LFP solar charging target SOC

[Capt.Don]

How do folks with LFP house banks handle routine daily charge/discharge cycle and battery state of charge? With solar, my battery charge/discharge cycles between 90%-100% daily. The Victron MPPT solar controllers are set at LFP 14.0v/13.3v.

Our boat is in a marina with sufficient solar to maintain the battery at 100% SOC while running the fridge and small electronics. What I'm finding is

1. fridge cycles on/off 3.5a (normal behavior)
2. solar powers the fridge during the day
3. solar tops-off the battery to 100% SOC
4. during the night, no solar, no shore power, the SOC drops to about 93%

Effectively, the battery is charging/discharging from about 90%-100% daily. I don't think it is best for the battery to be sitting at 100% SOC, perhaps 80% might be better? I'm wondering if I should lower the absorption voltage to around 13.5v to cycle the battery at around 80% SOC? Thoughts?
Thank you,
Don

[sailorboy1]

I always just left my batteries to cycle and charge with solar as you are doing. I think I turned down the voltage to 13.8 and the time hold as 0 because it didn't really matter if they got to 100% SOC instead of 95%. Every once in a while I would turn up to 14.4 and get fulluy charged, but considered this left over lead acid muscle memory.

[wholybee]

You could just drop the charge voltages a bit. To say, 13.8 charge, with 13.0 float. That way you never get to 100%. But, the difference is small because you are not storing them at 100%. If solar is your only charge source, what should be happening is that it charges to 100% (whatever that voltage is), and the stops charging, and begins discharging even if it is still during the day. It will discharge to the float voltage, and then hold that, as solar then powers the boat.

Also, make sure you have no absorption time set in the MPPT controller. I think by default Victron does include an absorption period, so you need to enable the advanced settings and turn that off.

[barnakiel]

(my observation, a lion battery, newest device)



I noted newer devices tend to stay below 100% when being charged. E.g. my laptop battery is sitting at around 95-98% rather than 100% when in any use (also at sleep and with the lid down). Now if I switch off, the batt charger reminds on for a brief moment. Next time I start the machine, the battery is at 100% but again the system will tune the charge cycle and remain at 95-98% throughout the day.


My conlussion : with a Lion battery, modern chargers will not keep the battery at 100% - intentionally. As my older machines used to hit 100% when in use and stay there for the whole cycle.


I do not know if this translates to LFP or not. Probably Google knows.


b.

[Capt.Don]

Thanks everyone. I dropped voltages to 13.8/13.2 with 0 abs time. We’ll see where SOC settles.
Don

[CaptainRivet]

Originally Posted by Capt.Don

Thanks everyone. I dropped voltages to 13.8/13.2 with 0 abs time. We’ll see where SOC settles.
Don

With 13.8V if you have a passive balancer you need to give it some absorption time, otherwise the battery is going out of balance over time, 300Ah 15 min 600 30min. Active balancer 0 is ok as it balancing during discharge from 13.8 to 13.6V.
13.2V float is good with offset 0.1V or even 0.2V which basically means it charging till full and then discharging till 13.0V till a new cycle starts, should be 80 or 75%SOC, like this it get all 2 or 3 days to 100% and then after short balance discharge again.

[Capt.Don]

Whether cycling at 80-90% or 90-100% probably doesn't matter much.

Regarding balancing -- I'm OK letting the solar charge to lower voltage (say 85% SOC) and having the alternator (Balmar 614) or occassional shore-power (ancient Heart Freedom charger) charge to 14.1v, letting the BMS balance the cells.

[sailorboy1]

Originally Posted by Capt.Don

Whether cycling at 80-90% or 90-100% probably doesn't matter much.
.

It doesn't

Don't worry about balancing, it is a science project type

[CaptainRivet]

Originally Posted by sailorboy1

It doesn't

Don't worry about balancing, it is a science project type

This you get completely wrong. That’s no science project but the must so your bank live a long life or not done properly the fastest way to ruin it.
If you have a significant imbalance you will have a very hard time with a passive balancer to get it back to balance and this will cost you approx 200 cycles lifespan.
So better keep ĺn balance.
Running the bank the first month at 14,4V with 1 or even 2h absorption makes sure it’s balancing properly, running the cells in (cells need several cycles till they operate to their best way) and making sure the top balance is as good as possible. Yes it should stay as at high state of charge. You cannot expect you get a perfectly top balanced battery and this makes sure you get it done.
Then you just need to keep the top balance and can go lower to 14,2V or 13.8V to optimize for battery life.
Not doing that, especially with dropins without external communication is the fastest way to ruin them.
And to see if the bank will get out of balance you charge one time with solar or shorepower till 14,6V with 1h absorption and note down the highest voltage cell and the deviation direct after cutoff.
Then run the bank 2 week with 13,8V and do the same again. is the cutoff voltage higher or the deviation bigger then absorption is not long enough to keep balance. You go up to 14,0V with same absorption time, is it still growing add 30min absorption. If no go to 14.2V…play this game till deviation is kept or getting smaller. That’s your absorption voltage and time that your battery live a long life in your installation.
Simple process and you do it once. each battery and installation is different. For some 13.8V works and some need 14,4V with a long absorption, this depends on cell quality, deviation and what balancer you have Passive/active/balancing current.

[barcoMeCasa]

Originally Posted by CaptainRivet

This you get completely wrong. That’s no science project but the must so your bank live a long life or not done properly the fastest way to ruin it.

Not if you get good cells. Just about to balance my Catl bank after 22 months without balancing & the cells are still with mV of each other.

Not a recommendation but just a data point that people seem to get freaked out about needing cells balanced every charge cycle. Maybe a hangover from the bad old lead acid days.

[CaptainRivet]

Originally Posted by barcoMeCasa

Not if you get good cells. Just about to balance my Catl bank after 22 months without balancing & the cells are still with mV of each other.

Not a recommendation but just a data point that people seem to get freaked out about needing cells balanced every charge cycle. Maybe a hangover from the bad old lead acid days.

Yes mine doesn't get to 100% either very seldomly as well oversized but that doesn't mean that's the case on his boat. And as he obviously doesn't know what his bank in his installation need in minimum to stay in balance he either needs to figure it out or play 100% save. And 13.8V with 0 absorption is not the 100% save way.

Every charge cycle is not necessary but even the best cells need from time to time a top balance. It's also important to know do you have parallel cells, if yes the more the longer must be the absorption as the parallel cells only even out 100% in the upper knee (important is even out under load/charge, not in 0 load). Even more important You have 12V=4 cells in series, that genuine stable when you have top notch cells as only 4 tolerances adding up, 24V or 8 in series means 8 tolerances adding up is already not genuine stable anymore means you need balancing even with best cells but that can be all 2 or 3 month a top balance if top notch cells. Worse is 48V or 16 cells in series means 16 tolerances adding up and thats causing it is genuine unstable and need already for top notch cells a good balancer. That's why also 24V is the best compromise on a vessel as the cells operate in emergency without BMS and balancer a long time but current level and losses are lower then 12V.

So first you need to figure out what your batteries need as charge voltage and as absorption in minimum to stay in balance in your installation. And that is not necessarily 13.8V with no absorption as a lot telling you, you need to figure it out in your installation what is needed.
If you don't know it's time you figure it out...

But a boat that stays unattended for month it's better to play save, force the battery to charge a bit deeper and have all 2-4 days a full charge with a short absorption if passive balancer who only works during charge, as the time to ballance is not enough from 13.6V to 13.8V during charge. Its enough if 0 if active balancer as they work during charge and discharge and has therefore double time plus has more power.
It doesn't harm or shorten the battery life if you do this but it harms it when it gets out of balance.

People damage the batteries more by trying to avoid a high state of charge because they read you need 13.8V...no you don't. It doesn't harm the battery or it's lifespan if you charge it to 13.8V or 14.6V (that's what it's made for) but it harms it if you keep it there=store it on this high state for a LONG time, either by a too high absorption time or the hit the knee a couple of times a day and short absorption periodes count together too. More important for semi storage=unattended over month is the low float voltage so the battery sits on a lower voltage level, that's where it spends the most time.

[sailorboy1]

Science project! People make themselves such slaves to their batteries.

[Capt.Don]

I'll monitor the individual cells for imbalance and occassionally rebalance with shore-power or alternator w/Balmar regulator at 14.1/13.4v. Thank you for your comments,
Don

[CaptainRivet]

Originally Posted by Capt.Don

I'll monitor the individual cells for imbalance and occassionally rebalance with shore-power or alternator w/Balmar regulator at 14.1/13.4v. Thank you for your comments,
Don

Ahh ok, then 13.8V without absorption is ok.