LFP maintaining during low load

[sailorboy1]

I currently am in a slip with lower house loads so have adjusted my solar to only charge to 13.4V for an hour and then drop to 13.3V float. Been 2.5 weeks like this and the batteries appear to be staying within 70% SOC in morning to 90-95% in afternoon.

Anyone see a problem with this? Should I maybe let it go lower once in a while and then recharge to 100%?

[Pete7]

I would carry on using the existing range, providing the cells remain in reasonable balance. little to be gained from stressing the cells going to 100%.

Pete

[s/v Jedi]

I would stop charging at 70% SOC while in the slip.

[Statistical]

I assume solar is your only charging (no shorepower)? There is no need to periodically go to 100%. LFP has no memory effect.

What you have is fine. If you want you could drop it a few percentage more so it peaks at <90%. Don't drop it so much that lowest SoC is <50%.

For long term storage a constant 70% SoC is ideal but you have only solar and nighttime loads and it looks like the daily swing is 20% so the goal would be around 70%. 70% to 90% or 60% to 80% something in that ballpark. 40% to 60% would be less than ideal so would 80% to 100%.

[sailorboy1]

I have shore power, but am not using the battery charger. If I did the batteries would stay at 100%.

Maybe I will turn down the solar to 13.2V. Buit think I will wait to see how the discussion goes.

[Statistical]

Originally Posted by sailorboy1

I have shore power, but am not using the battery charger. If I did the batteries would stay at 100%.

Maybe I will turn down the solar to 13.2V. Buit think I will wait to see how the discussion goes.

If you have shorepower why now just turning down the charging voltage on the shorepower charger and keep the batteries at roughly 70% SoC 24 hours a day?

[Chotu]

Originally Posted by Statistical

If you have shorepower why now just turning down the charging voltage and keep the batteries at roughly 70% SoC 24 hours a day?

That’s what I was thinking.

also, it might be handy for times like this to have some junky little 12 V battery. You could even take a dead one sitting by the marina dumpster. Somebody is bound to be throwing one out around there somewhere. Just unhook those expensive nice LiFePO4 batteries and use the 12v as the buffer in your system along with the battery charger on shore power.

I had to leave my boat for a few weeks to attend to some work. I turned off the solar system completely while it is at a dock and turned everything off to face the similar problem of being away

[sailorboy1]

Originally Posted by Statistical

If you have shorepower why now just turning down the charging voltage on the shorepower charger and keep the batteries at roughly 70% SoC 24 hours a day?

no setting for that and I don't feel battery SOC just sitting there is any better than cycling in 20% range

besides the solar is green energy just sitting to be used

[Statistical]

Originally Posted by sailorboy1

no setting for that and I don't feel battery SOC just sitting there is any better than cycling in 20% range

besides the solar is green energy just sitting to be used

Fair enough. I think where you are at is fine. Droppping peak SoC a bit might help but honestly calender aging is going to dominate (and is unavoidable) for boat owners who don't abuse their batteries and don't deep cycle daily. As long as you aren't holding charge at constant 100%, don't deep discharge below 50% and battery temps aren't over 45C the cycle life is going to be a lot longer than the calender life.

[Statistical]

Actually found this and looks like even 50% SOC shows low degradation which surprises me. Since you are cycling and not holding constant that should be the lower end of the range (i.e. 70% to 50% probably fine, 50% to 30% not so much).


https://www.sciencedirect.com/scienc...52152X21011889

[fxykty]

When we connect to shore power we set our charging voltage to 13.3V - that seems to settle the battery at around 50-60% SOC. If you can’t set your shore power charger voltage then just turn it off when you get to

Otherwise, we set our charging voltage to 13.8V. We set our charging profile to stop charging at 90% and restart charging at 60%. Generally we cycle between 30% and 90%, though if we have big draws after sunset we sometimes are at 10-15% in the morning before solar starts up again. Note that we have a constant 15-20A draw on our battery, so going down 20% overnight is usual. Once a month our BMS charges to 100%, indicated by 2.5% tail current, to recalibrate itself.

The joy of an LFP battery - SOC just doesn’t matter, as long as you stay out of the far limits and don’t hold charge at 100%. Or even if you do, a bit degradation in the battery and it will still outlast you! We’ve had our battery 3 years and we’re just over 200 cycles - I’m not worried if we limit our battery to 2000 vs 5000 cycles. We’re more likely to temperature age it faster, but that still will only mean that I don’t need to bother leaving the battery in my will.

[fxykty]

Originally Posted by Statistical

I assume solar is your only charging (no shorepower)? There is no need to periodically go to 100%. LFP has no memory effect.

What you have is fine. If you want you could drop it a few percentage more so it peaks at &lt;90%. Don't drop it so much that lowest SoC is &lt;50%.

For long term storage a constant 70% SoC is ideal but you have only solar and nighttime loads and it looks like the daily swing is 20% so the goal would be around 70%. 70% to 90% or 60% to 80% something in that ballpark. 40% to 60% would be less than ideal so would 80% to 100%.

You are recommending quite high figures. What is the problem with cycling around 50% - I thought that was the ideal for LFP? As for long term storage, around 50% is ideal and I note that’s the approximate voltage that new cells are charged to before being sent it to customers (at least, that’s what Winston does).

[Statistical]

Originally Posted by fxykty

You are recommending quite high figures. What is the problem with cycling around 50% - I thought that was the ideal for LFP? As for long term storage, around 50% is ideal and I note that’s the approximate voltage that new cells are charged to before being sent it to customers (at least, that’s what Winston does).

It seems you are right. I thought 70% was ideal but looks like per the data above 50% is. The only remaining question is the OP isn't holding at a constant 50% but cycling about 20% so which is better:

50% to 70%
40% to 60%
30% to 50%

my guess is the top one but I haven't seen any data. Most studies are either looking at depth of discharge or calender life not short cycling at different SoC.

Still LFP is pretty tolerant of SoC unless someone is overheating the battery and discharging very deep or holding a constant 100% not sure how much of this matter real world. For a lightly used (not liveaboard not daily deep cycling) boat the largest factor is probably temps at least any place warm.

[kmacdonald]

From what I've read, it is best to store at 40 or 50% SOC. 100% SOC is not good for them. The charge parameters you are charging with wont bring them to 100% SOC or if it does it wont be for long. I think you are fine with what you have done.

[s/v Jedi]

Like I wrote all the up in post #3: do not go above 70%.

check your equipment, maybe your battery charger can act as a power supply without a battery. If so, disconnect the LFP battery and enabke that power supply mode.

Another option is to use the flat voltage curve of LFP. This means you can discharge to below 70%, then lower the MPPT float voltage step by step until the battery stops charging, starts a little discharging and share the load with the MPPT. Let it stand there and observe. Most of the time, discharging stops within 8% or so when MPPT has taken over the load.

Then next morning after sunrise, when SOC comes up to 60%, lower absorption voltage until it switches to float. You now have a balance between 50-60%.