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

Take your time... I'll be here.

I haven't spent enough time in both camps to fully understand the Toyota mindset...
...but if members on other Toyota forums behave like they do here on PriusChat, I think that's the answer.

Based on my experience with the Toyota's hybrid electronics, there isn't much difference between Honda and Toyota hacking from a technology perspective. Honestly, the Prius is easier from a hardware perspective, as the only electrical interface I have to interact with is a single 8p connector, whereas the Insight has quite a few more other signals to spoof/appease.

It's funny because most people think that moving to a serial-based architecture makes things harder, but IMO it makes everything easier... with just three wires on the CAN bus, you can basically do anything. This reminds me of a video Andy did a few years back:

To clarify: Someone from Team Jack thought they were insulting me by branding me with that moniker. I thought it was funny, hence the reason it's now in my signature. Honestly it correctly describes my affinity, so not sure why they thought it would bother me. Bless their heart.

I'm the first to admit I've logged very few hours in the Toyota ecosystem. However, so far my career odometer is around 70k hours, during which time I've become exceptionally good at assimilating, abstracting, and applying knowledge. In other words, my inexperience with Toyotas specifically doesn't really matter much, because with enough experience it doesn't really matter what logo is slapped onto the device you're tinkering in.

For example, even though I'm new to Toyota, I'm probably the only person outside Toyota who has deciphered their gen3+ hybrid serial data bus. tbh it wasn't even that hard, although it did require focus for numerous hours. I'm retired, so I get ~80 hours a week to spend on whatever engineering projects I'm most interested in. Not having deadlines breathing down my neck is both a blessing and a curse, particularly since I'm almost always working on several projects simultaneously. Time is certainly my most limiting factor.

LiBSU only became my primary interest in the last couple weeks. Right now I'm ramping up to 50 hours a week in Toyota Land.

I don't have a concrete answer to your question. Ask me again next year and maybe I'll have more Toyota domain knowledge.

 

Are you intending to place an inline power MOSFET in series with the pack positive?
If so, any out-of-bounds excursion will disconnect the traction battery from the hybrid system, which is a jarring customer experience. We're talking MIL and car dead on the side of the road. That design could meet ISO26262 ASIL-C safety requirements, but it won't be pleasant.

Active balancing is overkill for automotive traction batteries.
Packs in poor health won't benefit much from active balancing.
Packs in good health will stay balanced with C/100 passive balancing (e.g. 50 mA for a 5000 mAh pack).
Active balancing will work, but it's not necessary.

Yes, LiBSU generates every battery parameter, and has absolute control over battery charging/discharging. Specifically, LiBSU can prevent pack current whenever it wants (e.g. cell full, cell empty, too cold, too hot, etc).

Moreover, LiBSU has numerous levers to pull to gradually coax the hybrid system into charging/discharging/idling the pack. This allows LiBSU to force/limit/prevent assist/regen without using the nuclear option (i.e. opening a high power MOSFET). Of course, if the SHTF, LiBSU can open the OEM contactors and assert the MIL, too.

 

What will it block, and how?

 

I always appreciate people sharing information, so thanks.

The 'happy path' range I've seen spans from 12.9 volts to 17.9 volts, which is close to your observations. However, note that this is only the happy path, when all cells are at the same voltage. In the video I posted in #6, I describe some of the pitfalls you can run into if all four (or five) cells in a given BSU voltage measurement aren't the same.

I see four problems with a 5S/tap setup:

1: You should characterize whichever cell you're using before you commit to 5S. In most NMC lithium cells, the resting cell voltage plummets as you drop below 5% SoC. For example, the 5AhG3 cell I use in my Honda Insight lithium battery product has the following resting Voc->SoC LUT:
Vcell -> SoC
3.22 -> 5%
3.17 -> 4%
3.10 -> 3%
3.05 -> 2%
2.95 -> 1%
2.70 -> 0%

2: Cell resistance increases dramatically below 10%. The Prius computer will fault out when it sees high resistance (e.g. voltage sag during heavy assist).

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3: In normal operation, you absolutely don't want to operate near a cell's maximum safe operating voltages (e.g. 2.75 or 4.25 volts).
In most lithium cells, cycle lifetime decreases dramatically below 10% (and also above 85%). For example:
-a cell continuously cycled from 10% to 80% might last QTY5000 cycles, whereas;
-a cell continuously cycled from 00% to 80% might last QTY250 cycles.

Cells typically like to stay between 10% & 85% SoC. You'll need to look at your cell datasheet to find cycle lifetime information. So then for my 5AhG3 NMC lithium cell, my product keeps cell voltage between 3.45 volts (10% SoC) & 4.05 volts (85% SoC). This is to maximize cell lifetime, and also go provide headroom to prevent the cell from over/under charging.

For maximum lifetime, you ideally want to keep your cells between:
-13.8 & 16.2 volts for a 4S/tap BSU blade setup (right in the middle of the BSU's 12.9 to 17.9 volt range), or;
-17.2 & 20.2 volts for a 5S/tap BSU blade setup (outside the BSU's range; the BSU will overdischarge a 5S NMC setup)

As you can see, without a replacement BSU designed for lithium, neither 4S nor 5S is suitable for use with the Prius' NiMH BSU.

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4: A 5S/tap configuration will leave the cells chronically undercharged. What happens when a customer turns their car off with the pack at 0% SoC (e.g. extended airport stay)? There's basically zero energy available in the pack, so the pack voltage will plummet rapidly over the next few days. I strongly encourage you to measure your cell's energy curve (in mAh) versus voltage... if you do the math right, you'll see that a 5S setup will only use a small portion of the cell's nominal mAh rating.

What is your 'soft' protection mechanism?
Please don't say active balancing (which doesn't work for reasons I described in my video in post#6).

Where does your BMS source energy if the pack is overdischarged? Please don't say "from other cells", because that energy isn't available even during the happy path case (i.e. all cells balanced, yet overdischarged).

Where does your BMS sink energy if the pack is overcharged? Please don't say "into the balance resistors", because they can't dump the 75000 mA the electric motor will dump into the pack if the OEM electronics don't know anything is wrong with the pack.

My point is that your 'soft' limits don't work/exist in this application.
Regardling 'hard' limits, a properly design high current MOSFET switch will add safety to your proposed system, but it's an inelegant solution. Are you sure that MOSFET can handle opening when Vds@t+ is 300 VDC? That' MOSFET pair (you need to block both directions) is going to need a chonker heat sink.

No issues here.