Battery voltage fluctuations when not being used

[harrisoncabintech]

Hello everybody,

This is my first time posting so please bear with me as I am new to the solar world. I installed a system at my off grid cabin, I have 1000w of renogy solar panels that go to a 60A renogy charge controller. The charge controller feeds 3 100ah Deespeak 24v lifepo batteries (all in parallel) that power a 4000w sun gold inverter charger. It's been running now for the last 2 months and I finally had to to review my findings that were tracked by my victron smart shunt. I am finding that while I am away I've noticed this pattern of the batteries getting up to there 29.2v max charge (yes seems high but battery manufacture said that's what it should be) and then drops down to exactly 27.9v for half a day then goes back up. I have attached a picture of the smart shunt reading. This occurs constantly while I am not using the system. Is this normal or is there so.ething going arry here that I might be missing. All batteries were top balanced before install. Any help would be great.

 

[sunshine_eggo]

Is the inverter on? If so, there's a load.

29.2v max charge (yes seems high but battery manufacture said that's what it should be)

Sometimes manufacturers don't know what's best for them.

and then drops down to exactly 27.9v for half a day then goes back up.

I am not seeing this at all. I'm seeing a gradual taper off until around sundown with a steeper slope taper overnight. Then when PV is available the following day, it charges again.

What is your float voltage, or were you tricked into not having a float voltage?

I have attached a picture of the smart shunt reading. This occurs constantly while I am not using the system. Is this normal or is there so.ething going arry here that I might be missing. All batteries were top balanced before install. Any help would be great.

First, LFP is like lead acid in that it need to be charged to a higher than resting voltage to take on a full charge. There is natural "settling" of the voltage when charge is removed.

Given that the cells are still > 3.4875V, your battery is still at an extremely high state of charge - pretty much 100%.

 

[mikefitz]

The Renogy charger, if using the default lithium mode, terminates the bulk stage when the battery volts reaches 'boost volts'. There is no absorbtion period or float voltage. Charging restarts if the volts fall below 'boost return volts' , (default 26.4 volts).
The graph shows the battery very quickly reaches 'boost volts' in the morning and charging stops. The 'surface charge' gradually bleeds off as the volts fall towards resting volts, overnight there is a steeper fall off due to charger loading.
The voltage on the battery pack would eventually fall and stabilise at around 27 volts if no load or charge occurred.

I cannot find the full instructions for charging the Deespeak battery but it seems from the Amazon site that a charge volts of 29.2 and a float volts of 29.2 is recomended. This seems way too excessive and a shure method to reduce battery life.
It seems with the application under discussion there is no daily load on the battery so it's constantly in a high state of charge, ( not good).
My advice, for what it's worth, would be to reduce the charge voltage whilst the cabin is occupied, boost volts, 28.4, and configure the controller for a float volts of 27 volts.
For the unoccupied period reduce the boost and float to below resting volts, say 26.5.

Mike

 

[harrisoncabintech]

Great thank you both for your comments. I wasn't aware of the settling the batteries would do and the graph I see makes sense now. I just got back from the cabin today and made the wise changes you both suggested in regards to full charge being at 29.2v as I was running my own parameters and not the renogy set lithium values as they were even much more aggressive then what I had. Cheers