Lithium Battery charging profile. And victron ‘trend’ charts.

Robermcm

I captured this screenshot of my 2023 tab 400 lithium battery (single 100ah) from within the victron app, looking at the mppt solar controller. It runs up to the top absorption voltage of 14.6v as specified in my ’user defined’ profile (using battleborn provided numbers), and then fades to the set float voltage of 13.5v. It does this every day pretty much if I don’t use the trailer or don’t use it much. You can see a little use (or, since it’s above zero, is that charging?) of the battery in red in this sample. 

Why does the voltage fade?  Is that as designed? The float voltage is lower than the absorption voltage, so is that not the battery numbers but the mppt charger numbers I’m looking at?



thanks much.  

MarkAl

Since the charger shuts off at night, when it powers up on the morning, it automatically begins in bulk or absorb mode and then transitions to float after a set time. If it didn't turn off at night it would stay on float. The charger "forgets" what happens or doesn't know when off. So it defaults to the topping off mode 1st. 

Robermcm

Makes sense. Thanks. 

Robermcm

Follow on question. 

I’ve put my trailer in storage when not preparing for a trip. It’s outside, but has a good overhead cover. It does prevent solar from working to keep the lithium battery charged (2023 tab 400, single bb 100ah lithium). 

I visited the trailer today. Was at about 13.6 float volts when i dropped it off five days ago. The voltage today is 13.15. That seems like a material drop over 5 days with all systems, other than solar, turned off. Is that normal for an idle lithium battery?  Edit: it’s been cold the last few days. I’m thinking that has reduced voltages materially. Question about storing battery during idle times remains. 

No shunt installed, but one ordered.  How should I store the trailer lithium battery in idle times? Take out and bring home? Charge fully and not worry, maybe turn off mppt controller? Can get cold here, but rarely below zero Fahrenheit. 

thanks much. 

dsfdogs

@Robermcm is your battery switch off, inverter off, and battery heater switch off? 

Robermcm

dsfdogs said:

@Robermcm is your battery switch off, inverter off, and battery heater switch off? 

Yes, all 3 are off. Only the solar controller is using voltage, unfortunately without much sun. If I turn solar charging off, does that effectively stop the controller from using battery?  I’m thinking not, because it still responds to the app via Bluetooth. Which would mean, for long storage, I’ll need to disconnect the controller wiring or,the battery negative to truly shut it off. 

pthomas745

The solar controller usage is extremely low.  It probably does not make much difference when considering the "normal" self discharge of a lithium battery that ranges (depending on how the internet feels from day to day) of only about 1-3 percent per month.

I have  LiTime and Redodo lithium batteries.  Both of them show the same behavior as you describe.  The "charged voltage" is only held for a very short period of time, and both wind up "resting" around 13.5 or so.

Lithium voltage state of charge charts for voltage are also a bit nuts.  The "discharge curve" for a lithium battery is extremely "flat", which makes voltage problematic for measuring capacity. 

I'll let this web page try to explain this better than I can.  The first section has a good description.

https://batteryuniversity.com/article/bu-903-how-to-measure-state-of-charge

Robermcm

excellent link. What I was looking for in battery voltage numbers education. 

Marceline

Robermcm said:

dsfdogs said:

@Robermcm is your battery switch off, inverter off, and battery heater switch off? 

Yes, all 3 are off. Only the solar controller is using voltage, unfortunately without much sun. If I turn solar charging off, does that effectively stop the controller from using battery?  I’m thinking not, because it still responds to the app via Bluetooth. Which would mean, for long storage, I’ll need to disconnect the controller wiring or,the battery negative to truly shut it off. 

That's weird. I've only had that kind of drop off when I left my trailer for many months this rainy winter (I usually camp year round). It's hard to explain. Are you 100% sure that your battery was fully charged when you dropped it off? If you looked at that reading when you were plugged into your vehicle or shore power, it wouldn't have been correct.

I think that you're going to have to fully recharge your battery from shore power or a charger, and then set the SOC to 100%. Then watch your SmartShunt or BMV carefully for a few days to confirm that something is drawing more than it should from the battery when everything's turned off.

Robermcm

agree. I bought a second battery and shunt, so will charge all of them up with my Noco genius 10, install, and see what happens. Hope to install a midpoint monitor on the second shunt tap too. 

Edit: midpoint tap is for two (or a larger even number?) of batteries in series. Since my measly two batteries are in parallel, midpoint voltage doesn’t seem to be relevant.