop,” Petersen said. “Different lithium chemistries yield different characteristics, some safer than others.”

Petersen asked, “Did Tesla choose the appropriate chemistry for in-home energy storage?” From phones and cars to computers and airplanes (JAL 787 battery fire), there is not a single lithium chemistry that is best for all applications with respect to safety.  

“No one wants a battery issue in their house,” said Petersen. “A good lithium battery system needs to include high-quality cells, a battery management system [BMS], cell balancing and a means to limit thermal propagation between the cells and to the outside.”

He added, “There is always some percentage of failures in battery packs and battery cells. As the percentages are slight, and small electronics like computers and cell phones do not have many cells or other components, we don’t see them that often.”

Systems like Tesla’s Powerwall and other battery-powered cars, however, can have thousands of cells and other components. Petersen explained that, without a system to prevent a chain reaction, one failure could cause a big issue. Therefore, assuming the percentages of single failure remain constant, the increased number of cells and components in the battery will increase the overall chances of failure.

“In the end, the cell chemistry, manufacturing processes [cell and battery system], safety systems, temperature and electronics will play a huge role in system performance and customer safety,” explained Petersen.

Tesla Powerwall System Needs an Inverter

Petersen was also surprized to see that the Tesla Powerwall is not sold with an inverter. He noted that a system like this will need an inverter to operate. He added that, “Prospective buyers will need to understand their specific energy usages and costs to properly size both the battery system and inverter to maximize their full benefit.”  

Furthermore, Petersen explained that inverters are typically 90-94 percent efficient. Given the 92 percent efficiency spec for the Powerwall battery, you will see an overall efficiency of about 83-87 percent. However, that will not take into account the efficiency of other connections.

Users will need these efficiencies and other information when sizing their inverter and assessing the economics of their Powerwall. “Until a specific application is fully evaluated, it is difficult to determine if the technology is economically viable,” said Petersen.

The Economics of a Tesla Powerwall

It wasn’t just the lack of an inverter that made Petersen question the economics of the Powerwall. There’s also the 10-year life guarantee of the battery.

Petersen said, “With the chemistry used, you can expect 800 to 1,000 full cycles. That isn’t really cutting edge in the lithium-ion world. If you cycle it fully, twice a day, for a year, that’s already 730 cycles.”

“I don’t know the warranty’s details,” he explained, “but, to provide a warranty for such a time, it must be prorated or a combination of limiting the amount of cycles, limiting the charge voltage, or limiting the depth of discharge. Or perhaps they just expect many users will not make a claim. If they do, then the price of the system will be much less expensive than it is today.”

“It’s about cost per life,” Petersen said. “If I spend $3,500 for battery that has a 10-year warranty life, then I will want to use it as much as possible.” To that end, Petersen brought up high-quality NMC batteries with 3,000 to 4,000 cycles, high quality iron phosphate batteries with 6,000 cycles, or even lithium-titanate batteries with 14,000 cycles. He suggested that these batteries might make more sense over time despite their higher initial costs.

The finances of the Tesla Powerwall are also dependent on its use case. Currently, the battery is marketed for three use cases: power shifting, green energy storage and backup power supplies.

If you were to use the battery for power shifting, Petersen said, “you will need to know what is the peak and off-peak price per kilowatt from the electric company. The difference can then be multiplied by the total amount of battery energy storage to determine how much money can be saved per day. This amount can then be divided into the total system cost [battery, inverter and installation] to determine the system’s payback.”

As power shifting may not be financially feasible for many households, Petersen suggested that off-grid green energy storage might be a better option. “However, this will typically need a higher cycle life for the battery,” he said. This is again problematic given the unknowns associated with the battery’s warranty.

Petersen suggested that the best option could be if the utility companies take part in the sale of the Tesla batteries and others like it. “After all,” he said, “it might be in their best interest to subsidize or lease the battery, inverter and installation. They could then maintain a grid to meet the lower level load requirements as opposed to one for the rare extreme condition occasions.”

“With the participation of the utility companies, and possibly government, the speed at which batteries, and Tesla, becomes accepted in the home or in local community storage will drastically increase,” explained Petersen. “This supply and demand could make Tesla’s gigafactory the first of many more to come. However, without utility or government involvement, or the ability for Tesla to get their battery prices lower, it could prove to be a costly investment.”       

Tesla was contacted to comment on Petersen’s claims. They did not respond.

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