Stop Buying Cylindrical Cells. You Are Getting Ripped Off.

This has been bothering me for years, and after a conversation yesterday I need to put it in writing. There are companies right now — GreenTec Auto, NewPriusBatteries, AliExpress, Alibaba and others — selling cylindrical NiMH cells as replacement packs for Prius and other hybrids. They're acting like these new batteries are equivalent to OEM, they're not. They're a step backward to a design Toyota and Honda both moved away from over two decades ago, and there's good reasons why.

When Honda and Toyota were first developing hybrid cars in the late 1990s, cylindrical DD-size cells welded into sticks was the most viable option. It was the technology of the moment and got the job done well enough to get the first hybrids on the road. But when Toyota launched the Prius to the global market, they ditched the cylinders and moved to a prismatic design. Toyota's own thermal evaluation research confirms the pack's innovative forced-air cooling system was a significant improvement thanks to that prismatic change. Honda followed suit not long after, abandoning cylindrical for their own NiMH architecture also with better cooling and less internal resistance.

Why prismatic are better than cylindrical:

• Failure points. The OEM Prius battery pack uses 28 prismatic NiMH modules. Each module contains six internally connected NiMH cells that are sealed and mechanically secured by design. Cylindrical-cell replacement kits reduce the pack to 14 larger modules, but they introduce approximately 169 external spot-weld connection points. Each of these welds becomes a potential failure point, especially over time as the pack goes through repeated heating, cooling, expansion, and contraction cycles. These spot-weld joints are a known weak area compared with the OEM sealed module design.
  
  
• Designed for lower-power applications. A traditional cylindrical cell is essentially a large jelly roll inside a metal can. The current must pass through relatively small internal tabs connected to the cathode/positive and anode/negative layers, which can create higher resistance and limit current flow under high-load conditions. Tesla introduced a tabless design with its 4680 cells to improve current throughput and reduce resistance, but even with that improvement, cylindrical cells still generally have much more current-path limitations compared with prismatic or pouch-cell designs.
  
  
• Volumetric efficiency. Prismatic cells fill roughly 72% of available volume versus around 50–60% for cylindrical — meaning more energy density in the same physical space: http://www.lifepo4batteryshop.com/blogs/cylindrical-prismatic-pouch-battery-comparison.html
  
  
• Pack integration. Prismatic modules slot cleanly into the OEM compression frame. The entire structural design of the battery enclosure assumes flat modules under controlled compression.
  
  
• Thermal management. The flat form factor allows controlled, uniform airflow between modules. With cylinders, you get hot spots — particularly in interior modules that can't shed heat as effectively. This matters enormously in a pack's longevity because it lives behind your rear seat and relies on cabin air to stay cool and has to deal with the challenge of sometimes the inside of a car becoming incredibly hot.
  
  
• Difficult to reverse back to OEM. Installing a cylindrical-cell kit will require removing the original Toyota wiring harness and hardware from the hybrid battery tray and retrofitting a different hardware designed specifically for cylindrical modules. This means that if you ever decide to convert the pack back to the OEM prismatic-module setup, which you will sooner than later, you will have to purchase a replacement wiring harnesses, copper bus bars, nuts, and all the other hardware. Those parts add up, so the conversion is not easy or inexpensive to reverse.

Over the years I've spoken with many hybrid owners who went the cylindrical replacement route. Getting barely 1~4 years out of a new pack before it starts throwing codes is not unusual. The voltage spread grows fast, internal resistance climbs, and the car starts throwing warning lights for too much voltage difference between blocks.

Just yesterday I talked with someone who paid GreenTecAuto.com more than $2K for what was sold as a brand new pack. When he shared the data the numbers looked like an old pack on its way out. When they went back to GreenTec, they were told there are no refunds. That's more than two grand gone and he also lost his old pack to them which represents a core deposit of $1350 he can't get back when he wants to buy a new OEM pack from Toyota.

Also for me personally, six years ago I was doing Honda cylindrical module reconditioning in my studio apartment in winter. I was working on Bumblebee brand battery sticks for a Honda Hybrid and during the third round of a low-amp discharge cycle, two separate welds let go and I had leaking electrolyte as a result. These were actual explosions at the weld points that were louder than a firecracker and I would of been injured had I been near the packs. I've not experienced this problem with other cylindrical manufacturers, but these reasons and likely others Bumblebee Battery went broke and were bought out.

What's more, NiMH electrolyte is a potassium hydroxide solution, and according to its own safety data sheets, contact with skin causes severe irritation and chemical burns. In my 8 years of working on prismatic battery packs I've never had a problem with electrolyte exposure, but with the cylindrical I've experienced problems more than once and so have others. Here's a photo of Jack of Dr. Prius App with his hand injury from a leaking cylindrical pack that took a full month to stop being painful and itchy.

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Lastly, I want to address the counter-argument some sellers use. Some point to the fact that some manufacturers like Tesla use cylindrical cells, and they claim cylindrical geometry provides better thermal management and lower internal resistance than prismatic, but as explained above these aren't anything like the cylindrical design that Tesla uses. And even worse, some sellers like Greentecauto put a warranty voided if seal broke sticker on the cover and don't tell the customer they're buying an inferior design and it's difficult to see what you bought if you aren't allowed to open the cover.

There's nothing Tesla-like with hybrid cylindrical modules. A Tesla's cylindrical lithium cells live in an active liquid-cooled system engineered from the ground up for that geometry. Your Prius uses passive forced-air cooling through a system that Toyota's own engineers specifically designed around prismatic modules. Dropping cylindrical NiMH sticks into that enclosure and claiming it's an upgrade on a failing battery pack is dishonest.

If you need a replacement pack for your Prius or hybrid, buy new or refurbished OEM prismatic modules or Sodium Ion from a reputable manufacturer. The original engineers made the right call when they moved away from cylindrical. The aftermarket vendors pushing cylindrical packs are not innovating, they're cutting costs by ripping people off with an inferior product.