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.

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.

Any advice appreciated.

My "normal" REC ABMS settings attached.

 

[Samsonite801]

Your BMS shouldn't be controlling the charging, BMSs should only be set for safety thresholds so the cells can't go too low or too high.

Your charger sources should at least be good enough to have custom charge user profile settings, so you can set desired settings (or have a comms link ability to be able to charge based on BMC SoC).. And charger settings should be set to maintain battery somewhere within the safety thresholds set on BMSs, so that they kick in and charge up to desired voltage or SoC. During normal operation, the BMSs should never see their thresholds triggered.

What type of chargers are you using?

 

[ohthetrees]

I respectfully disagree with that. I believe it is appropriate for an external BMS to be the "authority" on what charge/discharge treatment the batteries should experience. The REC Active BMS has a specific feature that allows it to communicate and control a Victron GX system. I don't think REC and Victron would have implemented this system if it was bad system design. From the REC manual (ignore the Wakespeed stuff, I don't have a Wakespeed):

Battery Pack’s Charging Algorithm:
The communication between the REC ABMS, the Victron GX device and the Wakespeed WS500 is
established through the CAN bus. All the parameters that control the charging/discharging behavior
are calculated by the BMS and transmitted to the GX device and WS500 unit in every measurement
cycle.
The charging current is controlled by the Maximum charging current parameter. It’s calculated as
Charge Coefficient CHAC x Battery capacity. The parameter has an upper limit which is defined as
Maximum Charging current per device MAXC x number of inverter devices STRN.

https://www.rec-bms.com/wp-content/uploads/2022/02/UserManual_ABMS_Victron_Wakespeed.pdf

From the Cerbo GX manual:

Example 1 - Managed CAN-bus batteries For example, in systems with an Managed CAN-bus BMS battery connected, the GXreceives a Charge Voltage Limit (CVL), Charge Current Limit (CCL), Discharge Current Limit (DCL) from that battery and relaysthat to the connected inverter/chargers and solar chargers. These then disable their internal charge algorithms and simply dowhat they're told by the battery. There is no need to set-up charge voltages or choose the charge algorithm type.

In my case, the BMS basically "asks" a Cerbo GX for a certain charge current if available. It is the responsibility of the Cerbo to coordinate among my 4 Victron MPPT and my Victron Multiplus Inverter/Charger, to provide as much charge current as possible up to the "ask" current.

I have programmed the Cerbo and the individual charge sources to do "the right thing" should they ever lose communication with the BMS.

In other words, in normal operation, charging is controlled by the BMS. If the Cerbo loses network connectivity with the BMS, it falls back to it's own "LiFePo4 friendly" charge profile. Should the Cerbo lose contact with any charge sources (MPPT, Multiplus) those sources fall back to their own LiFePo4 profile.


What I'm asking basically, is what charge voltage would charge a LiFePo4 battery that was at low SOC, but would never take it above, say 75% SOC. I understand that SOC can't be exactly calculated from voltage. I'm looking for aproximations. Basically, I'd like to keep the battery resting voltage between 3.0 and 3.3v/cell. I thought choosing 3.25v/cell as "End of Charge" would do that, but I was wrong.

 

[Samsonite801]

I respectfully disagree with that. I believe it is appropriate for an external BMS to be the "authority" on what charge/discharge treatment the batteries should experience. The REC Active BMS has a specific feature that allows it to communicate and control a Victron GX system. I don't think REC and Victron would have implemented this system if it was bad system design. From the REC manual (ignore the Wakespeed stuff, I don't have a Wakespeed):

https://www.rec-bms.com/wp-content/uploads/2022/02/UserManual_ABMS_Victron_Wakespeed.pdf

From the Cerbo GX manual:


In my case, the BMS basically "asks" a Cerbo GX for a certain charge current if available. It is the responsibility of the Cerbo to coordinate among my 4 Victron MPPT and my Victron Multiplus Inverter/Charger, to provide as much charge current as possible up to the "ask" current.

I have programmed the Cerbo and the individual charge sources to do "the right thing" should they ever lose communication with the BMS.

In other words, in normal operation, charging is controlled by the BMS. If the Cerbo loses network connectivity with the BMS, it falls back to it's own "LiFePo4 friendly" charge profile. Should the Cerbo lose contact with any charge sources (MPPT, Multiplus) those sources fall back to their own LiFePo4 profile.


What I'm asking basically, is what charge voltage would charge a LiFePo4 battery that was at low SOC, but would never take it above, say 75% SOC. I understand that SOC can't be exactly calculated from voltage. I'm looking for aproximations. Basically, I'd like to keep the battery resting voltage between 3.0 and 3.3v/cell. I thought choosing 3.25v/cell as "End of Charge" would do that, but I was wrong.

Well here is a screenshot from a spreadsheet I have showing voltage-to-SoC here (click on picture to zoom larger):

 

[Samsonite801]

How much (Wh) are those essential loads you must keep running when you're away? Have you tried doing a (from 50% SoC) capacity til BMS cutoff check, based on your normal loads you keep running while you're gone? ...To kind of gauge how much actual capacity is there to get enough charge through the day and carry it through the nights? Also to factor in potential days of bad weather...

Do you use VRM with your Cerbo for remote monitoring while you're away?

One time I had an event (violent windstorm) at my RV on the off-grid property where I was away for several days and the wind blew upwards over, and face down, 4 of my 16 solar panels, and my system ended up discharging all the way to zero (in running my essential loads), so when I got back I had a surprise kind of like you.

So my next step had been to setup my Venus OS on an RPi with VRM over my Starlink so I can keep an eye on the SoC whenever I'm in town for a few days. Not sure if you have Starlink or not out there on your boat, but something to consider if you're not using VRM for remote monitoring.

I guess you would also have to be close enough to the boat where you could react in case there was an event, or somebody who could go out and check on it for you while you may be far away (at least until you get your system issues all refined to where it could never have problems anymore)...

 

[codfish]

I use these experiments as my off season bible. Yes they are tiny cells, but chemistry is correct. They did constant voltage charge till zero current acceptance.

Charge voltage experiments with lithium iron phosphate batteries showing how capacity varies with charge voltage and higher cycle live with lower charge voltage

Engineering resources for designing equipment using lithium iron phosphate batteries from PowerStream

www.powerstream.com

note that 3.2v/cell is only 8% full, and 3.3/cell is only 20-30% full.
I’d go to maybe 3.35/cell And I’d test it for a few days while on board.

but… even 8% of your giant bank seems like it should have survived. Something else going on? test while on board.

 

[codfish]

“Lost contents of fridge” … on my boat the fridge is THE major consumer of watt-hours. Do you know the major contributors to your consumption?

 

[ohthetrees]

I use these experiments as my off season bible. Yes they are tiny cells, but chemistry is correct. They did constant voltage charge till zero current acceptance.

Charge voltage experiments with lithium iron phosphate batteries showing how capacity varies with charge voltage and higher cycle live with lower charge voltage

Engineering resources for designing equipment using lithium iron phosphate batteries from PowerStream

www.powerstream.com

note that 3.2v/cell is only 8% full, and 3.3/cell is only 20-30% full.
I’d go to maybe 3.35/cell And I’d test it for a few days while on board.

but… even 8% of your giant bank seems like it should have survived. Something else going on? test while on board.

This was a great link, just what I was looking for. As for why 3.25v wasn’t cutting it, I speculate that there was a cloudy week then the BMS cut the battery off, so it could never recover. I think I’ll do a little testing while I’m on board, maybe try 3.32v to start.

 

[MisterSandals]

As for why 3.25v wasn’t cutting it, I speculate that there was a cloudy week then the BMS cut the battery off, so it could never recover.

I wonder if you simplified your system and removed the "network-like" things, it would come back to life after being run low.
Have the BMS monitor cells and disconnect if low (this would preserve them to your comfort level and experience level. No comm to SCC.

Have your SCC float at your storage voltage (3.30V-ish?) and bulk to perhaps 3.40V.
When the sun shines, it charges and doesn't wait to hear from Cerbo or other pieces.

Just a thought. I like simple, especially while i am away.

 

[ohthetrees]

I wonder if you simplified your system and removed the "network-like" things, it would come back to life after being run low.
Have the BMS monitor cells and disconnect if low (this would preserve them to your comfort level and experience level. No comm to SCC.

Have your SCC float at your storage voltage (3.30V-ish?) and bulk to perhaps 3.40V.
When the sun shines, it charges and doesn't wait to hear from Cerbo or other pieces.

Just a thought. I like simple, especially while i am away.

I don't think I'm going to go in that direction. I really like how my system degrades gracefully if the network goes down, but if the network is up I have centralized logic. The problem is that I gave it a poor parameter for when I'm away. I think I'm on the right track to giving it a better parameter. In my case if the BMS disconnects the main contactor, it doesn't matter what logic the SCC are using, the battery is disconnected.

I'm doing some testing right now, I'm guessing that something in the 3.3-3.35 range is going to keep things happier. I also have Starlink now (only $65/month because I bought in Trinidad!) so I'll be able to remotely monitor via Victron VRM which will be a huge help. Thank you for the suggestion though.

 

[pvgirl]

Depending on the cost and/or difficulty in replacing these medications it may be simpler to just run the system as normal, and accept a possibly shorter battery life, in exchange for keeping medications safe.

 

[n2aws]

Depending on the cost and/or difficulty in replacing these medications it may be simpler to just run the system as normal, and accept a possibly shorter battery life, in exchange for keeping medications safe.

As another "out of the box" thought..

If you are leaving the boat for 4 months, can you just take the medication with you? Without knowing your circumstances, I wonder what medications you can just stop taking for 4 months, and then resume when you return... that needs to stay cold. I'd say take them ashore with you, and continue using them as needed/directed?

 

[Q-Dog]

Less use needs less solar. My inclination would be to shut down some of the solar charging and just let the system run normally. This should let the batteries cycle. I would of course test quite a bit before leaving it for 4 months.

 

[HarryN]

Similar to what @Q-Dog mentioned - you are just kind of getting away with your design vs what should be done.

For one, the loss of battery capacity due to keeping the system mostly full is real -but not as big of deal as you think.

With a solar charged system that is running some house loads - every night the pack will be slightly discharged so it won't be completely full anyway - for at least 50% of the time.

The easiest solution is to cut your active solar charging capacity by 50% while you are away and not change anything else. I would not go under 600 watts no matter what to keep the fridge going consistently.

A better solution is to use a solar charge controller that you program in new voltage limits.

 

[Q-Dog]

Similar to what @Q-Dog mentioned - you are just kind of getting away with your design vs what should be done.

For one, the loss of battery capacity due to keeping the system mostly full is real -but not as big of deal as you think.

With a solar charged system that is running some house loads - every night the pack will be slightly discharged so it won't be completely full anyway - for at least 50% of the time.

The easiest solution is to cut your active solar charging capacity by 50% while you are away and not change anything else. I would not go under 600 watts no matter what to keep the fridge going consistently.

A better solution is to use a solar charge controller that you program in new voltage limits.

Right. He wants to use "storage" parameters, but the batteries aren't being "stored." Storage means the batteries are shut down, disconnected, no power going in or out. As long as the batteries have a little bit of power going in and out they will be fine at normal voltages.

 

[ohthetrees]

Similar to what @Q-Dog mentioned - you are just kind of getting away with your design vs what should be done.

For one, the loss of battery capacity due to keeping the system mostly full is real -but not as big of deal as you think.

With a solar charged system that is running some house loads - every night the pack will be slightly discharged so it won't be completely full anyway - for at least 50% of the time.

The easiest solution is to cut your active solar charging capacity by 50% while you are away and not change anything else. I would not go under 600 watts no matter what to keep the fridge going consistently.

A better solution is to use a solar charge controller that you program in new voltage limits.

I appreciate your input, but I don't agree. You say I'm "getting away with my design" and that Q-Dog mentioned my design was wrong. Yet I cited sections of both the Victron Cerbo manual and the REC BMS manual that indicate my architecture is just fine. Nobody has mentioned a concrete reason, nor a source indicating that it is an incorrect architecture. I feel like there is a game of telephone going on here, where people are just repeating what they "heard". Specifically, so many people have built-in BMS in their drop in batteries that some best practices lore gets generalized (incorrectly in my opinion) to how people think about external BMS like mine. My external BMS has specific provisions for providing charge/discharge requests to the victron network via DVCC, while separately(!) controlling a battery dissconnect switch.

I am open to the idea that I shouldn't try to both "store" and "use" the batteries when I'm away since I still have refrigeration, and DC-DC chargers running, (I estimate these together to be roughly 15% of my normal daily load) and I should just leave my settings as normal.

In any case I sincerely apreciate you taking the time to share your opinions, whether I agree or disagree, it helps me think more carefully about my system.

 

[ohthetrees]

Right. He wants to use "storage" parameters, but the batteries aren't being "stored." Storage means the batteries are shut down, disconnected, no power going in or out. As long as the batteries have a little bit of power going in and out they will be fine at normal voltages.

You might be right about this. I'm an optimizer, and the idea of the batteries being at 100% by 10am worries me a bit, but maybe I'm being too much of a perfectionist. Or maybe I just need to find the "right" charge voltage! That will be an experimental process though.

 

[HarryN]

Perhaps then - just add a small power system to take care of your storage needs?

Something that does not use your larger battery pack.

Example - a couple of battle borns or even better - some AGMs, 400 watts of solar, and a bogart solar charge controller.

The AGMs won't trip off since there isn't any BMS and in the climate you are describing - a bogart SC 2030 is sufficient with just the built in settings.

High Efficiency Solar Charger Controller

Compact, powerful and smart solar charger controller for RV, off-grid. 30 Amps 12V and 24V charger for Gel, AGM, lead acid, lithium LFP. WiFi remote capable

www.bogartengineering.com

 

[hilo90mhz]

Don't underestimate Starlink as an energy consumer too - it uses more than most small refrigerators.