youtube video of replacement of nimh with lifepo4 hv batteries

If I wasn't separated by an ocean by those batteries I would of brought them already. I have to go through the process of trying to import them to australia as they weren't available here.

That lines up with the 202v-235v usable range I see for nimh cells with a low load placed on them.

Keep in mind there are different flavours of LTO chemistry. Mix and max cathodes and anodes for different properties. The packs we are looking at have a nominal voltage of 2.3v as they are high energy density variant. the 2.4v are a high power density variant. The voltage curves and properties are throw around interchangeably a lot online but they are different and very much make a difference in our case.
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the voltage curve starts dropping of rapidly after 2v. IF we are still using the oem bms and the voltage sense wires then the slight variations in cell capacity are going to show here. The weakest ones will drop off very quickly and that will cause a low voltage fault.

Between 2.5v-2v is where most of the energy is anyway. That's still 16.5ah usable per cell. or about 3700 watt hours ( 98s1p config) compared to the 500watt hours usable range of the nimh oem battery. Realistically tho, most of that isn't going to be used. without a dedicated bms to take advantage of all that extra energy. BUT that's not to say it doesn't have merit, They will outlast the car. They are good for 25 years. Plus that 98s1p configuration has a power output rating of 117KW watts as opposed to the oem batteries 36KW.
It will put up with all the abuse and outperform anything else.

Cutting open one of those 24 packs to get 2 cells out is going to be a HUGE PITA. they are glued in on 5 sides , the top has welded busbars. 4x24 +2 to make 98s.
I think they just fit in the space above the spare well. all that is out of my prius at the moment whilst reconditioning the nimh battery.
Oh Balancing is going to be a thing for these as well as the oem system wont do that. Add a super capacitor balancing board to each cell that comes comes on at 2.5v to bleed down and you should be fine. Charge the pack to 245v for a top-mid balance

 

Yes the nimh ecu does coulomb counting. Then the soc is computed. But it assume that the battery have 6.5 Ah usable (when new)
The range will be rounded 40 - 80 % SOC.
In your case the voltage calibration will be used when your cells will have lost a lot of their capacity or their voltage be too high/low.
So with new LTO cells you will only use 40% of 6.5 Ah. The voltage range will be very reduced compared to 6.5Ah nimh cells.
When reconnecting the 12V battery, the Soc will be set at 65%, so your LTO cells need to have a "middle range" voltage at that moment.

If you want to use more Ah you need a SOC spoofer. For example BMS2 that Normann made, device that is used in diy Gen2 phev. This device use the total voltage (also temperature...) and compute the SOC, DCL and CCL.

I think using 98 LTO will work because I have replaced the 168 Nimh cells with 70 A123 Lifepo4 20Ah cells in my gen2 prius. And that work. I have connected the 15 senses wire each 5 x A123 LIfepo4 cells.
But after driving, reading the 14 voltages monitored I saw sometimes bad cells voltages, I think this was EMI problem. But the nimh ecu don't throw dtc.
My purpose was not to use only these 70 cells, I add 140 more just connected on + and - pôles. Now I don't have bad cells voltages.
So be careful, if possible don't change the way/length the 15 wires goes physically. This explain why the minus power wire is shielded.

It will be better connecting the HRC 120 A fuse at the same place that Toyota does !
This is because the nimh ecu, when you have high amperage consider the fuse internal resistance.
Then it does a voltage spoofing for this bloc that could be wrong if the fuse is at an other place.

Remember also that the nimh ecu also does insulation reading.

And of course add a bms, the nimh ecu accept up to 1 - 1.2 Volts delta between the 14 groups. It can't mesure each of your 98 cells. The Nimh balancing is made by over-charging cells.

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