4s BMS 120a LifePo4 Battery Management System for 12v DIY Batteries, Programmable, Bluetooth Included.

(13 customer reviews)


  • Includes a Bluetooth communication module.
  • BMS For 4 cell, 12v volt Lithium-iron-phosphate DIY batteries
  • 120 amp continuous charge or discharge current
  • Low Temperature Charging cutoff at -1c
  • Fully programmable with IOS app, Android app, or Desktop app

The high current wires will be hydraulically crimped into a single tinned copper lug on each side. – Not available with the threaded terminals or no-wires options.

SKU: 4sBMS Category:


User Manual: Click here to download the manual (PDF)

3D model: click and drag to rotate, or download the STEP file.

BMS For 4 cell, 12v volt Lithium-iron-phosphate DIY batteries.
120 amp continuous charge or discharge current.
Low Temperature Charging cutoff at -1c.
The Apps allow you to monitor SOC (state of charge), Current, Voltage, cell balance, temperature, and more.

Bluetooth Module is included, Balance wires are included, USB module is an additional purchase.
Fully programmable.

If you choose to add crimped copper lugs, the high current wires will be hydraulically crimped into a single tinned copper lug on each side, and finished with heat shrink tubing.

Final assembly, testing and calibration is done in Naples, Florida. Units with 8 gauge wires and/or crimped lugs are assembled in Denver, CO or Naples,  FL. The BMS is manufactured in China.

To view or change Parameters, there are 3 ways to connect to the BMS.
1. Bluetooth module to IOS app. The app is on the Apple app store, it is free to view operational data, but costs $6 to view and change parameters.
2. Bluetooth module to Android app. (download this version, don’t get it from the play store)
3. USB module to PC/desktop application. This application is free

IOS app is on the Apple app store. Search for “xiaoxiangBMS

Click here for Software Downloads and Support Resources.

A great resource for general DIY power education is www.Mobile-Solarpower.com

FYI for experimenters: The BMS communicates with a 5v TTL UART at 9600 baud. It is directly compatible with Arduinos, etc.
We have written a new Arduino library for the BMSs, now available on the downloads page. The beta version allows you to read all the monitoring parameters and switch charge and discharge on/off. Future releases will have full read/write capability. A python library is also in the works.

Additional information

Weight 1 lbs
Dimensions 6 × 4 × 1 in
Wire Size:

10 gauge x 12" (discontinued), M6 Threaded Terminals, 8 gauge x 12" (Standard), 8 gauge x 24" (+$22), No wires (-$8)

13 reviews for 4s BMS 120a LifePo4 Battery Management System for 12v DIY Batteries, Programmable, Bluetooth Included.

  1. RONALD MARK HYMAN (verified owner)

    First, Overkill Solar, Steve, does a great job at customer service. Very, very patient answering all of my beginner questions and holding my hand through the purchase process. I received my fortune cells and 2 BMS’s quickly and extremely well packaged. I had my BMS’s upgraded with heavier and longer cables and the quality of the work is outstanding. And note that the customer service has continued cheerfully after the sale. I heartily recommend Overkill Solar as a go to source for Lithium cells and Battery Management systems.
    Ron in Mexico

  2. Randall Reed (verified owner)

    Bought this bms after my Daly was doa. Great product and easy to use. Very fast shipping. Great resources on site. Thanks

  3. Dave

    Just finished installing two 120a BMSs on my batteries. Followed Steve’s directions and was pretty straight forward. I got the 8 AWG longer wires which worked well for me. Blue tooth connect with the App was flawless. Discharging batteries now to see how things go.

    I have it mocked up on a bench before it goes in the van to work out bugs.

  4. Dave

    Installed two 120a BMSs for my batteries. Super easy to install and the Bluetooth connect was perfect. Adjusting the settings for my battery was easy once I paid the $5.99 for the deluxe version of the app. Discharging now to see how things track.

  5. Lynn

    Happy with the BMS, works great, easy to program and the IOS app works well. This is my 3rd BMS of this type. I have a questions about the app. What do the length of the blue bars mean? They don’t correspond to the voltage. And the “balancing” note does not appear every time the voltage delta is exceeded. One battery has issues with one cell getting out of balance. Both at the high end and the low end of the voltage cycle.

  6. Marc (verified owner)

    Steve at Overkill Solar provided stellar support, way beyond what I have ever experienced elsewhere. Not only did he answer my beginner’s questions promptly, but when the temperature sensors broke off one of the two BMSs I had ordered, he immediately shipped another one totally free of charge to Curacao where I was with my boat. He even took care of customs fees.
    I’ve had the two BMSs operating for over a month now and they perform flawlessly.

  7. James Hansen

    I just purchased.

  8. dennismichaud83 (verified owner)

    I built two 24 volt batteries with 8s BMS’s. Everything works really well and I get full capacity out of the batteries. I have been using them for a couple months now and I am really happy with what I got. Steve answered all of my beginner questions very patiently. Good people, good product!

  9. Scott (verified owner)

    Been running the 120a BMS for about a month now and it has been great. Solidly built and easy to use. I love using the app. It is very easy configure even as a relative beginner and the range on the wireless connectivity is fantastic. The information it provides on the charge of the cells and the battery as a whole has been great to have with out needing a bunch of other equipment. Steve has been fantastic as well. Lots of confirmations on my order as it was shipped gave great confidence and it was shipped super fast. I could not be happier with this product and the excellent service.

  10. david.white (verified owner)

    I am new to these batteries and as such I started with an inexpensive Ali Express BMS. They (2) arrived and I started testing. ZERO support from the manufacturer, they could not answer the most basic questions about set up.
    I found Overkill through a bunch of internet searches and started asking questions. Steve was great, I ordered 2 of this BMS (4s, 120 A). They arrived, the documentation was awesome, left very few questions. The questions I did have to ask were answered quickly and completely. I have been testing the BMS’s now for a couple of weeks and they work great. I tested every protection parameter and in each case the BMS behaved as I would think it should. Can’t recomend high enough.

  11. michael sullivan

    i am very lucky to have found this bms… still in the very early learning stage’s BUT appears to be going well . thank you overkill for doing it right . mike uk

  12. John (verified owner)

    I was in desperate need for a BMS and Steve was able to get me one and after installing it on my 280ah battery I built all I can say is it’s fantastic. The capabilities of this thing is the best you are going to get. I felt I was taking a big risk purchasing these 280AH cells but with this BMS I have complete confidence in my battery that I wouldn’t have with a lesser BMS.

  13. Michael (verified owner)

    Very satisfied with this 12V BMS for my 4S LiFePO4 battery build.
    Works as advertised. Adequate documentation.

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