Continuously float or cycle LiFePo4 in backup application?

[robwolff3]

I have a 280ah 48v LiFePo4 in a backup configuration with my grid-tied MPP LVX6048WP. The battery bank sits at a fullish charge waiting for a grid outage and then backs up my critical loads. Not wanting to over wear my batteries in this application its seems best to keep them at 80% or 90% SOC as opposed to say 100%. The MPP seems to have two options, continuously float the batteries at a particular voltage all of the time or turn off the charger once the desired voltage/SoC is reached. When the charger is turned off the MPP continues operate in bypass mode from the grid and it always draws about 88w from the battery bank. Over the course of about a week this slowly draws down the SoC of the bank 10%. At this point I would have to reengage the charger and top off the bank so it remains ready for a grid outage. A sensible cycling seems like between 80-90% SoC?

I'm just not sure if cycling between 80-90% SoC once a week or just continuously floating the batteries will wear them less.

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On a side note if you want to go further down this analysis with me. I landed on a backup application because my 14 kWh bank barely covers my critical loads for 24 hours in the winter, definitely not all of my house loads so if I discharged my bank during non-solar hours to offset grid usage and hit a non-sunny days streak here in the Midwest, I may end up in a spot with a depleted bank not ready to back up my critical loads and have to run my generator.

My TOU rates are 12c 7pm-11am and 22c 11a-7p, so I could try beating the peak rate but then do I charge the bank with off peak immediately after or wait till solar the next day. What do other people do that are grid tied with a smaller battery bank?

 

[MisterSandals]

I'm just not sure if cycling between 80-90% SoC once a week or just continuously floating the batteries will wear them less.

That is the conundrum! I am not sure that cycling them with either keep them fresh or wear them out.
I'd try to figure out what state you would NEED them in an emergency and go with that. And in special cases like an impending storm or event with increased chance of power loss, charge them up full beforehand.

My first goal for a backup system would be ensuring readiness.

 

[Ampster]

There is a chemical degradation that occurs more rapidly at when a Lithium battery is at higher voltage. I believe the wear will be greater with a constant float charge. Cycling once a week is probably preferable. However, unless inverter overhead causes 10 percent SOC decline in a week, normal SOC loss in one week should be a lot less so even weekly would not be necessary. One might be able to find a rebulk setting that would be based on voltage or SOC.

 

[wopachop]

Curious why disconnecting the battery at around 70% charge would not be considered? Especially in the summer? Like others mentioned when a big storm is predicted you have plenty of days to hook it back up and charge to 100%.

 

[Ampster]

Curious why disconnecting the battery at around 70% charge would not be considered? Especially in the summer? Like others mentioned when a big storm is predicted you have plenty of days to hook it back up and charge to 100%.

I charge every day to 3.45 volts per cell which is around 99-95 percent. Because my inverter is in Constant use, they quickly settle yo 3.3 volts. Any number could be considered based on use case and user preference. I discharge daily to 60-70 percent which still gives me a reserve for power outages. The original question was about float charging Lithium. I don't float but I used a long Constant Voltage (Absorb) stage when charging from solar

 

[robwolff3]

Curious why disconnecting the battery at around 70% charge would not be considered? Especially in the summer? Like others mentioned when a big storm is predicted you have plenty of days to hook it back up and charge to 100%.

Its a good suggestion and something to consider.

I guess the only downside I see is that if you physically disconnect the batteries to prevent the 90w constant draw, you loose your automated switch to battery power, which is a great benefit of having a hybrid inverter and a battery bank. When the power goes out I would have to go to the inverter, flip the battery disconnect on and hit the button to start the inverter. For context, my grid power in rural Michigan goes down 2-3 times a year.

 

[Hedges]

I think what you'd like to do is fully charge to ensure balanced, then draw down to lower SoC, then "float" in the sense of zero current in or out.

Keep them cool/cold to reduce degradation, even if at 100%. Chest fridge (e.g. extra thermostat on power cord of freezer)?

Regarding your inverter I read, "Grid interactive (grid injection) capable"
Do you have net metering, or can you get on reasonable terms?
Given that you've bought a battery that ought to last up to 16 years cycling daily, can you use it to store PV generated power off-peak and then export during times of peak rates? That could let you receive 1.5x to 3x the kWh you produced, in exchange for wear on batteries (which may have cost you $0.05/kWh of cycle life.)

 

[robwolff3]

I think what you'd like to do is fully charge to ensure balanced, then draw down to lower SoC, then "float" in the sense of zero current in or out.

I have been investigating the battery bank resting voltages after drawing down to SoC's increments, so I could hopefully float with zero current in / out. I need to do more work here. Last time I floated them at 53.3v (I think) they very slowly over the course of a week or more charged up above 90% SoC.

Keep them cool/cold to reduce degradation, even if at 100%. Chest fridge (e.g. extra thermostat on power cord of freezer)?

They sit at a constant 63 F in my basement all of the time. Sounds pretty cool to me?

Do you have net metering, or can you get on reasonable terms?

Effectively, Yes. DTE here has a "Distributed Generation Program" and I am exporting excess generation. I get about equal credit for generation import and export, but I do have to pay cash for transmission cost of the import. So reducing import, especially on-peak, could be advantageous. On-peak and off-peak rates start this month so im waiting to see how that pans out on my bill.

With my smaller battery bank maybe there is a middle ground where I discharge down to 60%, 50% or 40% SoC when the sun doesn't cover my energy usage on-peak (and off-peak afterwards), wait till solar the following day to recharge the batteries off peak, like you suggest. Maintaining a minimum SoC in case of a power outage in the evening or at night. Could even include a override if inclement weather is coming up on the forecast to keep the bank changed like @MisterSandals suggested.

Seems like most people build larger battery banks so they get the best of both worlds, beating on-peak rates / self consuming more of your solar and outage protection.

 

[Hedges]

See if you can cycle the battery, starting to discharge at 4:00 PM or whenever peak rate time, aiming to hit 20% SoC by 9:00 PM or whenever end of peak. Of course that may leave you in the dark if grid fails at night, but if still down the following day you would be on your own PV + battery until it returned.

Assuming your PV is less than 100% of usage (which of course means when grid is down you would have to forgo heavy loads), this could let you earn credits to cover more kWh consumption than you produce. The battery should earn more than it costs this way.