Fluctuating AGM voltage during Bulk stage

[vas]

I have completed an installation of 75/10 MPPT tied to 100W solar panel on PV side, and to 100Ah AGM battery on the battery side. When performing tests I noticed that during bulk stage the MPPT output voltage continuously fluctuated between ~12.8V and ~14.0V - see screenshot below.

What could be the potential reason for such fluctuations?

[pwfarnell]

A screenshot of your mppt settings, a history plot showing the PV input voltage and current at the same timeframe as the plot above and your panel details would be useful to give a diagnosis otherwise it is just guesswork.

[vas]

Hey @pwfarnell , thanks for getting back to me. Attached is a screenshot of solar vs battery voltage, along with “battery” sections settings.

[pwfarnell]

Thanks for the data, my thought was panel voltage but that is staying good at around 18 to 19V.

Another request, a plot of the battery voltage and current on the same plot. Also, do you have anything connected to the load output. If you do have something connected to the load output, what is it, what is its power requirement and how is the load output controlled.

[vas]

Thanks for looking into it, @pwfarnell ! Here is battery voltage vs current.

As for the load output - nothing is attached to the load terminals.

[pwfarnell]

That is really unusual, I had expected the current to go up and down as the voltage cycled up and down.

The only things that I can think of are

1. Do you have any loads on the battery or just the solar charger and the battery.
2. you have a loose connection or crimp in the wiring that or some other resistance that heats up and cools down. Notice that as the current reduces the cycles take longer. This could be in the positive or negative wire.
3. Do you have a resetable breaker like either of the ones shown below. These are not very good and they work by heating up to break the current, one of these could show this behaviour.