Pack Architecture

Your Tesla battery pack is made up of thousands of Panasonic (primarily) 18650 lithium-ion cells in a series-parallel arrangement.  Some packs in our Model S/X family have 14 modules but most are 16 modules.  Some are software locked by Tesla to have a lower capacity than a 'larger' pack that contains an identical physical arrangement of cells and modules.  The older basic packs have 74 Cells in a Brick and newer packs have 86 Cells in a Brick (tighter physical density in the same form-factor).  There are 14 and 16 module 90kWh packs at 350V and 400V so there are LOTS of variables but their capacities are all measured in kWh with the smallest in our group being only 40kWh and the largest in our group being 100kWh.   For purposes of this example, I will describe the 85kWh pack, which was the 'big' one used from 2012-2015.

First, some definitions:

• Series and Parallel - In the simplest terms, a bunch of batteries connected in parallel connect all the + terminals together and all the - terminals together and a bunch of batteries in series connects one battery's + to the next battery's - and that battery's + to the next battery's -, etc.  Batteries connected in parallel, share their voltages (i.e. jump starting a car) and batteries connected in series add their voltages together (i.e. a 3V flashlight bulb running on two 1.5V AA cells).  A series/parallel arrangement like our battery packs have combines the two connection methods.  Wikipedia can give you more details.

• Cell - This is the individual 18650 battery 'cell' that is a cylinder 18mm in diameter and 65mm tall.  This cell is used in many rechargeable flashlights and your grandmother would say it looks like an oversized AA 'battery'.  A fully charged cell is a little over 4 Volts and the battery is considered empty a little below 3 Volts (approximate/ballpark).  The actual 'full' and 'empty' Voltage values for various generations of cells are a little different based on the cell chemistry so please excuse the approximation.

• Brick - A brick is 74 (in our 85kWh example) cells connected to the same plate in parallel.  The 74 + terminals are connected to the same metal plate and the 74 - terminals are connected to a different metal plate on the other end of the cells.  The brick still has the same 4 Volts as a single cell but with 74 of them in parallel, they store 74x more energy.

  • ScanMyTesla Confusion!  SMT incorrectly calls a brick a 'Cell'.  The BMS/BMB and SMT can't give you individual discrete data on individual cells but it does on bricks - it just calls them 'Cells'.

• Module - The module is made up of six of our bricks in series connected though a series of top and bottom plates.  Brick #1's + plate (with 74 battery + wires) is connected to the + terminal of the module.  The - side of Brick #1 is connected to a plate that has those 74 connections but that same plate ALSO connects to the + side of Brick #2's Cells that are next door but upside down in relation to cells in Brick #1.  There are 148 Cells connected to that plate (74+ and 74-).  The - side of Brick #2's Cells are connected to a plate that shares a connection with the + side of the 74 Cells from Brick #3.  Brick #1, 3 and 5 are facing 'up' and Brick #2, 4 and 6 are facing 'down'.  When we finally get to Brick #6 (which has cells that are upside down relative to #1), those 74 - connections are connected to the - terminal of the module.  The Module is approximately 24 Volts (6 Bricks x 4 Volts) and made up of 444 individual Cells.  

  • As mentioned - packs of this generation made after 2015 have 86 cells in a brick and 516 cells in a module.  This gave us the 16 module, 400v 100kWh packs and the 14 module, 350v 90kWh packs.

• Pack - The full High Voltage Battery Pack is made up of 16 of these modules in series -- one module's + is connected to the next module's  - and so on.  The MODULE numerical order is NOT the High Voltage bus order.  16 Modules at approximately 24 Volts give you approximately 400 Volts total.  Module #1 is on the left side at the back and #7 is at the front on the left.  Module #8 and #9 are stacked in the 'hump' at the very front (8 on the bottom and 9 on the top).  Interestingly, Module #9 is the only one in the Pack that is mounted upside down even though it is identical to the others in every way.  Module #10 is across from #7 and #16 is at the back across from #1.  In the 14 Module packs, there are no modules in the hump so the consecutive numbering is maintained with #8 being across from #7 and the final Module (#14) being across from #1.  The exact routing of the HV bus (which + goes to which -) is explained below.

Isolation Fault

What does that even mean?  The first thing to understand is that an isolation fault makes EVERYTHING about working around high voltage more dangerous.  The easiest way to explain the basic issue is to take a 9V battery and a simple volt meter.  Touch a probe to + and the other probe to - and you read voltage.  Hold either probe to one of the terminals and the other to the side of the battery, and you get nothing.  The energy inside the battery is isolated inside and not connected to the case.  This is the way the 9V battery and all batteries including our Tesla pack should be.  If everything is the way it normal is, you could theoretically stand barefoot on a wet piece of metal and touch the outside of the pack and there will be no voltage present.

Now image, you touch the + of the 9V with one probe and the case with the other probe and you get voltage.  That would be bad because that means part of the battery circuit is making contact with part of the case.  The energy inside is no longer isolated inside the battery.  This is not good.  A 9 Volt battery is cheap and disposable and won't hurt you so they don't have complicated isolation circuits to detect that kind of problem... but Teslas do.  We don't need to understand all the sorcery going on inside Tesla's safety circuits but what we do need to know is that an internal isolation error (internal means inside the pack) means all of our slow and methodical safety protocols should get slower and more methodical.

Don't try that 9V trick inside the case of the Tesla pack.  The isolation circuit does allow a tiny bit of HV (high voltage, low amperage) out to test for isolation faults.  If you touch part of the HV bus and any part of the metal case, you will get zapped (i.e. taser painful, but generally not death).  Treat every touch-point as the potential deadly danger it is until you get multiple modules removed and the total potential voltage is in the safe range.

If you missed it from the safety section, please watch this video on loss of isolation by Weber Auto on YouTube

High Voltage Interlock Loop (HVIL)

The HVIL is a safety system that 'loops' all of the HV components together in series.  If any of the components thinks there is an issue, it will 'open' the loop which will open the contactors in the pack and pretty much nothing will work.  When this happens, there will be errors on the MCU and in service mode that try to identify what component is having the issue.  Sometimes the information is more granular than others.

You can manually open the HVIL when you are working on this by unplugging the 'Fireman's Loop' at the righthand base of the windshield.  This loop is there for First Responders to literally cut if the car is in an accident but we unplug it as part of our service routine.

Note - an error in the HVIL is different than an isolation fault so the troubleshooting, diagnosis and repair is different.