Plug in Prius Gen 3 (2012) EV range extension with BYD blade LFP cells

[Csaba Zagoni]

Hello, this is my first post on this forum. I'm from Romania, so I apologize for my sometimes bad English.
I just purchased a Gen 3 plug in Prius, and thinking about building a battery pack with the BYD blade cells to match the voltage of the Li-ion battery that is already in the car.
The Li-ion pack is 4 stacks in series of 14 cells 20Ah each, totaling 4*14*3.7=207.2V and around 4 kwh.
The pack I want to build would be:
Variant A: 4 stack of 16 cells 138Ah each totalling 4*16*3.2=204.8 V and around 28 kwh. Each stack with own BMS
Variant B: 2 stack of 32 cells 138Ah each totalling 2*32*3.2=204.8 V and around 28 kwh. Each stack with own BMS
I figure if I match the voltage before I conect them in paralel than the 2 batteries should work fine.
What I don't know if the Prius measures the voltage on the DC bus and works in EV mode until the voltage drops to a certain level, or measures the battery pack cells directly. Even than it should work because the voltage in the original battery will drop only when the voltage in the extension battery drops also.
So the range will be displayed wrong, but the car should be able to EV only mode for more than 100 km. Charging will be also like 13 hours, but can be done at night with no problem, and probably won't be necesary every day.
Before I start sourcing the parts, I ordered ODB2 interface so I can monitor for a while the voltage values at cell level and pack level.
What are your thoughts on this. Is it doable without interfering in the CAN comunication, can it work?
What pc software do you recomend to diagnose the car, that can see the battery data as well?

 

[sunshine_eggo]

I would hesitate to mix NCM and LFP together. NCM has a very linear voltage curve where LFP is flat. LFP voltage is very mushy when it comes to charging, i.e., it will settle notably farther than NCM.

As such, targeting a nominal voltage is not the right way to go. You want your fully charged NCM voltage to be nicely divisible by 3.45V - where LFP can be charged to about 98% SoC.

I would guesstimate 66 cells assuming a peak charge of 4.10V/cell on the Prius pack. I'd use a 66 cell Orion BMS and use with single cells rather than try to shoehorn 16S packs in there.

Your proposed method of operation is very similar to how many if the plug-in kits for the standard Prius worked, but those where NiMH packs, and they had a pretty crude SoC computation.

 

[Csaba Zagoni]

Thanks for the quick reply

I would hesitate to mix NCM and LFP together. NCM has a very linear voltage curve where LFP is flat. LFP voltage is very mushy when it comes to charging, i.e., it will settle notably farther than NCM.
As such, targeting a nominal voltage is not the right way to go. You want your fully charged NCM voltage to be nicely divisible by 3.45V - where LFP can be charged to about 98% SoC.

My thinking was that the extension battery working can be limited by it's own BMS.
Between 2,5-3,45V the Li-ion and LFP work together in paralel. Under and above those limits Li-ion works alone.
But again, I don't have enough knowledge about how exact the BMS can work, so the LPF pack can be protected all the time.
I'm looking forward to put a diagnostic tool on it so I can see the voltage data, the realtime high and low limits between which the battery is working.

I would guesstimate 66 cells assuming a peak charge of 4.10V/cell on the Prius pack. I'd use a 66 cell Orion BMS and use with single cells rather than try to shoehorn 16S packs in there.

Thanks for the suggestion, I found a EU dealer for Orion BMS right in my country . It is much more expensive than the ones I was looking at till now, but i'll give it a tought

 

[Vladan]

I would hesitate to mix NCM and LFP together. NCM has a very linear voltage curve where LFP is flat. LFP voltage is very mushy when it comes to charging, i.e., it will settle notably farther than NCM.

As such, targeting a nominal voltage is not the right way to go. You want your fully charged NCM voltage to be nicely divisible by 3.45V - where LFP can be charged to about 98% SoC.

I would guesstimate 66 cells assuming a peak charge of 4.10V/cell on the Prius pack. I'd use a 66 cell Orion BMS and use with single cells rather than try to shoehorn 16S packs in there.

Your proposed method of operation is very similar to how many if the plug-in kits for the standard Prius worked, but those where NiMH packs, and they had a pretty crude SoC computation.

Chaba, I did the similar LFP addition, but with a gen 2 Prius that had the EV mode with all it means. I used it 80% for city driving with great pleasure and benefits. LFP pack was with own bms, all was great, but i used diodes to allow current to go single direction to NiMH pack only. All was perfect but one thing was not: I did not use (and had to take out the LFP pack every winter - being the worst job)! Our winters in Serbia and your in Romania are similar or same. If that doesn't bother you, it is ok, your concept will work. I had 60Ah pack (10P 6500Ah cylindric cells) then 76S them. LFP chemistry allows you to have a lot of "hidden" capacity with relatively low voltage that doesn't exceed Prius standard settings which you do not need to touch at all. Charging profile (long + low voltage + low amps = means slow) will help you. If you slowly saturate the cells you can have 95% or more percent capacity with 3,4V per cell which is a warranty for a LONG, LONG life of LFP cells. So all that is doable, but winter time (even if you have a warm garage) remained to be of concern for me. I never got into trouble since I was extremely careful and daily thinking of temperatures. Of course, also summers that are hot but that is less of problem, since the car was strictly in shade and during 3 years there was almost no cell-degradation at all. In my opinion the only serious minus was winter (which is also not short) so I finally took those cells and added to my solar system at home, and sold Prius that was already old-but still good. The LFP cells are after that, still living a healthy life after 3 years in my cellar, working with other LFP packs of my Solar System. I hope this helps you to make your decision. Good luck!

 

[black_jmyntrn]

oh wow! I feel I am at home here!

The 2024 goal is 50kWh+ and using slightly more space than the OE modules take up. I have a plan for this and its using the newer lighter battereis Toyo/Lex has released. Also fast charge upgrade, did you know the Japan spec Prime 2017 fully charges in 16 minutes?

The last few years ive been experimenting with the batteries in general and also the prius hv battery all while taking my Prius places those with 4x4 go. Currently have 2 85ah 12V each with a 2000W Pure Sine Inverter, check this out... cooking on an air fryer while using a HV charger.

 

[Vigo]

^ That is AWESOME!!! Thanks for posting about it.

I briefly owned a Highlander Hybrid with the electric rear diff. I had previously had a GS450h and i know you could put that one into a maintenance mode that disabled all traction control. I wonder if the same was possible on the highlander, making it not suck off-road. But still very cool that you're taking your FWD Prius on trails!