Toyota Prius Owners Are Swapping Tired Old Batteries For Lithium-Ion Ones And Seeing Big Gains

The Toyota Prius was the first hybrid to market when it went on sale in 1997 (though the Honda Insight beat it to market in the U.S.) and the main model line has spawned five generations of cleaner, more efficient vehicles. As you might imagine, that leaves thousands of older Prius models driving around with high miles on the clock and tired batteries. Salvation is at hand for the tinkerers out there, though. Some owners are now electing to pursue battery upgrades for their aging hybrids to unlock better fuel economy and slightly improved performance.

The key is that until the fifth-generation model, most examples of the Prius were built using older nickel-metal hydride (NiMH) technology. While these were historically cheaper than newer lithium-based batteries, they make a tradeoff in performance. NiMH battery packs are larger and heavier than lithium packs that store the same amount of energy, and generally can’t deliver anywhere near as much power.

Thus, with aging, old-technology batteries, early Prius models are ripe for a battery upgrade. Project Lithium, also known as NexPower, is the main player in this space, offering a range of lithium cell upgrade kits for a wide variety of older Prius models. Beyond that, the company also offers upgrade kits for other Toyota and Lexus hybrids like the Camry Hybrid, GS450h, and the Alphard and Vellfire vans.

It all comes down to drivability and efficiency. With a weak or dead hybrid battery, a Prius can be a lot less fun to drive every day. It forces the car to rely more on its ICE combustion engine, and there’s a loss of the beneficial low-down torque from the electric motor when the battery can’t deliver. For a tired old car, a lithium battery upgrade can fix all those problems. It can even go further because, with additional battery capacity, the motor can work for longer before the saved juice in the battery runs out. That’s really helpful when going up long hills, or when trying to drive the car longer distances in EV-only mode.

Jack, the man behind Project Lithium, openly shares information on the project on his YouTube channel.  In one video, he walks through the process of upgrading his 2006 Toyota Prius. After 150,000 miles, the original NiMH battery was dead. He ably demonstrates how to remove the stock battery and replace it with the new cells. The Prius’s battery is at the rear of the car, just behind the rear seats. It’s accessed by pulling out the back seats, along with a bunch of panels, trim and carpets out of the trunk. It’s important to know what you’re doing during the removal process to avoid any electrical safety issues, but Jack makes it look easy. He pulls out the pack, noting that the stock part weighs a hefty 80 pounds or so.

Out of the car, the construction of the battery is obvious. It’s made up of multiple separate blocks all connected together. In total, it adds up to an NiMH pack with a tiny capacity of just 1.31 kWh when new. Compare that to the hefty 100 kWh pack you’d find in a Tesla Model S Plaid.

As Jack demonstrates, the interconnects between the cells are easily removed to break the pack into its individual modules. It’s very simple compared to modern EV packs, which are full of cooling channels to enable higher performance and power delivery. These modules are removed from the battery frame, and replaced with the new modules from Project Lithium, which are full of high performance lithium cells. The new battery modules can then be hooked up with the stock interconnects, the battery frame can be bolted back together, and the whole assembly is ready to go back in the car. As a bonus, the more energy-dense lithium cells mean the pack weighs roughly half as much as the stock unit. Once installed, the battery’s vital signs can be inspected via the Dr. Prius phone app.

It’s not a project that you should take on if you’re completely unaware of the dangers of electricity, but it’s not beyond the realms of the educated amateur mechanic. Alternatively, Project Lithium has a network of installers that it works with to provide customers an easier option. The packs from Project Lithium start as low as $2,455 for the Prius C, which only needs 10 drop-in replacement modules. A Toyota Highlander or GM Tahoe hybrid uses double the packs, and the upgrade kit costs closer to $5000 when it’s in stock. That’s not a bad price for a brand new battery pack, even despite the low capacity. After all, it took significant effort in research and development to engineer the packs as a near-drop-in replacement.

Performance benefits are slight, but noticeable, according to the company, and many forum anecdotes concur. This is because the Prius motor and supporting electronics were only designed to draw so much current, based on the capabilities of the NiMH battery. However, where the lithium battery upgrade comes in handy is that it can keep the motor running at full power for much longer than the stock pack. Thus, it won’t necessarily provide much more punch, but what punch is on offer from the stock drivetrain will be on tap for longer. It’s a boon in sustained high-load scenarios like going uphill.

The killer app is in fuel economy, however. TheChip, owner of a third-generation Prius raved about the upgrade on the Prius Chat forums. “I managed to get 54 mpg on my round trip, even with some leadfoot driving. Previously, I’d have been around 38-42,” he noted. Do the maths, and that’s 26% less fuel, a healthy win you can take to the bank. He also noted the ability to get a full 3 miles of all-electric driving, something non-PHEV Prius models never really excelled at. Previously, he couldn’t even achieve a full mile. Other owners have posted smaller but still noticeable gains. On Reddit, owners with the lithium packs noted mileage gains on the order of 10%, or roughly 4-5 mpg.

A small additional benefit is that the extra battery power can come in handy for accessories, too. Some owners have enjoyed the ability to sit in their cars for a quarter of an hour with the AC on, with the engine dutifully remaining switched off. It appears that longevity hasn’t proven to be an issue in the wild, either, as some have had the batteries in use for several years now, racking up thousands of miles.

Gains to fuel economy are great, but it bears considering the costs. The kits from Project Lithium cost a few thousand dollars. So, for example, let’s say you live in California, where the gas is $4.80 a gallon, and you do 15,000 miles a year. At around 45 mpg, you’re using about 333 gallons of gas a year. Bump that to 50 mpg, and you’re only using 300 gallons of gas a year. So, saving 33 gallons at $4.80 a gallon nets you $158. Even if you were only getting 40 mpg before, you’re only saving $360 a year.

So, ultimately, swapping out the battery for better fuel economy maybe isn’t the smartest investment. Really, the biggest gains to be had are for a Prius with a dead battery. Not only will fuel economy improve, but the car will be much more drivable when it can properly use its electric motor again. Hence the tagline: “Fall in love with your hybrid again!” In that situation, even a reconditioned NiMH pack will be a boon, but the lithium upgrade is a nicer way to go. Oftentimes, it’s desirable when repairing a car to put in something better if technology has moved on. Think of it as akin to fitting electronic ignition to a classic muscle car that still has points.

Ultimately, it’s neat to see an aftermarket company spring up to provide better, more potent battery solutions for hybrid vehicles. Once upon a time, hybrids and EVs were these new, shiny things. Now, they’re becoming familiar to us, and a grassroots industry is springing up to support, maintain, and even improve them. As enthusiasts, that’s something worth supporting, especially when it’s keeping more of these cars on the road for longer!

Image credits: Project Lithium/NexPower/Dr.Prius