why would battery death in one cell bank force the car into the pits if they are two laps from finishing endurance?
Youre' right, it won't, because you still have an engine. I stand corrected.
Let me add that, in my experience, cells do not suddenly go bad (the BMS makes sure that they are treated well). What happens, instead, is that a connection in the pack loosens or breaks. So, design your pack well, and build it well, using a torque wrench.
First, let me define "battery death in one cell bank" as: "one of the cells develops a failure that results in its resistance becoming very high, or in its capacity becoming noticeably lower". (The latter issue becomes the former issue once that cell is empty.)
Losing a connection in the pack has the same effect, but at least a reverse voltage on that connection is not a safety concern the way a reversed cell is.
Then, look at 4 topologies:
- two completely separate packs,
- two separate batteries that are then connected in parallel,
- a single pack with cells directly in parallel,
- a pack made up of half as many cells that are twice as big.
Under ideal conditions, all topologies are identical: all the packs have exactly the same voltage, total capacity and total current delivery capability.
But, if a single cell goes bad, the effects are different for the various scenarios.
Topology 1: two completely separate packs.
The battery with the bad cell cannot deliver any current (without a BMS, the voltage across the bad cell would be reversed, with worrisome effects; with a BMS, the voltage across the bad cell would drop below the limit the moment you tried to use that pack, and the BMS would shut down the motor controller). Regardless, you'd be running with only one motor (which will make the car pull to one side), and 1/2 the torque.
Topology 2: two separate batteries that are then connected in parallel.
As soon as you draw current, the pack voltage drops (say, 10 V). In the good battery, the drop is shared evenly. The battery with a bad cell cannot deliver current, so its cell voltages remain constant; the result is that the entire voltage drop appears on the bad cell, reversing it (-10 V in the example). The BMS will see that, and shut down the ENTIRE pack. So you're left with no electrical power. (Without a BMS, the effect on the reversed cell will be nasty.)
Topology 3: a single pack with cells directly in parallel.
The bad cell cannot deliver any current, but its buddy can. That pair of cells is the limiting factor to battery capacity and to battery current. Both motors continue to work, though the max torque is now 1/2 as much. But at least the torque will be balanced.
Topology 4: a pack made up of half as many cells that are twice as big.
The bad cell cannot deliver any current, so nor can the entire pack. So you're left with no electrical power.
So, the best topology is #3. Not ideal, but better than the alternatives.
But, Formula Hybrid requires you to use a fuse for every parallel connection, making a topology #3 pack cumbersome and expensive.
So, for you Formula Hybrid teams, that leaves topologies #1 and #4. The question is: "what's better? Losing electrical torque on just one side, or losing the entire electrical torque (evenly on both sides)? If the former, topology #1 is better; if the latter, topology #4 is better.
(Again, this is only considering the bad cell / bad connection issue. When looking at other issues, other factors may favor other topologies.)