Description
User Manual: Click here to download the manual (PDF)
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 Naples, FL. The BMS is manufactured in China.
To view or change Parameters, there are 3 ways to connect to the BMS. All 3 applications are free.
- iPhone App: Overkill Solar Mobile App
- Android App: Overkill Solar Mobile App
- USB module to PC/desktop application.
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.
Eric Hansen (verified owner) –
Package arrived in good time (shipped to Canada) with no damage. Installed the 120amp 4s BMS on a DIY Lifepo4 280 ah battery. It was easy to set up with the app and has worked flawlessly!
Adam MacDonald (verified owner) –
Great to have the peace of mind, education, and support afforded by the team at Overkill Solar. I built 3 DIY 300AH batteries with Ali Baba cells from Basen (great quality by the way). The BMS arrived very well packaged and with great instructions on how to assemble the battery. I’m up in Canada and my heart sank a bit when I saw UPS arrive (usually they charge insane import fees from the US). Luckily it seems like the shipping included these fees! The three units I ordered arrived in little under a week, really quick.
Now some negatives: the Chinese software is beyond janky. This isn’t the fault of the guys at Overkill Solar, but be warned that especially for multi-battery units, you should buy a Victron smart shunt or something similar. Relying on this software will screw you over at some point. Plus, switching between multiple BMS’ in the software is not always as easy as it should be. I sort of expected this, but just keep it in mind. Oh, and do NOT press the lock button no matter what, haha.
I would upload an image of my build, but I don’t see an option for it. I will say these BMS units will be my go to in the future when I build another 24 batteries.
Harrison Clark (verified owner) –
I’ve been using Overkill BMSs for about six years. Out of seven units, just one was faulty. I shipped it back on a Tuesday and had a replacement on Friday!! I’ve tested each one right up to its max rating and only that one had an issue. They run very cool.
The new BT app is a HUGE improvement. Like the old one, it tends to update the SOC via the voltage lookup table unnecessarily such as during storage and when changing settings. It does not do this in routine usage so this is a nuisance and not a major problem. That’s my only gripe though I’d sure like to see all four of my 12V batteries summarized on one screen. Nobody seems to do that.
The balancing current of this BMS (12V one at least) seems designed for 100 Ah batteries but is sufficient on larger batteries if the batteries are well behaved. It’s fine on cells like the Calb SE200 Grade A cells. On the typical “blue” 280 Ah grade B cells a more aggressive setting may be necessary (start balancing at a lower voltage and target a smaller delta voltage). The unit has this flexibility.
Where even more balancing is needed, one can charge to a lower bulk voltage (around 13.8 or 13.9V) and hold voltage there for an hour or two via the “absorb” setting before dropping to float voltage. This will add balancing time and still get the battery to 100% SOC. This is an option if your solar controller has a “user” mode with this flexibility and you have enough solar to complete the charge before sundown.
BTW, after using grade B 280 Ah “blue” cells for a few years and then upgrading to Calb SE200 cells, I’m not looking back. The SE200 cost per Watthour is higher but worth it. Compression is built-in, capacity is predictable, terminals are stout, battery construction is simpler, etc. I do hope that Overkill will stock the SE200 someday.
Bob Staples (verified owner) –
Fast shipping. Bought 2 of the 120a bms and some heavy custom wires. Had a bms go bad in one of the 3 100ah batts i got. I connect in series for a Minn kota 24v 80# trolling motor. So i basically did a total rebuild of 2 of my 100ah batts including top balancing. I prob could’ve made one 24v battery, but its nice to be able to have a 12v battery’s ready for the fish house in the winter. I bought a couple battery cases that the tops screw on from china, unfortunately not available in the usa. These are nice for accessibility if i have a problem. Which i don’t think i will with these bms’s. Got both batts put together last weekend, and i gotta say they work great. The app works great. I was able to name each batt to decipher which is which. The custom made wires they made me with different sized lugs on each end worked perfect for my situation. Overkill has some good products.