My version of Li-ion modules for Toyota/Prius NiMH

A year ago I met the owner of a Toyota Auris hybrid who challenged me to make something to adapt lithium to his car, and as the owner of a lithium battery manufacturing company, I accepted the challenge.

I was already familiar with NextPower Cell, but in Europe (I'm from Spain), it's almost impossible to find. I bought a couple of individual modules to see how they did it, but I didn't like either the cell type or their construction.

I decided on 21700 high-discharge Li-ion batteries with copper screws. After passing all the tests required to be sold in Europe (CE and IEC), this is the result.

We come from this thread
Lithium or sodium for replacement | Page 5 | PriusChat

 

we were discussing this project in another thread. I'm leaving a screenshot of what I'm responding to in this message.

The car only measures voltage every 12 1.2V series. If that isn't a problem, it isn't in my version.

Many have criticized me in other forums regarding the lack of balancing and the risk of the cells exceeding 4.2V. The truth is, this is the result of not understanding everything that's happening overall.

Project Lithium uses Lifepo4 technology because it's safer than Li-ion. This safety comes from its lower energy density, which makes it safer compared to other technologies, but it has certain problems. Lifepo4 has a very long lifespan, but its maximum discharge is about 2C (if a battery has 10Ah, its maximum discharge is 20A). The car can demand up to 100A for a few seconds of discharge and about 50A for charging. It's simply a technology that can't tolerate the usage it's being put to. Furthermore, the voltage doesn't match. An original module has a nominal 7.2V, while LiFePO4 has a nominal 3.2V. You can either use 2S and get 6.4V, or 3S and you end up with 9.6V.

LiFePO4 has another problem: it doesn't tolerate overvoltage well. When exposed to overvoltage, the battery tends to swell and even explode, something that doesn't happen to Li-Ion. Plus, Project Lithium modules don't respect the car's cooling system.

The bottom line is that Project Lithium's bad reputation stems from insisting, insisting, and insisting on a "safer" technology, which ceases to be safe when they demand 5 times what the battery can offer.

Now my version: My idea was to create a cheaper replacement for the original Toyota replacement, not to improve fuel consumption, not to make the car more responsive, nor to create a BMS system with active/passive balancing. I just wanted a cheap replacement.

Seeing the problems Project Lithium had, I decided on Li-ion. Why?

1st: It matches the voltage perfectly. An original module is 7.2V (6 cells of 1.2V). Li-ion has a nominal voltage of 3.6V, 3.6*2 equals 7.2V. Perfect fit, no problems with the BMS.

2nd Discharge: 3P of 21700 cells rated at 50A deliver a peak discharge of 150A with a capacity of 12Ah. The car requires a maximum of 100A, so the margin is healthy.

Furthermore, since the cells are cylindrical, the module has vents both at the bottom and top, allowing air to flow through while respecting the car's cooling system. Tested during the Spanish summer, and with the temperature sensor inside the module, it never exceeded 45°C.

Regarding the danger of not having a balance, a well-made battery doesn't need any kind of balancing, and cheap plates like the ones that could fit inside the module are so poor and slow that, in my experience, they damage more than they fix. My project wasn't about creating a new BMS; that would only make the project more expensive and prevent any workshop from wanting to install it.

It would be nice to be able to upgrade your new car with that technology, yes. But it's not necessary in this case. Since the voltage curves of Ni-Mh and Li-Ion are the same, if there were any errors, they would pop up just like they do in the original battery. I've been testing for months without any errors precisely because they meet all the above requirements.

Furthermore, Li-Ion doesn't explode, burn, or combust under overvoltage. For example, this appears in a datasheet.

9.1 Overcharge test
Test method: Cell is to be discharged at a constant current of 0.8A to 2.5V. The cell is then to be charged with 20V and 18A. Charging duration is to be 7 hours.
Criteria: No fire and no explosion.
Overcharge test must be performed with the UL1642 standard.

For all these reasons, it infuriates me at the bad reputation this type of project has received due to poor chemical decisions from the start.