Well, just under the back seats is where the battery pack is housed, but it should be sealed up. The fact that you can SMELL something is pretty terrifying, but fortunately you're driving a 2014 Spark EV with A123's Lithium Iron Phosphate cells. They're very safe cells that do not have a runaway thermal reaction and combust like Li-ion does when things go wrong (like overcharge, reverse voltage, or internal short circuit). They will certainly vent toxic gasses though if there is some kind of short circuit, so you really don't want to breathe that...
This morning, it was completely dead--no response to the key or anything. I jumped the 12v battery and the car started [...] but the smell is still happening
It would be nice if you mistook the source of the smell as coming from the back seats, and it was actually your AGM battery doing the venting and going bad, and the HVAC was pulling the acidic and acrid smell through the cabin and settling in the rear... When you accelerate, more wind could make its way into the cabin, causing you to feel like the smell was more intense on acceleration.
Do you know when the 12V battery was replaced, is it original?
Do you detect the smell in the engine compartment after the car has been on for a couple of minutes? The Spark EV could initiate an equalization charge on the AGM (aux power unit sends 14.7-15V to the battery) causing venting in order to balance the cells.
I think that the smell intensified when I drove in low and accelerated but am not sure.
Normally if there is such a low voltage on a bad cell to the point that it is venting, the BMS should detect the conditions that allow this and cut power immediately. When you say it starts, has power steering and everything, it sounds like you still have propulsion. I want to say that's good news, but there's just not enough information.
Do you have no check engine light and diagnostic trouble codes? You can request them from Onstar if you subscribe to the service and don't have an OBD II tool.
Again the fact that you are describing a cell failure and are still able to drive (with no codes?) doesn't make sense. I've never heard of a BMS or the HPCM2 allowing this, they are very strict, no-nonsense computers that prioritize locking you out when they detect a whiff of bad cell (pun slightly intended).
The BMS should fail its battery pack voltage checks if the voltage on a single or multiple cells is below 2.5V, a flat 0V, or negative from severe capacity loss. If it allows you to drive, the rest of the cells in series with this bad cell would punch current through it and reverse charge it, decomposing the electrolyte, generating gas, and venting.
If you have a bad cell in your battery and take it to a dealer, they'll opt to replace the whole pack which is out of warranty now and the job is worth 2-3x the car's trade-in value... It would be a good idea to investigate this asap at the dealer, and by immediately examining the voltage of the 96 cells in the high-voltage pack with an OBD II tool and the Bolt EV PIDs with TorquePro yourself after you book an appointment. I would avoid driving it just from the toxic fumes alone, but also because until you know what's happening to cause the smell, something untoward could be happening. If yo have to drive it, like to the dealer, roll all the windows down or choose to tow.
If you were driving at a low state of charge, this would be the point that LIFEPO4 cells would swing out of balance. A damaged cell with less capacity can have its voltage drop significantly lower than the rest of the series-connected cells and become overdischarged, and start venting. But again, the BMS should prevent this.
Possibility #2 is that the cells are being overcharged which could also cause venting, but this is usually constrained to when you're charging the cells, and the BMS should prevent this.
Possibility #3, and you should really have a check engine light for all of this, is that the battery doesn't have any coolant and the cells are overheating as well as the materials surrounding them (glues, thermal compounds, foam).
You could check your coolant levels to confirm this, pop the hood, and look at all 3 of the coolant bottles right at the front of the engine compartment. Look for the fill line embossed on the container (just like you see inside laundry liquid detergent caps) and compare that line to the liquid line on each of the bottles. If it's significantly lower than the fill line on the bottle, the coolant might be too low to circulate and the liquid cooling wouldn't be functional. That said, if the coolant pump failed or was seized up, this could also result in overheating, yet the coolant levels would appear normal. To check the pump you could try briefly plugging in and charging and listening for the pump whirr.
Was there ever any smell when you charged?
Please report back to let us know what you discover.