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

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Vencedor, Sep 19, 2025 at 3:59 PM.

  1. Vencedor

    Vencedor New Member

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    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
     

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    #1 Vencedor, Sep 19, 2025 at 3:59 PM
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  2. Vencedor

    Vencedor New Member

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    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.
     

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    #2 Vencedor, Sep 19, 2025 at 4:40 PM
  3. Brian1954

    Brian1954 Senior Member

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    I have copied and pasted the posts from the other thread into this thread so that everything is together.

     
    #3 Brian1954, Sep 19, 2025 at 4:54 PM
  4. PriusCamper

    PriusCamper Senior Member

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    Seems like high energy density like lithium-ion equals more safety issues not less? Maybe you're saying its a more reliable high energy density for this application? But in terms of a fire that destroys the car I always thought the higher the energy density the greater the risk?
     
    #4 PriusCamper, Sep 19, 2025 at 6:09 PM
  5. Tombukt2

    Tombukt2 Senior Member

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    So we're yo get one?? I'll run the shiet out of it.
     
    #5 Tombukt2, Sep 19, 2025 at 8:49 PM
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  6. mudder

    mudder Active Member

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    This is a false equivalency. NiMH's end of charge behavior is drastically different from lithium. Specifically, NiMH cells can 'perpetually' turn excess charge energy into heat, whereas lithium cells cannot.

    We aren't criticizing you, we're advising you that safety issues exist with your product as described (which I will do in more detail below).

    Many LiFePO4 ('LFP') cells can handle more than 2C.

    While I agree NexPower's V1 through V2.x cells were undersized for traction batteries, they could probably still handle at least 20C.
    I certainly agree with you that NexPower used their LFP cells beyond the manufacturer's specified C rating.

    Right, which is why NexPower created a 2xBlade (i.e. the width of two OEM blades). This allows a 5S LFP 2xBlade to approximate two 6S NiMH blades.

    The bigger issue with LFP is its REALLY flat midband voltage curve (e.g. from 30% to 70% SoC). This makes it difficult to estimate Voc->SoC.

    I agree LFP cells don't tolerate overvoltage very well.
    But this is also true for ALL lithium chemistries (including LFP, LCO, NMC, LTO, LiS, etc).
    In fact, of the commercialized lithium chemistries, LFP tolerates overcharging 'the best' (but still not well).

    The short answer here is that all lithium chemistries are damaged by overvoltage. That's why a per-cell BMS is absolutely critical for safety in lithium systems.

    I don't think NexPower exceeded the LFP cell's rated charge/discharge by 5x.
    In my estimate, they were more like 1.5x. Your point still stands, though.

    If your only design goal is 'cheap', then I suppose little effort was spent on 'safe'?
    A lithium pack without a BMS is not going to age well.
    I recently worked with another company who defended their lithium product's lack of BMS by saying "it will last at least until the warranty expires"... their warranty period was just one year. Yikes!

    Which specific cell make and model are you using? Or what specific chemistry is it? Note that your answer is irrelevant because ALL lithium cells require a BMS when sold as products to consumers. However, knowing your cell chemistry will at least let me advise you on exactly how dangerous what you're doing is.

    OK, but what happens when a cell fails and the OEM BMS doesn't catch it until the cell goes into thermal runaway? This is the exact issue I harped on for months with NexPower's lack of BMS.

    IIRC, Toyotas can easily pull 150 amps.
    In my reverse engineering, I note that the OEM current sensor in a Gen3 Prius can measure up to 205 amps of assist. Seems odd that Toyota would go through the additional engineering effort to measure 205 amps if they only ever pull 100 amps (as you claim).
    I am not a Toyota person, so I leave the actual current values to those who know in this community.

    This is only the case when the cells are new, and even then it's not guaranteed. That's why you need a per-cell BMS with supervisory control.

    While a well-made battery doesn't require high current balancing, per-cell balancing is absolutely required to handle cell aging/heating/etc. Typically you only need C/100 passive balancing on a well made pack.

    I don't understand your statement. Please elaborate.

    It would also make your product safe (if properly implemented).

    You haven't convinced me that your lithium pack is safe without a BMS.
    A properly designed BMS is required for safety on all lithium batteries sold to consumers.

    No, this isn't the case. "Testing for months" isn't long enough. Think about your customers ten years from now. What's going to happen when those cells degrade at different rates for a decade? The OEM NiMH BMS will only alert the driver when a 1.2 volt delta is measured. Please watch this video I made last month for a better explanation, including example scenarios the NiMH BMS won't catch:


    Please post the datasheet so I can explain how you are misinterpreting what they're saying.
    NMC lithium cells will absolutely go into thermal runaway if you overcharge them.

    Please send me your cell and I will record a video of it going into thermal runaway. I have all the test equipment to do this safely.

    Sorry science infuriates you...
    ...as I said at the onset, we are trying to educate you as to why your product is unsafe as described.
     
    #6 mudder, Sep 19, 2025 at 9:58 PM
  7. PriusCamper

    PriusCamper Senior Member

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    It's for EU not for the so called "new world." And I'd love to see proof that EU certified this product for sale. I mean why not a link to buy if OP isn't lying about EU certification?
     
    #7 PriusCamper, Sep 19, 2025 at 11:29 PM
    Last edited: Sep 19, 2025 at 11:40 PM
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