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Discussion Starter · #1 ·
OK folks, after a lot of consideration, and a desire to have 100kwh of battery in the truck, I have come to the conclusion that the Volt packs won't cut it, mostly for form factor reasons. They are a PITA to make fit in the space allotted and cooling is also complicated. Therefor, I am contemplating using Lifepo4, 100Ah cells to make the pack. I can do it and it is a lot of cells but the price is right and they are tested cells. No cooling needed. 112S3P seems to work. Am I crazy. Standard cell charge/discharge current is 1/3C. I need a bit of guidance before I bite the bullet.
 

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OK folks, after a lot of consideration, and a desire to have 100kwh of battery in the truck, I have come to the conclusion that the Volt packs won't cut it, mostly for form factor reasons. They are a PITA to make fit in the space allotted and cooling is also complicated.
Well, Volt packs are designed to fit in a specific way into a plug-in hybrid, so they are not optimal in any way for a battery-electric truck. They have been readily available and the modules are relatively easy to work with - explaining their popularity - but what is available has changed over the years.

I'm surprised that you find the cooling to be complicated. Other than no active cooling at all, there can't be anything much simpler than connecting up a pair of hoses to each module and circulating the coolant.

The obvious updated alternative to Volt modules are Bolt modules (same cell manufacturer, same general module construction, much larger capacity, more appropriate energy and power density) but I have no idea how available they are or what they cost... and they would need cooling plates.

Therefor, I am contemplating using Lifepo4, 100Ah cells to make the pack. I can do it and it is a lot of cells but the price is right and they are tested cells. No cooling needed. 112S3P seems to work. Am I crazy. Standard cell charge/discharge current is 1/3C.
The recent common thinking here has been that LiFePO4 prismatic cells are relatively expensive for their energy and power capacity... but if you have a suitably good deal available, they would still make sense. The use of individual cells makes it reasonable to configure your desired energy capacity for your desired voltage. Of course using individual cells means a lot of interconnections and custom BMS wiring.

1/3C? At 3.2V (nominal) and 100 Ah per cell, 112S3P would be about 107 kWh, so 1/3C would be 36 kW. If that's a continuous rating then okay, but peak demand of this vehicle will be much higher than that. Would you mind sharing the cell specs or a link to them?

The other obvious no-active-cooling option is Leaf modules (of any generation).
 

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Discussion Starter · #3 · (Edited)
Max continuous discharge is 1C and max instantaneous discharge is 3C. My issue with Leaf modules is that it is luck of the draw, you don't know the SOH of the pack until you test it. This will be a big decision for me because the cost is about the same as a used Tesla 85kwh pack and those are hard to find in Canada anyway.

The form factor issues with the Volt packs revolve around splitting it up to fit in the available places in the truck. These Lifepo4 cells are very modular and easy to make custom boxes for.

It seems my local supplier can provide me with 72Ah cells which have a better energy density (and therefor a smaller size). The tradeoff is that the original ones are rated at 3000cycles and these are at 2000 cycles. They also have a higher continuous and max discharge rate.
 

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Max continuous discharge is 1C and max instantaneous discharge is 3C
Ah... "1/3C" meant 1C and 3C, not one-third C - that's better!

It seems my local supplier can provide me with 72Ah cells which have a better energy density (and therefor a smaller size). The tradeoff is that the original ones are rated at 3000cycles and these are at 2000 cycles. They also have a higher continuous and max discharge rate.
Is the pack size with the 100 Ah cells a problem? Less mass would be better, but I don't know how much difference there is.

I don't know how much the cycle life matters. Has anyone fully cycled a DIY EV's pack the rated number of times in the life of the vehicle?
 

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Discussion Starter · #5 ·
Ah... "1/3C" meant 1C and 3C, not one-third C - that's better!


Is the pack size with the 100 Ah cells a problem? Less mass would be better, but I don't know how much difference there is.

I don't know how much the cycle life matters. Has anyone fully cycled a DIY EV's pack the rated number of times in the life of the vehicle?
The 72Ah cells would take up 1.8m2 and the 100Ah would take up 2.8m2, both at roughly the same depth. It would mean 112S4P for the 72Ah instead of 112S3P for the 100Ah
 

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Max continuous discharge is 1C and max instantaneous discharge is 3C. My issue with Leaf modules is that it is luck of the draw, you don't know the SOH of the pack until you test it. T
Last I knew Wolftronix sold a pack sniffer that you just plug onto the Leaf pack, it would wake up the internal BMS and read the state of each pack of cells out. Pack Sniffer / Controller Version 2 I went to a junk yard and they were happy for me to test all their packs, write down the numbers and take the pack of my choosing. Yes, a couple of the packs had a bad module or two, and they all had different SOC. In the end my pack was 3.97±0.005 per cell when I picked it up, and 3 years later when we finally got it mounted and turned on they were 3.96±0.01. That is amazing in my opinion. We have had zero issues with the pack ~14 months of near daily use and 1000's of miles.
 

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I'm a little puzzled by these specifications. It says "more than 3000 cycles under the designed working condition", but then it says "Life Cycle @ 0.3C 80% DOD". If the "SOC usage window" is "10% ~ 90%" as stated, then what is more severe than "designed working condition" about "0.3C 80% DOD"? Is the "designed working condition" a trickle?

The energy density looks good, and the maximum continuous discharge rate is reasonable, but the maximum discharge rate for a shorter period is not given. With even one motor (but especially two) and about 100 kWh capacity, you'll go well past 0.3C (30 kW) at least intermittently, but if the maximum burst discharge rate is really 3C that's lots.
 
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