On the Battery page, we want to find the CAC values.  CAC stands for Calculated Average Capacity.  As the name implies, this value is calculated by the BMS over time.  It is measured in Amp hours (Ah).  A lower CAC imbalance is better indicating the bricks all store a similar amount of energy.  The capacity is the key here.  Have you ever had a failing rechargeable flashlight battery that charges up full to x.x Volts but dies really quickly when you use it?  This is what the BMS is calculating for the capacity of each Brick in the pack.  It measures Voltages at various states of charge (SOC) and compares them with Voltages at other SOCs and as the pack charges and discharges to calculate the CAC.  It does this continually and as one or more Bricks start to lose capacity, it compares the weakening brick to the strongest brick and adjusts the calculated imbalance value.  When that value gets big enough or changes quickly enough, it will throw on of the 'worst' errors - the dreaded BMS_u029 code.

It may be counterintuitive, but a weaker brick/cell with less capacity will have a higher Voltage at a high SOC and a lower Voltage than the neighbors at a low SOC.

On the BMS tab, you get a listing of the 'Cell' Voltages and the 'Cell imbalance' listed in Milli Volts (mV).  You want that number as low as possible but it will always be greater than zero.  In our example here, 8mV is pretty good.   8mV means the difference between the highest brick value and the lowest brick value will be .008 Volts.

This is also a good time to remind readers that SMT inaccurately identifies these as 'Cells' while the readings are actually of the bricks of 74 cells in parallel.  Refer to Basic Architecture tab if you missed that information.

As you know from the Basic Architecture information, Cell (Brick) 1 through 6 are in Module #1.  Cell 7-12 are in Module #2 and so on.  This is important because many of the failure modes discussed later are obvious when you look at the SMT cell values and see one or two of the values in a specific module are simply wrong or missing completely.  The most common bogus values come from the last two bricks in each module.  If your 5th and 6th bricks in a module are wacky, that is actually great news!  That almost certainly means your battery cells are fine and the problem is in your BMB board.  This is the most common but also the most 'fixable' of all the v1 pack errors.

When something goes wrong with the car, one or more alerts pop up on the MCU screen as 'Active Alerts'.  They can provide some descriptions but Service Mode offers better resolution.  To enter Service Mode, go to 'Software' on the MCU.  Press and hold 'model' (i,e. P85) for 3 seconds.  A password box will pop up - type 'service' and you will be in Service Mode.

As mentioned, any High Voltage error including the BMS will cause the battery contactors to open and not provide power to the 12V battery.  The 12V will quickly go dead in several minutes to a few hours.  Service mode will provide EVERYTHING wrong it can find, in no particular order or relevance.  When you have BMS errors, you can essentially ignore warnings about 12V.

Take photos of what is on the screen.  Don't count on remembering it later.