Voltage difference terminal - bus bar, potential galvanic corrosion

CU17

New Member
Joined
Jan 12, 2021
Messages
4
Dear Community,



I have run into the problem of the terminal voltages differing from the bus bar voltage.



To give you some background on my setup:



I have a 16s Lishen 272Ah (280Ah) system with batteries from Shenzhen Basen. As a sidenote: They tested between 282 and 287Ah.

My system is supervised by a Batrium WatchMon 4 with Longmons (I had that one leftover from my 18650 battery built) and the Batrium controls cooling fans and a relay (Kilovac EV200). I have also designed a little pre-charge circuit to prevent high inrush current. The battery is connected to a Sunny Island 6.0.



After about a month of the system running, I also compressed the cells due to the aforementioned difference in voltage between bus bar and cell terminal voltage. I first thought this was due to the cells slightly expanding and contracting, changing the contact area between bus bar and service. However, this turned out to potentially not be the case.



Removing the washers holding down the bus bar and moving it a little bit on the cell terminal and re-fastening it fixes the problem for a couple of days/ weeks, but the problem also always occurs with different cells and I can’t clearly define a pattern, if it mostly happens on the positive or negative cells.



The cell voltages measured at the bolt screwed into the cells always measure a correct and reasonable cell voltage but the BMS (measuring on the bus bar) measures a sometimes 100mV higher or lower voltage (charge – discharge at 70A ) than what the actual cell voltage is, causing it to stop the charge / discharge or limit the current.



I would obviously like to have a fix that doesn’t involve unscrewing and re-fastening the bus bars every couple of days and I am not sure if the slight changes in size of the cells (even if the cells are somewhat compressed) or some form of galvanic corrosion causes the problem.
 

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Scph9002

Solar Enthusiast
Joined
Aug 28, 2020
Messages
164
Try with space between each cell to let them expand freely. Its not like not compressing cells will make them not last for thousands of cycles and plenty of years :p

Maybe even stressing the terminals like that is even worse
 

Bob B

Photon Sorcerer
Joined
Sep 21, 2019
Messages
3,885
If it were my pack ..... I would remove all bus bars .... compress the cells .... thoroughly clean all cell contact pads and bus bars .... re-assemble with a thin layer of noalox on each connection point.

There are a lot of posts with info about cleaning your contact points.
 

Roswell Bob

Solar Addict
Joined
Dec 5, 2020
Messages
575
Location
Warner, NH
Dear Community,



I have run into the problem of the terminal voltages differing from the bus bar voltage.



To give you some background on my setup:



I have a 16s Lishen 272Ah (280Ah) system with batteries from Shenzhen Basen. As a sidenote: They tested between 282 and 287Ah.

My system is supervised by a Batrium WatchMon 4 with Longmons (I had that one leftover from my 18650 battery built) and the Batrium controls cooling fans and a relay (Kilovac EV200). I have also designed a little pre-charge circuit to prevent high inrush current. The battery is connected to a Sunny Island 6.0.



After about a month of the system running, I also compressed the cells due to the aforementioned difference in voltage between bus bar and cell terminal voltage. I first thought this was due to the cells slightly expanding and contracting, changing the contact area between bus bar and service. However, this turned out to potentially not be the case.



Removing the washers holding down the bus bar and moving it a little bit on the cell terminal and re-fastening it fixes the problem for a couple of days/ weeks, but the problem also always occurs with different cells and I can’t clearly define a pattern, if it mostly happens on the positive or negative cells.



The cell voltages measured at the bolt screwed into the cells always measure a correct and reasonable cell voltage but the BMS (measuring on the bus bar) measures a sometimes 100mV higher or lower voltage (charge – discharge at 70A ) than what the actual cell voltage is, causing it to stop the charge / discharge or limit the current.



I would obviously like to have a fix that doesn’t involve unscrewing and re-fastening the bus bars every couple of days and I am not sure if the slight changes in size of the cells (even if the cells are somewhat compressed) or some form of galvanic corrosion causes the problem.
Have you been able to resolve problem?
 

CU17

New Member
Joined
Jan 12, 2021
Messages
4
Have you been able to resolve problem?
Yes, shortly after writing the post I fixed the problem (at least at hasn't reoccurred since then).

I removed all busbars, bolts and washers and cleaned them and the cell terminals thoroughly. For the busbars, botls and washers I first used a steel sponge (do not do this on the terminals - the risk of shorting something is too great) and then used fine grit sandpaper. I used the same sandpaper on the terminals to clean them.
The last and probably most important step is to apply Noalox to each bolt, terminal and busbar. I also used the sandpaper to do this. You only need a very small film per terminal.
I'd first clean all the busbars, nuts and washers, and then clean one battery terminal, apply the Noalox and install the busbar before moving to the next. That ensures as little corrosion as possible depending on the temperature and humidity conditions you're working in.

Hope this helps
 
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