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.
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 email@example.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.
Good news, all orders on overkillsolar.com can now be tax free!
How is this possible you ask? Because we are no longer shipping items from California, we are not obligated to collect tax for any states other than Florida. *
But what about Florida sales taxes? The Great State of Florida™ offers a sales tax exemption for any purchase of equipment to be used in a solar power system. Read tax information publication TIP_19A01-09 from the Florida DOR for more details: Florida Solar Tax Exemption (TIP_19A01-09)
We now have this tax exemption applied by default on the checkout page for all orders that are shipping to Florida. If your order is NOT to be used in a solar power system, you must uncheck the checkbox so that the proper sales tax can be applied to your order. Otherwise we will record your certification and acceptance of the tax exemption.
*Note: As always, all individual taxpayers are expected to file and pay “use tax” on items that they purchased online if the merchant doesn’t collect taxes for their home state. You do pay your use tax on out of state purchases, right? RIGHT?
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.
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.
We chose these 2 models because they both have a well formed sine wave output with minimal distortion, and a reasonable price.
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.
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.
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.
Run time until inverter cutoff at 5% SOC
Inverter only, no load. (standby power)
100w equivalent led lamp
60w equivalent led lamp
Small Air compressor (note 1)
Desktop Computer (note 2)
Hair Dryer, labeled “1875”
Vacuum Cleaner, (note 5)
20″ floor fan, high speed
20″ floor fan, low speed
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.
Run Time until inverter cutoff at 18% SOC
Inverter only, no load. (standby power)
Microwave oven rated 1150W (note 4)
5,000 BTU window AC (note 3)
Laptop charger rated 64w
55″ TV and sound bar w/sub (note 6)
Hair Dryer, labeled “1875W”
Vacuum Cleaner, (note 5)
20″ floor fan, high speed
20″ floor fan, low speed
1. California Air Tools 1P1060S, rated 4.5 amps 2. Small form factor desktop PC, Windows 10, Intel i7, idle, with 3 monitors, plugged into an APC UPS 3. Cool-Living CLW-15C1A-JA09AC window air conditioner. 5000BTU/h, rated 4.0 amps 4. GE Microwave JES1657SM1SS, Rated cooking power 1150 watts 5. Shark NV70 31, Upright household vacuum with brush roll powered, rated 10 amps 6. Element 55″ Roku TV and old Vizio soundbar with bluetooth surround and subwoofer, playing Bob’s Burgers at party volume.
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.
30 Nov 2022: The latest update adds “background location” permissions. This was necessary for some (not all) Android devices to discover the Bluetooth modules.
17 Nov 2022: The first release has all the basic functions that allow you to view your BMS status, change parameters, and calibrate.
The Android app requires a bunch of permissions to work correctly. Please allow precise location, media/files, and Bluetooth discovery. All this is necessary for Bluetooth to work on all Android versions. We DO NOT collect any personal information with the app, and it does not track your location in any way.
It also automatically unlocks password protected BMS firmware, and will resolve any issues with lost or incorrect BMS passwords.
Note: the password/PIN function has been completely bypassed by this release. If you are concerned about unauthorized access to your BMS via Bluetooth, you should unplug the Bluetooth module when it’s not in use.
UPDATE: We think we have fixed the bug that stops some Android devices from finding the Bluetooth modules. Anyone with this issue should update and try again. Your app should say “V1.1.1 -V10” at the top of the HOME tab. If not, uninstall and reload from the play store.
The new app is still in the beta test stage. We have found that some Android devices are unable to find the Bluetooth modules using our new app. The issue is with specific permissions that need to be requested by the app.
If you have a bug to report, please email Support@OverkillSolar.com and describe the bug plus which device and Android/iOS version you are using. Thanks!
HOWEVER no, it is not capable of doing an initial balance on new cells.
The balancer works by connecting a tiny bleed resistor to the cells with the highest voltage, and the excess energy in those cells turns into waste heat. This is a slow process. The intention is that the BMS can maintain the balance on the cells as they slowly drift over their lifetime.
A batch of new cells needs to be top-balanced before they can be expected to charge properly as a battery pack.
Why top balance?
Why? Because of the nature of the LiFePO4 voltage curve. At the top end of a charge cycle, the cell voltage spikes quickly, and charging must be stopped to prevent damage to the cells. If one cell is at a higher state of charge, (in terms of amp-hours or coulombs), even by a small amount, it will spike while the other cells are still in the “bulk” phase of their charge cycle. On the graph below, the red line is the highest cell, which triggers a “cell overvoltage” alarm before the pink/green cells get to a full charge. The BMS must then disconnect to protect the high cell, and the battery pack will be at a lower voltage than expected. You want all the cells to spike up at the same time, and the only way this can happen is for them to be well balanced.
There are several ways to manually balance cells, depending on what equipment you have access to.
Method 1 – cc power supply
The best way in my opinion, is to use a regulated power supply to charge the cells to 3.65 volts each. The cells would be connected in parallel as a single cell and charged together (without the BMS), then re-assembled into the series-connected pack with the BMS.
Will Prowse demonstrates in this video:
Method 2 – manual bleed resistor
Cheapest way: Connect a load to the high cell in your pack to quickly bleed off the excess energy. I tried this method using a random car light bulb with some alligator clips on the leads. You need to watch the cell voltages closely because its easy to go too far.
Method 3 – passive equalization
What does NOT work is the old recommendation of connecting your new cells in parallel and letting them passively equalize for hours or days. This does not work because of the flat charge curve. They are at almost the same voltage even if they are far apart in state-of-charge. Basically the cells don’t know that they aren’t balanced unless you can push them into the very top end of the charge cycle.
What about cell matching? Cells have a certain internal resistance. Grade-A cells are tested at the factory to confirm that their internal resistance is acceptable, usually <1 milliohm. If your battery pack is made of grade-B cells or cells of different ages or if they have been damaged before, then they are not matched. Mismatched cells will quickly become unbalanced when the pack is cycled. This is one reason why you should pay for good grade-A cells.
I bought 4 of the very cheapest low grade garbage cells from aliexpress, just for experimenting. I balanced them several times, but after even 1 cycle of charging and discharging, they are way out of balance. This is because they are not matched at all. Some cells have a high internal resistance, so they get hotter than the better cells, and this puts them at a lower state of charge. If you are trying to use crappy cells like this, you will only be able to charge them up to ~80% to avoid constant cell over-voltages. This might be good for a big cheap solar storage bank, but it can cause big problems for a pack that you cycle daily, or use with large loads.