Solar charge voltage of LiFePo4 for off-season

[ohthetrees]

I live 8 months a year on a 50' sailboat with 1800w of solar, and a 13.5 kWh bank I made from bare cells. My BMS is an REC Active BMS controlling the whole charge system via a Victron Cerbo GX. It is performing absolutely fantastically when I live aboard.


The problem is my "layup settings" for when I leave the boat for the hurricane season and head back to the USA for 4 months. I want to leave the batteries at roughly 30-50% SOC which is how I understand the batteries are "happiest" for long term storage. The problem is that I need to leave a few systems running (a couple of DC-DC battery chargers for start batteries and bow thruster batteries, and a small fridge for our medications) which are maintained by the solar panels. Normally my BMS "End of Charge" setting is 3.5V/cell which sets SOC to 100% and ends charging. When I left last year I set "End of Charge" to 3.25V/cell which equates to roughly 40% SOC. The loads when I'm not aboard are very low compared to our normal loads, so I thought this setting would be adequate to keep the battery at a partial charge while I was gone. However, when I came back, the battery was disconnected by the BMS because of under voltage, and SOC was 0. The lithium batteries are 100% fine because the BMS did it's job, but we lost contents of the fridge, and the lead acid batteries maintained by the DC-DC chargers.
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I think what is going on is that 3.25v/cell just isn't enough for the cells to take a real charge from solar during the day, and even though loads were low, they just weren't recovering during the day from the overnight loads. The 3.25v setting probably would have been adequate with a power supply plugged into the mains, but we are solar only. So my question is, since this BMS doesn't allow my to set a target SOC directly, what settings can I use to keep the batteries at partial SOC while I'm away? Obviously 3.25v is too low, but 3.5v is too high.
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​Any advice appreciated.
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My "normal" REC ABMS settings attached.

[Caz]

Have you posted your question on this REC user forum

[ohthetrees]

I had no idea that forum existed, thanks for the tip.

[Stu Jackson]

If you put aside V alone what is your energy USE vs. energy INPUT from solar? Daily on a good day, daily on a cloudy day, weekly with various mixes?

[smac999]

Setting the absorb and float to ~13.3v. Should keep them around 80%.

It’s possible you just did not have enough solar over the winter. Especially if a fridge is left on.

Do you have the cerbo hooked up to the vrm? Then you would have all the Data loging. And be able to check it from home. You can also adjust the mppt settings remotely. Though your bms may be overiding the mppt settings.

For example my small home system in the winter. 200w solar is not enough to keep the cerbo and bvm running. Only 0.2a draw. Because it rains everyday. The battery just keeps dropping each day.

[hjohnson]

I've got a similar setup (though I depend on shorepower). I just go into my Cerbo and set the DVCC override on the system voltage to 13.3v. This lets the charge drop down to around 60% which is comfortably in the flat part of the curve.

This override works with both the inverter/charger and my MPPTs. No need to mess around with the settings on my REC ABMS.

[ohthetrees]

Originally Posted by hjohnson

I've got a similar setup (though I depend on shorepower). I just go into my Cerbo and set the DVCC override on the system voltage to 13.3v. This lets the charge drop down to around 60% which is comfortably in the flat part of the curve.

This override works with both the inverter/charger and my MPPTs. No need to mess around with the settings on my REC ABMS.

That is exactly the solution I arrived at after reading the link provided above by Caz. I've been testing it a bit, and so far so good! Thanks for the input!

[tanglewood]

Originally Posted by hjohnson

I've got a similar setup (though I depend on shorepower). I just go into my Cerbo and set the DVCC override on the system voltage to 13.3v. This lets the charge drop down to around 60% which is comfortably in the flat part of the curve.

This override works with both the inverter/charger and my MPPTs. No need to mess around with the settings on my REC ABMS.

I did the same. It's a very handy way to control this is you are using DVCC for all your charge control. And it can be remotely adjusted further, if desired.

[sailingharry]

It sounds like tweaking the voltage up just a touch maybe the solution. And an easy solution. But here is an alternative.
Run the batteries down to 60% before you leave. Then completely isolate the lfp battery and shift your limited loads and your solar directly to the lead acid start battery.
On my boat, still 100% agm, I could do this easily. Pull the fuse to the house back, and it is 100% isolated. Then close the bank cross connect switch (this is an emergency only switch, it is a key switch with the key screwed to the bulkhead beside it. It takes very deliberate action to close that switch!). Bingo, all of my charge sources and all of my house loads are connected directly to my start battery. Presumably charge settings would need to be adjusted to account for different chemistries.
While I am not LFP yet, I've spent a lot of time learning for the upcoming conversion. The fundamental battery wiring listed above would not change. I have thought about the wiring I just suggested should a problem occur in my existing bank, and feel the redundancy and backup alignments would be equally useful with LFP.

[Pete7]

This was quite an interesting experiment and perhaps brings into doubt the advice to store at 50%.

https://www.youtube.com/watch?v=znWo...withWillProwse

[Wandering1]

That was one guy doing one thing on one battery.
Manufacturers suggest storing at 30-50%. If I was going home for 4 months, I would just deplete the lithium to 50% and turn everything off. 3% per month self discharge will not cause a problem.
There are some interesting technical studies around that suggest lifepo4 cells will last their longest kept at 30-50% charged. These are studies independent of the Manufacturer

[hjohnson]

Originally Posted by Wandering1

That was one guy doing one thing on one battery.
Manufacturers suggest storing at 30-50%. If I was going home for 4 months, I would just deplete the lithium to 50% and turn everything off. 3% per month self discharge will not cause a problem.
There are some interesting technical studies around that suggest lifepo4 cells will last their longest kept at 30-50% charged. These are studies independent of the Manufacturer

In my case, I like to have my monitoring system up and running, even when not aboard. Bilge Level, temperature/humidity, tanks (not that I really need to know that when not aboard), and weather data off my masthead sensor. All of that sucks about 20W (including LTE modem, AIS transponder, and other sensors).

[h20man]

Originally Posted by Stu Jackson

If you put aside V alone what is your energy USE vs. energy INPUT from solar? Daily on a good day, daily on a cloudy day, weekly with various mixes?

What about Birds sitting on panels (with an H after the S).


That is what is taking my panel output from 1.7KW down to 200W.


GRRR.

[Wandering1]

Originally Posted by hjohnson

In my case, I like to have my monitoring system up and running, even when not aboard. Bilge Level, temperature/humidity, tanks (not that I really need to know that when not aboard), and weather data off my masthead sensor. All of that sucks about 20W (including LTE modem, AIS transponder, and other sensors).

Why? You aren't there. Why do you need to know these things?

[sailingharry]

Originally Posted by Wandering1

Why? You aren't there. Why do you need to know these things?

For me, I don't "need" to know these things, and certainly won't pay the LTE modem fee to find it out. But I know I am, at my core, an unusually frugal person.

But boats are toys, and there isn't much about them that we "need" to do.

Wind is interesting. I have a weather station at my house, and it tells me rather useless stuff like wind speed and direction, and humidity, and solar intensity, and lots of interesting stuff. So it's fun to look at.

Bilge level is more than fun. If I were to have a monitoring system, and my boat were a distance away, I'd REALLY like to know if my bilge pump just cycled for the 5th time this hour.

Humidity is also more than fun. High humidity can be an indication of issues -- leaks, open hatches, etc.

Volts can be good to know -- I know people who have had shore power trip off and come back to the boat with a completely flat battery. Over on a solar forum, a fellow had his LFP batteries go way overvoltage and do damage (BMS failure of some sort) -- knowing volts would have helped him!

AIS is fun too. I often open Marine Traffic just to see what's going on out on the Bay.

The point is that if you already have all the sensors, and are either on shore power or have adequate solar, it's already there. If you either have already bought an always-on internet connection, or have linked to the marina wifi, it's free. Even if you only check it once a month, why not?