battery and BMS both read 14.4v but terminal lugs (past the BMS) reads 14.12v? Is that normal?
- Thread starter bordenov
- Start date
just sitting yea. Idk if the connections after the BMS aren't perfect, the BMS is going bad, or if this is completley normal lol
Steve_S
Offgrid Cabineer, N.E. Ontario, Canada
Depending on what the Terminals are made of, some voltage drop is to be expected but generally isn't very large. Terminals are most often made of brass (a very highly recycled metal) with some being better quality than others, they can also be plated which can also affect voltages depending on what the plating is, ie: nickel, chrome or tin.
I saw somewhere that .1- .5 voltage drop is normal? How can I check to see my BMS is ok? The BMS itself reads the correct voltage of my battery it’s only after the BMS that the voltage drops .3v.
Hmm yea now after using my batt a bit the measurements are 12.99 from the lugs after BMS and 13.5v before the BMS lol seems like mines maybe shot?
Bud Martin
Solar Wizard
- Joined
- Aug 27, 2020
- Messages
- 4,844
So the battery teerminals are not connected to any at all, correct?
The meter has 10MOhm input so to read open Voltage of 0.3V < than the Voltage at the battery terminals (before BMS), there has to be high enough resistance in the BMS's MOSFET's and other connections to cause Vrops like that.
i just assembled the battery so no it hasn't been hooked up to anything, although I just was messing with my temperature controller and silicon heating pad to get the settings right so now the voltage of the batt is 13.5v while the terminal lugs post-BMS are reading 12.99v. This is my second battery since the first one I built didn't have anything insulating the cells from each other and so two of them rubbed against each other and started leaking electrolyte. I just assembled this new one but with the old BMS I was using so maybe something happened while it was connected to my old battery.
Since the difference is now .5v instead of the .3v it was before I'm a bit more concerned lol. I don't remember there being much of a difference with my first batt but I'm not entirely sure.
Bud Martin
Solar Wizard
- Joined
- Aug 27, 2020
- Messages
- 4,844
So now you see 0.5V difference without load, that does not sound right, I suspect measurement error.
What Voltage do you see when put the meter probes between P- and B- of the BMS?
Your meter is on 20VDC scale?
You mean C- and B-? At least that’s what’s on mine. It measures .21v.
I just rechecked and yea now it’s saying 13.2v after the BMS and 13.5v before.
Sounds like perhaps some calibration is in order?
what do you propose?
There are others on this forum that could better answer that. There should be a calibration potentiometer you can adjust. You want an accurate meter or two to verify and probably at 70F.
I've done it many times at work but I have never done a DYI BMS. You should have a manual or factory support for this. I would call the place you got it from and start there. It needs to be accurate to protect the cells.
Do you have a calibrated volt meter? How do you know that yours is correct?
Similar threads
