🔋 The Extended Battery Decision (Poll: Which battery option suits you best?)

[ElectricShitbox]

I got by with 18kwh for a few years, before the pack in my spark started to fail. I'm quite comfortable with a 53kwh pack in the Slate, lol. That is really the far low end of what I would want for this type of vehicle though.
I am tempted by the 84kwh battery despite the added cost and weight for a couple reasons though.
First being longevity due to cycles. My spark's tiny battery got cycled hard because I was daily using most of the range. With home charging and 53kwh, this isn't too bad, but it's basically a non-issue with 84kwh.
Second reason is voltage. I saw somewhere that both packs use the same modules, 5 for standard 8 for extended. So presumably the extended pack operates at a higher voltage, which explains why both packs list the same 20-80% charging times. The amperage limit is probably the same. Getting more range in the same amount of time means the big battery will road trip better, not that either is a cannonball record contender.
Third is to support a potential future front motor. With the additional power demand and loss of efficiency from a second motor, that small battery starts to become a limiting factor. I wouldn't be surprised if a theoretical future dual motor conversion would require the big pack.

For the record, my ideal battery would be 60-70kwh and LFP. Highest voltage that is practical.

 

[Driven5]

I saw somewhere that both packs use the same modules, 5 for standard 8 for extended. So presumably the extended pack operates at a higher voltage, which explains why both packs list the same 20-80% charging times.

Connecting batteries in series increases the voltage while capacity (amp hours) stays the same. Connecting batteries in parallel increases the capacity (amp hours) while voltage stays the same. The larger battery pack can add 3 modules in parallel with the original 5, which would have the same voltage and proportionally more amp hours.

Each module can accept the same maximum charge rate. So as long as the charger amperage capacity exceeds the combined limit of all modules, such as at supercharging stations, the charging times at the same voltage will still be the same with the additional module capacity. When the module charge rate limit exceeds the charger capacity, such as level 1-2 charging at home, the charge time is proportional to the total battery pack capacity.

 

[ElectricShitbox]

Connecting 3 additional modules in PARALLEL to 5 existing modules will result in fire, so I assume that's not how they're doing it. They've at least implied they're using using identical modules the way the GM Ultium system is, where the Blazer pack is the same as the Equinox pack, but with additional modules, and so on up their range of models. The blazer has a higher voltage and therefore a higher charging rate.

Each module will be X number of amp hours, and they will be in series. The amp hours of the pack doesn't increase, but the capacity does because the voltage increases. Kwh = amp hours x voltage.
The max charging speed increases because the maximum amperage stays the same but the voltage is higher.

 

[sodamo]

Connecting 3 additional modules in PARALLEL to 5 existing modules will result in fire, so I assume that's not how they're doing it. They've at least implied they're using using identical modules the way the GM Ultium system is, where the Blazer pack is the same as the Equinox pack, but with additional modules, and so on up their range of models. The blazer has a higher voltage and therefore a higher charging rate.

Each module will be X number of amp hours, and they will be in series. The amp hours of the pack doesn't increase, but the capacity does because the voltage increases. Kwh = amp hours x voltage.
The max charging speed increases because the maximum amperage stays the same but the voltage is higher.

So why result in fire?
Not an engineer, but batteries have been a mainstay in my life these past 20 years.
in earlier time our 6 or 12 volt would be combined in series to get either 12, 24, or 48 v as the system required, then those series units would be combined in parallel for increased capacity. These days our LiFePo batteries come from the factory as 48v with caveat NOT to add in series, but parallel only. I currently have 15 units at 48v, 3.8kw all in parallel. as for the Slate increasing the voltage not capacity doesn’t seem logical. But then I’m not an engineer.

 

[ElectricShitbox]

Ah, I see where you were coming from where it would be each module being full pack voltage. Those you could just keep stacking in parallel, but then each module needs it's own BMS to balance the individual cells in each module. That adds a lot of weight, cost, and complexity that makes it unlikely that's how Slate would be doing it.
The source for my "5 or 8 modules" claim was a Sandy Munro tour of the Slate factory that I watched a few months ago.
So I went back and re-watched the section where they briefly discuss the battery. In it they say the small battery is 52.7kwh and uses 5 modules, and the big battery is 84.3kwh and uses 8 modules. That works out to 10.54kwh per module for both packs. Now, they don't directly say that they're the same modules in both packs, so they could be different cells in a different number of series that adds up to the same capacity. However, they confirm in that video that they are NMC pouch cells from SK On assembled into "proven modules".

So that's the end of everything I *KNOW*
The rest is what I *THINK*

I'm going to assume that SK On is selling them a module that is already used in an existing EV by a domestic manufacturer that they supply. (I haven't gone down that rabbit hole yet, will probably report back later)

I'm also going to assume it's the same module, and the modules are arranged in a 5s or 8s configuration.

Since Slate has confirmed Tesla Supercharger access, I'm going to assume that peak voltage in the big pack is under 500 volts.

So I think each module is 14s. Since they're NMC cells, let's call it a 4.2v peak per cell. That makes the big pack 112s total with a peak voltage of 470.4, and the small pack a 70s system with a peak voltage of 294.

If we call the safe cutoff voltage of 3.0 per cell, the big pack bottoms out at 336v and the small pack at 210v (barely above the bottom of the CCS protocol, and I think J3400).

If I'm doing the math right, that would put the capacity of each cell around 200amp hours, and would probably be a few pouch cells in parallel, as is typical.

 

[ElectricShitbox]

Here's the video I reference, and they discuss the battery around the 32 minute mark.

 

[sodamo]

Yes, I remember that video. My take on the battery upgrade, given Slate will be producing only, identically configured blank Slates is the blank Slate rolls of the line with a 5 module battery. The battery upgrade will be someone drops the battery cover, inserts 3 modules, makes software adjusts as necessary, if necessary. To me dropping the standard battery, replace with extended doesn’t make sense.
Still disagree series vs parallel upgrade. I think if that were true, voltage would be the topic of upgrade, not kw. Again, my own system as reference, 1 battery is 48 v/3.8kw. Bank of 15 is 48v/57 kw, not 720v/3.8 kw which would fry my system not extend it’s time of use (mileage). so fire with series, not paralle.

 

[ElectricShitbox]

For what it's worth, Slate has claimed that the battery upgrade will not be a DIY thing.

 

[Driven5]

The more I think about it, the more likely I think the modules are in series. EV's use 3.6V cells in a mix of parallel and series to create a low-voltage module, and the low-voltage modules are then connected in series to get the pack voltage... Except the really big 200kw packs being 2x 100kw sub-packs stacked in parallel.

I'm not sure why my brain jumped to full-voltage modules, other than compared to the 20% voltage difference in some other EV modular pack options, a 60% voltage difference seems much more significant.

I relish the idea of the small battery meaning less weight, and not wasting money and resources on capability I don't need, but the implications on heat management and upgrade potential could be the single factor that makes me consider the larger pack here.

 

[sodamo]

For what it's worth, Slate has claimed that the battery upgrade will not be a DIY thing.

True, but that doesn’t negate my theory of how, just who.
of course my hope it be done in a safe enough matter to be DIY, bearing in mind that DIY doesn’t mean any breathing body, some skill required.
when I increased my battery bank of 12 to 15 had my installer do it, not because I couldn’t but for the cost it made sense. Yes, i also know those 48v/3.8 kw units are built of smaller units in series.