Voltage swing/ripple under load?

[Bryant Tonkin]

Hi,

I'm starting to test my battery and inverter setup and I'm noticing quite a bit of voltage swing/ripple being displayed on the SBMS0. I'm not sure if it's to be expected or if it's a sign of something wrong. 

Here are videos of the SBMS with load and without load so you can see what I'm talking about. I've also hooked up my oscilloscope and noticed a ~1.2V ~120Hz ripple on the total battery voltage.

• With Load (YouTube)
• Without Load (YouTube)

Here's my setup. 

• 8 x 280Ah LiFePO4 batteries in series
• Samlex 4024 inverter
• 3/0 wiring (~3ft long leads for testing) 
• My thermal camera shows that the only "hot" component is the 300A ANL fuse. Everything else is at worst warm.

Is some level of ripple to be expected and maybe the interface of the SBMS0 just makes it look extreme due to its sensitivity, or is something off on my setup? Are any of you experiencing something similar? 

Thanks!

[Bernd]

To me this looks as on my SBMS0 system with 8s 150Ah cells. If you look at the SOC curves of a cell there are several lines, voltage without charge/discharge, one with load and one with discharge.

under load the voltage might drop e.g. by 0.1V and while charging it will jump +0.1V, depending on the cell for sure...

But I think your cells are working within their limits.

[Bernd]

BTW: nice spider app :-(

[Dacian Todea]

Bryant,

Yes both of those videos show normal expected behavior. As you have an inverter and that is 60Hz the current to load will go trough zero 120 times a second so what you see on the DC side of the inverter in therms of current will be something like a rectified AC again with current going to almost zero 120 times a second and peak current quite a bit higher than the average current (that 143A the SBMS0 measures).

The fact that there are differences between cells under load means that either contact resistance between cells is a bit different or less likely some cells have higher internal resistance than others.

The ripple is from that 120Hz variation in current that creates the voltage fluctuation and the fact that SBMS sampling rate is not perfectly matching  so there is a bit of alias.

What happen with the LCD ?   I can send you and LCD if you can solder that.