Lifepo Major Voltage Sag

[wpg205]

I'm running a 48v system with 16 Eve 280 ah cells (ordered from Basin) and during testing, I'm having issues with the voltage sagging significantly under any moderate load. For example, under a 700 watt load the voltage will drop from 53v down to something like 48v (I'm measuring at battery terminals, not at inverter).

I'm trying to identify potential causes. The cells are balanced and within 10 mv of one another. Under load, some cells seems to drop more than others, but not enough to explain the large drop in pack voltage.

My question is, on a per cell basis how much should I expect voltage to sag under load? Additionally, what could be causing the voltage sag? I suspect it is either bad cells or bad busbars/connections. I know at least one of the busbars that came with the cells had major issues, and now I'm suspecting more are (I've ordered copper bar to make my own). But besides bad cells or bad busbars/connects what else could it be? Has anyone else had similar issues?

Thanks for any thoughts.

 

[HRTKD]

Start with the easy stuff first. Bus bar connections are probably the cause of 50% of the voltage/cell issues seen on the forum.

Clean the terminal and bus bar surfaces with acetone. If the surfaces are not smooth, use a fine grit sandpaper. Scotch Brite can be used to remove any oxide layer. Dust off any oxides that were removed. Apply a thin layer of antioxidant and make your connections.

 

[DThames]

Start with the easy stuff first. Bus bar connections are probably the cause of 50% of the voltage/cell issues seen on the forum.

Clean the terminal and bus bar surfaces with acetone. If the surfaces are not smooth, use a fine grit sandpaper. Scotch Brite can be used to remove any oxide layer. Dust off any oxides that were removed. Apply a thin layer of antioxidant and make your connections.

Please let me add that you might be able to spot this with a meter if you measure from screw head to screw head on the two posts that are connected by a bus bar. There should be no voltage between those screw heads. If there is, you have a connections that is less than good.

 

[wpg205]

Thank you both. It was just terminal connections. I tightened them up drastically and sag went from 4v to .2v. I'm still waiting on new busbars to clean to the terminals, but I will do that as well. Thanks again.

 

[DThames]

Thank you both. It was just terminal connections. I tightened them up drastically and sag went from 4v to .2v. I'm still waiting on new busbars to clean to the terminals, but I will do that as well. Thanks again.

The terminal holes can strip out. A torque wrench is helpful.

 

[Steve_S]

Also before you install new busbars, homemade or otherwise, be CERTAIN that there are no burrs or ridges around the holes, if there are file them down so they are perfectly smooth. Those burrs can cause all sorts of issues.

 

[wpg205]

Thanks for the tips. I'm ordering the torque wrench now and will be lightly sanding the terminals. For the busbars I'll file then sand them, I know they need to be perfectly flat. I just didn't realize the degree to which sort of bad connections can lead to voltage sag under load. Its incredible important and I don't think emphasized enough.

 

[HRTKD]

A plastic, or other non-conductive, straightedge across the terminals could alert you to a situation where the tops of the terminals aren't all in the same plane.

 

[GSXR1000]

use studs and nuts

 

[wpg205]

I am using studs and nuts already, they just were quite loose on the terminals. In terms of the torque wrench, I've heard 35 foot pounds for the nuts, is there any specific recommendation for the studs or is it the same?

 

[HRTKD]

I am using studs and nuts already, they just were quite loose on the terminals. In terms of the torque wrench, I've heard 35 foot pounds for the nuts, is there any specific recommendation for the studs or is it the same?

35 inch pounds is appropriate for terminals with threads in them. Note, that's inch pounds, not foot pounds. This usually requires a 1/4" torque wrench. I use a digital torque wrench to ensure I'm not just in the ballpark, but at a very specific torque setting.

 

[wpg205]

Thank you for the correction! Thank god I didn't do foot pounds.