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Is it bad for na LFP battery to be on a solar charge controller for a long time with no load?

tomashubelbauer

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Hi, I have a miniature solar setup with a few panels, an SCC and a 206 Ah LFP battery. I used to have the setup power 24/7 lights in my shop but I disconnected the load some time back due to making changes to the shop lighting and I never got back to reconnecting it yet.

I use a Victron SCC and the VictronConnect app has been showing the battery was bulk charging for like two months and then for a few days in a row, it would also show absorption and float charging phases were happening. But after some time, the battery is back to showing bulk charging only.

I am wondering why the battery hasn't stayed on float after it reached 100 % charge. It has internals heaters and the temperatures were around and below freezing a bit for some of the days of the last month, so maybe some of the energy went into the heaters. But still I'd expect it to be on float almost all of the time since it is not being discharged by anything other than its natural self-discarge and maybe the heaters. Oh an also the Cerbo GX and the SCC I guess, but again, is the load of those devices large enough that it would seriously cut into the state of charge of a 206 Ah battery such that it would then bulk for days again?

Is the natural self discharge of LFP batteries large enough that you'd expect they don't remain on float once fully charged and still on the SCC?

Is it bad for LFP batteries to stay on 100 % charge for too long without any loads connected all the while still being connected to the SCC?

Am I better off disconnecting the battery while it is not being in use rather than having it be connected to the charger? And should I discharge it a bit first, like to 80 % or something? I am not eager to do this since the battery runs the Cerbo and I use Victron remote monitoring to keep track of how much sunlight my panels are getting even despite not making use of that energy other that keeping the already charged battery topped up. I want to collect this data to be able to compare winters year over year.
 
What voltages are we talking about?

100% SOC and low temperatures are not a problem. With high temperatures (over 45° C) and 100% SOC and storage for long times LFP will deteriorate faster.

The Cerbo will drain the battery slowly, so you need to keep it attached to a SCC or charger.

Keep the battery at a voltage of 3.35 V per cell, which is the resting voltage. Set the SCC to this voltage for bulk and float.
 
It is a 12 V battery and I have two 175 Wp 10 A solar panels hooked up to the SCC in series going to the SCC. The SCC is configured correctly for a 12 V LFP system so AFAIK it should be charging the battery with the correct algorithm. I am glad to hear the temperature is not a problem, I got the internal heater battery version because I was worried about charging it at freezing. I am mainly flabergasted as to why it stays in bulk, not float, when the battery SOC is 100 % for days in a row.
 
status.png shows the charger is enabled and charging but in bulk state. settings.png shows the float should start at 13.5 V and absorption at 14.2 V. We see that the battery is not at that voltage, but it has no load and history-recent.png shows daily there is energy going into the battery but not out of it. This has been the case for like 6 weeks now yet the battery is still in bulk state. I could accept that the battery was never fully charged and it just takes forever because it is the winter and it is cloudy and the energy gains from the two tiny panels are low. But history-float.png shows that at some point float and absorption were reached and that was the case for several days in the row. Where has the energy from the battery gone? I can only think of the internal heaters but I have no way to monitor if and when they even come on and especially the recent two weeks of winter here where I am were super mild.
 

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Consumption is only registered when the load is connected to the load output of the SCC.
The load of the internal heaters is not logged in the SCC.

In the float-history I can see that the absorption voltage of 14.2 is met.
In the other history it’s not. So not enough solar to resupply the energy of the heaters.

But at 13.8 the SOC is somewhere between 95-100%. So your battery looks good!

You can lower the absorption voltage to 13.8 if you’re fine with not changing to 100%.
In that case it will change from bulk to float.
 
Thanks for taking a look at this! I didn't realize that while there is no load the battery voltage is actually a useful indicator of the SOC like with lead batteries. I got so used to not relying on it for LFP because it is (to my knowledge) too inaccurate while under load that it didn't cross my mind to try to determine the SOC using the voltage at all.

I am still a bit mystified where the energy went since from the historical graphs it shows the voltage was at some point high enough to enter float and absorption but I guess just powering the Cerbo and maybe the internal heaters if they came on was enough to draw some energy out of the battery and make it enter bulk state again.

I am relieve there is nothing wrong with the battery, these things are pricey so I try to be extra cautions with my care of them. :D
 
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