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New DIY 12V Build - low battery voltage from BMS issue

ski3pin

New Member
Joined
Sep 20, 2021
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18
Location
West Slope Siera Nevada Range
I have assembled my first DIY lithium 12v battery with 4 3.2V 120ah cells using xiaoxiang 4S 120A Li-ion BMS. I followed the overkill solar directions. I've top balanced successfully and then connected the BMS and final assembly. I have this issue - the cell voltage measures 13.6V at the cell terminals but through the BMS at the final terminals the voltage is 11.9V. The android BMS app confirms battery voltage is 13.6V. I've double checked the balance wires and overall assembly and can find nothing. What I'm I missing, what should I look for? I appreciate the help.
 
I have assembled my first DIY lithium 12v battery with 4 3.2V 120ah cells using xiaoxiang 4S 120A Li-ion BMS. I followed the overkill solar directions. I've top balanced successfully and then connected the BMS and final assembly. I have this issue - the cell voltage measures 13.6V at the cell terminals but through the BMS at the final terminals the voltage is 11.9V. The android BMS app confirms battery voltage is 13.6V. I've double checked the balance wires and overall assembly and can find nothing. What I'm I missing, what should I look for? I appreciate the help.
Are you using the latest version of the app that's available from Overkills website? And are you using Android? I had a similar problem when I first assembled my battery and discovered there was a lock/unlock in the app settings. Once I unlocked it the BMS came to life at the output. But I am using Android. The latest version of the android app has the ability to turn off charging and discharging separately from each other.
 
Are you using the latest version of the app that's available from Overkills website? And are you using Android? I had a similar problem when I first assembled my battery and discovered there was a lock/unlock in the app settings. Once I unlocked it the BMS came to life at the output. But I am using Android. The latest version of the android app has the ability to turn off charging and discharging separately from each other.

Are you using the latest version of the app that's available from Overkills website? And are you using Android? I had a similar problem when I first assembled my battery and discovered there was a lock/unlock in the app settings. Once I unlocked it the BMS came to life at the output. But I am using Android. The latest version of the android app has the ability to turn off charging and discharging separately from each other.
I just double checked, I do have the latest version but I've gone through the settings and cannot find a lock/unlock in the settings. Under the menu heading "Battery State" near the bottom I find "locking" with a "no" to the right. I take this to indicate this BMS cannot be locked, to my way of thinking. I do see on the dashboard where charging and discharging can be turned on or off as you indicate.

I turned on and off the items I could and found no change to the low voltage reading.
 
Is there a mismatch between your LiFePO4 cells and a Li-ion BMS?

Is there a mismatch? Please explain your thoughts on this as I could be missing something. I based my parts purchases on what a fellow camper owner bought and assembled. In his application all is working well and he is happy. This is also very similar to the several batteries I've watched Will Prowse assemble in his videos.
 
I can only think a bad crimp on the lead that goes to the positive end of the battery. Either that, or if the Wire connector pugs into the BMS, perhaps it is loose or has a bent contact.

If you have done a multimeter test across the battery, but Overkill reads something different poor connection is all I have.
 
I've gone through the settings and cannot find a lock/unlock in the settings.
That was on the older app versions. A photo would be helpful to confirm you have everything hooked up directly. :)
 
First off, thank you all for your replies and suggestions. It is very helpful to be able to see this from many different angles.

Tomorrow I will closely inspect crimps and take a look at the balance leads' push connections into the BMS. I'll also some photos.
 
Tomorrow I will closely inspect crimps and take a look at the balance leads' push connections into the BMS. I'll also some photos.

I redid each crimped connector and the push connector is fully seated with no change to the low voltage difference. Here are photos of my battery for your critique.



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I assume it's due to welded studs. They way they do welded studs may reduce the contact area. Just a guess.
Hmm, that seems unfortunate for surface area. I wonder if the raised material is as conductive as the terminal post. Also wondering if drilling larger holes to sit down on the terminal post would make a difference.

I'd at least keep an eye on it, especially if you're not getting the amps (or they get warm) that you are expecting.
 
Can you show pictures of the app display.? There is a lock in the driving mode mode and the possibility of switching on/off the charge and discharge paths.
The app has a driving mode and a monitoring mode, it may be in the driving mode. This is selected in the 'app setting' page, you need to be in the 'monitoring mode'.

Mike
 
Can you show pictures of the app display.? There is a lock in the driving mode mode and the possibility of switching on/off the charge and discharge paths.
The app has a driving mode and a monitoring mode, it may be in the driving mode. This is selected in the 'app setting' page, you need to be in the 'monitoring mode'.

Mike
There is no lock mode with the latest version of the android app. Instead there are independent on/off modes on the main screen for charging and discharging and the OP has indicated he sees those settings. So he is not locked out.
 
Thanks for taking a detailed look. There is a small diameter shoulder on the terminal that the bus bars are seated against. Since I'm getting 13.6V at the cells, can the bus bars be an issue?
Did you try putting a charge on the battery through the BMS?
 
Hmm, that seems unfortunate for surface area. I wonder if the raised material is as conductive as the terminal post. Also wondering if drilling larger holes to sit down on the terminal post would make a difference.

I'd at least keep an eye on it, especially if you're not getting the amps (or they get warm) that you are expecting.
The raised material is aluminum. The stud is stainless steel. I believe these laser welded studs have been tested up to 150 amps with little heating at the stud.
 

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The raised material is aluminum. The stud is stainless steel. I believe these laser welded studs have been tested up to 150 amps with little heating at the stud.
The studs with the square base seem better, but I mostly see the raised round studs.
 
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