Most packs have only 2 contactors. Not 2 per cell.
The only way to have spare cells that are not in active use all the time is to physically disconnect the cells from the rest of the pack.
The only way to do that is to have contactors at each “end” of the cell, or cell pack, that you want to switch in and out.
Car packs are ~360-800v nominal depending on the car/pack.
To get to those voltages with the normal cells (~3.2-3.7v nominal)you need between 95 and 250 cells in series (wired one to another directly, all the power goes through all the cells).
Let’s do an example. The simplest pack possible. A 95s1p meaning 95 cells wired negative to positive in a single line. A contactor at each end to cut power to the car for safety.
This is the simplest pack. Also the lowest range and worst for cell wear.
So say you want to “double” the range?
You “simply” build an entire separate pack, and drop it next to the first with it’s own set of contactors, right?
But in that case you have doubled the amount of interconnect bus in the pack(the wires to get the high current out of the battery), as well as contactors.
You could get to the same power storage (range and longevity) by making a 95s2p pack with one set of contactors.
So instead of 2 lines of cells, you connect each cell to it’s partner with a small piece of wire then connect that to the next cell in the pack.
This means you don’t need the extra long wire from the back to the front of the pack for the second set. The tradeoff is you can’t physically disconnect the second cells, but you don’t need the weight and complexity of extra contactors, and the long wire for second cell set.
So what’s the actual benefit of physically disconnecting the second set of cells?
When one battery dies in the 95s1p pack, the whole pack is useless, as all the power from the remaining 94 cells must travel through the one high resistance cell.
In a 95s2p pack each cell only has to take half the current of the entire pack (improving as you go up in parallel cell count).
You would be able to run one 95s1p dead, then switch to the other and keep driving till that is dead. But the efficiency of that is actually less than you get if you just had one 95s2p you ran from full till dead.
So again, being able to physically disconnect some cells in the pack only adds weight, complexity, and risk.
The Tesla car with “less range” that can be “unlocked” is literally just a software setting that limits the charge/discharge voltage of the entire pack, not switching in and out battery cells physically.
That’s a lot of words to say, what you said is absolutely how battery packs work to limit total failures.
Can you provide a link to a battery pack design that is one total massive cell like you’re saying instead of multiple individual? Other than some thing like a AA or AAA battery….
How much do you think a length of wire weighs compared to the weight of the pack? You’re making to seem like it will add 30% weight here or something, the weight is negligible, why are you even mentioning it?
Not sure where you’re getting “one total massive cell” from anything I wrote.
Every pack is made of a bunch of smaller batteries. You can’t get 400v without batteries in series, from batteries that only make ~3v.
Just saw your last paragraph edit.
It’s a car pack, every ounce matters, and doubly so when it only adds complexity, reduces efficiency, and reduces reliability.
And an estimate of weight of the extra interconnect… let’s say it’s 8’ from back to front of the pack, a 350v pack and a 250kw motor. This means minimum of 715 A.
Busbar that is rated for 700-800A @30c rise has cross section of 1/4"x2". For the 8’ length that means we have 48in^3 of copper. That is ~16lbs of copper alone. Not counting the contactors, insulation, etc.
It is, especially when the choice that leads to that extra weight is less reliable, less efficient, and more costly. All things you don’t particularly want in a car
So let’s get back to the real discussion on how the packs actually work.
Can you explain how a microcontroller is supposed to put cells in and out of the circuit?
Most packs have only 2 contactors. Not 2 per cell. The only way to have spare cells that are not in active use all the time is to physically disconnect the cells from the rest of the pack. The only way to do that is to have contactors at each “end” of the cell, or cell pack, that you want to switch in and out.
Car packs are ~360-800v nominal depending on the car/pack. To get to those voltages with the normal cells (~3.2-3.7v nominal)you need between 95 and 250 cells in series (wired one to another directly, all the power goes through all the cells).
Let’s do an example. The simplest pack possible. A 95s1p meaning 95 cells wired negative to positive in a single line. A contactor at each end to cut power to the car for safety.
This is the simplest pack. Also the lowest range and worst for cell wear.
So say you want to “double” the range? You “simply” build an entire separate pack, and drop it next to the first with it’s own set of contactors, right?
But in that case you have doubled the amount of interconnect bus in the pack(the wires to get the high current out of the battery), as well as contactors.
You could get to the same power storage (range and longevity) by making a 95s2p pack with one set of contactors.
So instead of 2 lines of cells, you connect each cell to it’s partner with a small piece of wire then connect that to the next cell in the pack.
This means you don’t need the extra long wire from the back to the front of the pack for the second set. The tradeoff is you can’t physically disconnect the second cells, but you don’t need the weight and complexity of extra contactors, and the long wire for second cell set.
So what’s the actual benefit of physically disconnecting the second set of cells?
When one battery dies in the 95s1p pack, the whole pack is useless, as all the power from the remaining 94 cells must travel through the one high resistance cell.
In a 95s2p pack each cell only has to take half the current of the entire pack (improving as you go up in parallel cell count).
You would be able to run one 95s1p dead, then switch to the other and keep driving till that is dead. But the efficiency of that is actually less than you get if you just had one 95s2p you ran from full till dead.
So again, being able to physically disconnect some cells in the pack only adds weight, complexity, and risk.
The Tesla car with “less range” that can be “unlocked” is literally just a software setting that limits the charge/discharge voltage of the entire pack, not switching in and out battery cells physically.
So… As you said
That’s a lot of words to say, what you said is absolutely how battery packs work to limit total failures.
Can you provide a link to a battery pack design that is one total massive cell like you’re saying instead of multiple individual? Other than some thing like a AA or AAA battery….
How much do you think a length of wire weighs compared to the weight of the pack? You’re making to seem like it will add 30% weight here or something, the weight is negligible, why are you even mentioning it?
Not sure where you’re getting “one total massive cell” from anything I wrote.
Every pack is made of a bunch of smaller batteries. You can’t get 400v without batteries in series, from batteries that only make ~3v.
Just saw your last paragraph edit. It’s a car pack, every ounce matters, and doubly so when it only adds complexity, reduces efficiency, and reduces reliability.
And an estimate of weight of the extra interconnect… let’s say it’s 8’ from back to front of the pack, a 350v pack and a 250kw motor. This means minimum of 715 A. Busbar that is rated for 700-800A @30c rise has cross section of 1/4"x2". For the 8’ length that means we have 48in^3 of copper. That is ~16lbs of copper alone. Not counting the contactors, insulation, etc.
So 1% of a 1600lb pack? Wow so detrimental….
It is, especially when the choice that leads to that extra weight is less reliable, less efficient, and more costly. All things you don’t particularly want in a car
So let’s get back to the real discussion on how the packs actually work. Can you explain how a microcontroller is supposed to put cells in and out of the circuit?