I am putting together a thread with information for people who are considering poking at the battery pack. My experience is with the Roadster but most of this information will hopefully be relevant for other EVs as well.
The Roadster pack is 99S/69P - each 4V brick consists of 69 cells in parallel, then there are 99 of these bricks in series.
The range is determined by the weakest brick (ideally)
When charging or driving the current through each brick is the same as they are wired in series. The car doesn't have the ability to top up specific bricks when charging or to pull more energy from stronger bricks when driving.
The car monitors the voltage of each brick.
Driving stops when any brick reaches the minimum allowed voltage.
Charging stops when any brick reaches the maximum allowed voltage.
The weakest brick (the brick with the lowest usable capacity) will be the first to reach the minimum allowed voltage when driving. This same brick will be the first to reach the maximum allowed voltage when charging... ie it is the fastest to discharge and also the fastest to charge due to being the lowest capacity brick. Or at least this is the goal as it is the best case for range.
To help ensure this the battery management system inside the battery pack has the ability to turn on small bleed resistors on a brick-by-brick basis. Once charging is complete it can selectively discharge some bricks slightly so the next charge will be more even.
Do not attempt to charge a 0V brick!
Your car has been sitting for too long and is bricked.
The first thing to do it check the voltage on each individual brick. This can be done with the pack still in the car uses these boards: GitHub
If any brick is reading 0V (or close to it) then the brick is dead and can't be recovered (seriously). A dead brick means you will have to replace the entire sheet that contains the brick. In the case of the Roadster there are 11 sheets each containing 9 bricks... the sheets can be replaced but the bricks inside the sheets can't.
But you are thinking if I put a low current into the brick, even if one cell is shorted the worst case is that all the current goes through that one cell and a 100mA into a 1Ohm shorted cell is only 10mW of heat... harmless. Find the bond wire with the slight voltage drop and cut it to disconnect the bad cell.
The problem...
If you cut the bond wire to the shorted cell the brick will start to charge (or maybe there is no shorted cell to begin with and the brick starts to charge). All is good for a bit - the voltage slowly rises as the brick charges. You will probably get to a volt or so then something happens... the voltage starts dropping rapidly. One cell that was accepting the charge just suddenly shorted. The heat generated is no longer based on the controlled current you are putting in but is now determined by how much the other 68 now slightly charged cells can deliver into the now shorted cell.
I did exactly this, but not giving up I found the newly shorted cell and cut its bond wire. Repeat. Repeat. Repeat. Repeat. Repeat. Repeat. Repeat. 8 times before I gave up the voltage reached hopeful levels then crashed back down requiring another bond wire in the same brick to be cut.
It's dead.
The Roadster pack is 99S/69P - each 4V brick consists of 69 cells in parallel, then there are 99 of these bricks in series.
The range is determined by the weakest brick (ideally)
When charging or driving the current through each brick is the same as they are wired in series. The car doesn't have the ability to top up specific bricks when charging or to pull more energy from stronger bricks when driving.
The car monitors the voltage of each brick.
Driving stops when any brick reaches the minimum allowed voltage.
Charging stops when any brick reaches the maximum allowed voltage.
The weakest brick (the brick with the lowest usable capacity) will be the first to reach the minimum allowed voltage when driving. This same brick will be the first to reach the maximum allowed voltage when charging... ie it is the fastest to discharge and also the fastest to charge due to being the lowest capacity brick. Or at least this is the goal as it is the best case for range.
To help ensure this the battery management system inside the battery pack has the ability to turn on small bleed resistors on a brick-by-brick basis. Once charging is complete it can selectively discharge some bricks slightly so the next charge will be more even.
Do not attempt to charge a 0V brick!
Your car has been sitting for too long and is bricked.
The first thing to do it check the voltage on each individual brick. This can be done with the pack still in the car uses these boards: GitHub
If any brick is reading 0V (or close to it) then the brick is dead and can't be recovered (seriously). A dead brick means you will have to replace the entire sheet that contains the brick. In the case of the Roadster there are 11 sheets each containing 9 bricks... the sheets can be replaced but the bricks inside the sheets can't.
But you are thinking if I put a low current into the brick, even if one cell is shorted the worst case is that all the current goes through that one cell and a 100mA into a 1Ohm shorted cell is only 10mW of heat... harmless. Find the bond wire with the slight voltage drop and cut it to disconnect the bad cell.
The problem...
If you cut the bond wire to the shorted cell the brick will start to charge (or maybe there is no shorted cell to begin with and the brick starts to charge). All is good for a bit - the voltage slowly rises as the brick charges. You will probably get to a volt or so then something happens... the voltage starts dropping rapidly. One cell that was accepting the charge just suddenly shorted. The heat generated is no longer based on the controlled current you are putting in but is now determined by how much the other 68 now slightly charged cells can deliver into the now shorted cell.
I did exactly this, but not giving up I found the newly shorted cell and cut its bond wire. Repeat. Repeat. Repeat. Repeat. Repeat. Repeat. Repeat. 8 times before I gave up the voltage reached hopeful levels then crashed back down requiring another bond wire in the same brick to be cut.
It's dead.
