Battery modules from Model S and Model X cars come in several revisions, known as Rev A, Rev B, and Rev C. They look very similar in photos, but the balance wire connections differ. Use this guide to identify your revision before ordering a BMS kit.

Outside

Inside

If you have the chance to remove the old Tesla BMS board, It’s easier to see the differences.

Misidentification or Misrepresentation?

Unfortunately it seems to be quite common for sellers of these batteries to send a different revision, or even show photos of different revisions on the same eBay listing. If you end up with a different battery that doesn’t match your BMS kit, contact support@overkillsolar.com and we will exchange the adapter board.

For example, I found these photos on eBay- one shows a pallet of Rev A modules, and the next photo is a close up of a Rev B module:

Tesla battery capacity

Each car battery can be broken down into 16 modules. Each module is a 6 cell Lithium-ion battery that puts out 24 volts, and they weigh about 65 pounds each (30kg).

The capacity of each module when new can be found by dividing the car’s advertised battery size by 16. Example: for an 85kWh (kilowatt-hour) battery pack, each module holds 5.3kWh when new. The capacity degrades with age, as all batteries do.

We have also seen examples of modules that were damaged by leaking coolant, which corrodes the bond wires to each of the individual 18650 cells. Modules in this condition will have a significantly lower capacity than a healthy module from the same car.

Model 3 / Model Y batteries

Model 3 and Model Y cars have a different type of module. The car battery contains 4 large modules, each of them has 24 cells and they put out about 96 volts. We do not recommend these modules for DIY projects due to the extreme danger of working with 96 volts DC.

This PDF describes how to modify an Overkill Solar BMS for alternative cell counts.

Overkill Solar normally stocks 3 basic BMS models, which are configured for the most common Lithium Iron Phosphate (LiFePO4) battery setups: 4 cell 12v nominal, 8 cell 24v nominal, and 16 cell 48v nominal.

When using a different cell chemistry such as classic lithium ion, the number of cells needs to be adjusted to reach a usable system voltage. Common setups are 6 or 7 cells for a 24v system, and 12 or 13 cells for a 48v system.

Download the PDF instructions to read on: BMS_configuration_for_other_cell_counts-4S_and_8S-Overkill_Solar_LLC.pdf

Adapting an 8s BMS for 6s Tesla modules

Our Tesla BMS kits include a 8s BMS which has been preprogrammed with the parameters for 6 cell Tesla modules, and the included adapter board makes the necessary connections to make the BMS work with 6 cells. If you already have a BMS in the 8 cell configuration, you can adapt it for 6 cells by shorting 2 pairs of the balance wires together as shown in the above PDF, and in the photo below:

After making these connections, you can load the configuration file “6s_Tesla_Li-ion” from the Overkill Solar mobile app.

This is a Portable battery pack that we put together for demonstrations. We found this unused Bosch tool case that was just the right size for a Tesla Model S battery module with the BMS installed. It has a power switch on the outside which is connected to the BMS’s SW input, and the indicator light is connected to the 24v output (C- to B+). The output connector is a 120 amp Anderson connector mounted flush on the end, and we attached matching connectors to a suitable power inverter and charger.

This setup provides 3.6kwh of portable power that looks right at home on any job site.

Because 6 cell Lithium-Ion batteries have an end-of-cycle voltage of 18v, inverters designed for 24v systems may cut off early, reducing the available energy from the battery.

In this experiment, we connected 2 different 24v pure sine inverters to a used Tesla Model S battery module, and checked the power consumption of a variety of constant loads.

The battery for this test is a used 2014 Tesla model S module which delivered 3,632 watt-hours during a full cycle load test.

Note that the 2 inverters have different non-adjustable cutoff voltages.

The AIMS documentation specifies “Input under-voltage alarm 19.6 ± 1VDC”, and as tested it alarms at 19.5v, which is at 5% SOC on the test battery.

The WZRELB documentation specifies “Low Voltage Alarm 19.5-21.5v”, and it started beeping at 20.3v, which is at 18% SOC.

The cutoff voltage was tested using a variable DC power supply and a 23w light bulb loading the inverters.

Conclusions

This demonstrates that careful selection of your equipment will help get the most out of your
used tesla battery, without paying a premium price for programmable inverters.

The inverter with a lower cutoff allows you to utilize the full capacity of your battery.

On the other hand, you may prefer the inverter with a higher cutoff, which may extend the cycle life of your battery by not deeply discharging it.

A few surprises stand out in the test data.

First, the microwave oven rated “1150 Watts” could not be started using the 1,500 Watt inverter. When powered by the larger inverter, it pulled 1,888 Watts from the battery, nearly twice the power on the sticker! This is likely due to a large reactive component in the current flow used by the microwave. Because we measured the load using actual DC power flow from the battery, the extra energy must be dissipated by either the inverter or the oven itself. Further testing is needed to determine the most efficient way of powering a microwave oven on inverter power.

The 20″ floor fan only consumed 20% more power at high speed vs low speed, but it moved a lot more air. This fan uses a shaded pole induction motor, which is inefficient at low speed. Running one fan at high speed is much more efficient than running 2 fans at low speed.

Another interesting data point is the hair dryer. It consumed 86 Watts more when running on the AIMS inverter (which is actually 388 watts more than the rated output). This is because the AIMS inverter was supplying 117 Vac, versus 115 Vac from the WZRELB inverter. Since the hair dryer is a nearly 100% resistive load, it’s power consumption is directly proportional to the RMS AC voltage. Most of the other items tested showed little variation between the 2 inverters.

Test Data

Table 1: Average run time using the AIMS 1500w pure sign inverter

As tested, the AIMS 1500w pure sine inverter alarms at 19.5v, which is at 5% SOC of the Tesla battery.
The Tesla Battery will deliver 3450 watt-hours before this inverter’s cutoff voltage.
Wattage is measured from battery draw, including inverter efficiency losses.

Table 2: Average run time Using the WZRELB 2500W pure sign inverter

As tested, the WZRELB 2500W pure sine inverter alarms at 20.3v, which is at 18% SOC of the Tesla battery.
The Tesla Battery will deliver 2,987 watt-hours before this inverter’s cutoff voltage.
Wattage is measured from battery draw, including inverter efficiency losses.

External product links on this page do not contain affiliate trackers- we will not make a profit if you buy either inverter.

LiFePO4 is the chemical formula for the cathode material in a Lithium-Iron-Phosphate battery.

Lithium iron phosphate exists naturally in the form of the mineral triphylite.

LiFePO4 is sometimes abbreviated as LFP.

LiFePO4 chemistry offers a considerably longer cycle life than other lithium-ion chemistries. Under most conditions it supports more than 3,000 cycles, and under optimal conditions it supports more than 10,000 cycles. NMC (Lithium-Ion) batteries support about 1,000 to 2,300 cycles, depending on conditions.

LiFePO4 cells experience a slower rate of capacity loss (a.k.a. greater calendar-life) than lithium-ion battery chemistries.

The major differences between LiFePO4 batteries and other lithium ion battery types is that LiFePO4 batteries contain no cobalt (removing ethical and economic questions about cobalt’s availability) and have a flat discharge curve.