The engineer system has 40AH Lithium batteries in a 48V pack.
Thundersky shows a 5000 cycle life if cycled 70%, so the 40AH becomes 28 useable ah
There boost converter is probably running at 80% efficiency so it will reduce the useable AH to ~22AH
The engineer system has to boost the ~48V PHEV battery voltage to ~144 which is ~ a 3 X boost, so the current will be 3 X what is delivered to the stock battery.
The effective capacity when converted to 144V will be 22AH/3 = ~ 7.3 Ah of PHEV capacity at 144V
Buck Inverters work like a step down transformer.
As they step down the voltage they increase the current by the same factor.
They increase effective AH since they boost current while dropping voltage.
The PHEV pack must be 10 or more volts higher than the battery it is charging, so we need lots of cells.
The stock second generation Prius HV battery is 209V which is a perfect voltage for charging an Insight 144V pack with a buck converter. With the Insight pack full at ~170V, and the Prius pack depleted at ~ 180V, we always will have that voltage differential which is important for Buck converters operation.
My first approach is going to be to start with stock surplus Prius battery packs, which come with precharge and main contactors, current sensor , temperature sensors, and a built in prewired BCM, for~ $500 They would be tested rebalanced and repaired if necessary.
I will effectively recycle prius packs into Insight PHEV packs.
A simple inexpensive CAN battery controller interface would take full advantage of the Prius BCM, for safe reliable battery care during charging and discharging.
This Prius pack while only 6.5AH, is at 209V nominal.
209/144 = ~1.4 boost in current, effectively making the 209V 6.5 Ah pack look like a 9.4AH 144V pack.
Assuming we want long cycle life from the Prius pack, and we limit the discharge of the prius pack to 60%, we would get an effective 5.4AH boost in useable storage capacity at 144V
The buck system using prius cells will deliver 5.4 Ah peak output up to 25A
The engineer system delivers 7.3Ah . peak output less than10A.
The ultimate PHEV pack.
40AH 209V lithium pack, 70% DOD = 28AH useable for max cycle life.
deduct Buck converter efficiency losses 90%+ = ~25AH
Buck current magnification 25 * 1.4 = 35 effective AH at 144V
A 40 AH lithium pack would deliver 35 effective AH at 144V
Over 10 times the stock capacity
You would be able to drive for hundreds of miles with MIMA while in PHEV mode, cruising in the 100mpg club.
Battery life factors:
Buck battery current @ 10 A to stock pack
7.5A from a 40ah Lithium pack is a mild load negligible heating
7.5A from a Prius NIMH pack is a very mild load negligible heating
Boost Battery current @ 10 A to stock pack
Drawing ~35 amps from a 40AH lithium is a heavy load will heat up