Charging LiFePO to 80%/discharge to 20% settings question

[TotalKool]

Hello everyone. I've looked over several posts and charts on this forum, but I am very new to solar and don't want to damage my batteries. Sorry in advance if I am requesting something already discussed.
I hope to verify these settings to make sure I am charging my LiFePo batteries to 80% and discharging to 20% for extended battery life. This is a backup system I am putting in after our "Snowvid" episode in Texas last year.

I have two SOK 12v 206ah batteries that I top balanced, wired in series (24v inverter). I have a Victron BMV-712 Smart monitor and a Victron MTTP 150/60 solar charger / currently a four panel array of 200w 12v 10a wired in series (soon to add another four of the same series, then in parallel).
Settings in question:
Charger settings, absorption 26.6v ? Float 26.5v?
Monitor settings, charged voltage 26.6v / discharge floor 20% / Tail current 4% / Peukent Exponent 1.05 / Charge efficiency factor 99% / Current threshold 0.10A

Thanks in advance,
Brett

 

[sunshine_eggo]

First, you're being paranoid. Quality cells last thousands of cycles if you keep them in the published limits, typically 2.5-3.65V/cell.

Period.

Now with that said, charging to 80% based on voltage is almost impossible except as determined for your own system. Furthermore, charging to a lower peak SoC at a lower voltage means you are going to take a LONG time to hit that point if you try to set a lower absorption voltage - might not get there in a whole day of charging.

If you truly want to be able to hit 80% in a meaningful way:

Fully charge your battery
Sync BMV to 100%
Discharge your battery to below 60-70%
Under good solar conditions, charge your battery and note the voltage AND current at which your battery hits 80%.

This is the absorption voltage and tail current for the MPPT.
You then set the BMV to 0.2V below the MPPT and the tail current a little higher.

This relationship may change over time. LFP that's not fully charged experiences a shift in the already loose voltage to SoC relationship. 80% will no longer be 80% 100 cycles from now. A charge to true 100% is needed to restore the original relationship.

Alternatively, just set your absorption to 27.6 and your float to 27.2. Yes, this will take you well above 80%, but it does so at a much slower rate/lower current and is far less stressful to the cells, which will give you improved life over charging to peak cell voltage. This eliminates the issue in the prior paragraph.

Cut your discharge off at 24V, and that will keep you in the 10-25% range on discharge (% depends on the load and your system).

 

[John Frum]

This charge to 90% SOC and discharge to 10% SOC idea is just not practical(unless loads and chargers can talk to the BMS).
If the cells are DIY you want to be charging into the high knee on a regular(daily) basis to maintain the top balance.
However... charging full and discharging to the point where the weak cell diverges from the pack does make sense to me.
That regime is going to likely be very close to an 80% cycle and is likely what was originally intended before the internet got all over this idea.

 

[TotalKool]

First, you're being paranoid. Quality cells last thousands of cycles if you keep them in the published limits, typically 2.5-3.65V/cell.

Period.

Now with that said, charging to 80% based on voltage is almost impossible except as determined for your own system. Furthermore, charging to a lower peak SoC at a lower voltage means you are going to take a LONG time to hit that point if you try to set a lower absorption voltage - might not get there in a whole day of charging.

If you truly want to be able to hit 80% in a meaningful way:

Fully charge your battery
Sync BMV to 100%
Discharge your battery to below 60-70%
Under good solar conditions, charge your battery and note the voltage AND current at which your battery hits 80%.

This is the absorption voltage and tail current for the MPPT.
You then set the BMV to 0.2V below the MPPT and the tail current a little higher.

This relationship may change over time. LFP that's not fully charged experiences a shift in the already loose voltage to SoC relationship. 80% will no longer be 80% 100 cycles from now. A charge to true 100% is needed to restore the original relationship.

Alternatively, just set your absorption to 27.6 and your float to 27.2. Yes, this will take you well above 80%, but it does so at a much slower rate/lower current and is far less stressful to the cells, which will give you improved life over charging to peak cell voltage. This eliminates the issue in the prior paragraph.

Cut your discharge off at 24V, and that will keep you in the 10-25% range on discharge (% depends on the load and your system).

Thanks for the quick reply.
"Cut your discharge off at 24V, and that will keep you in the 10-25% range on discharge" Is the cut-off controlled by the by the discharge floor setting?

 

[sunshine_eggo]

Thanks for the quick reply.
"Cut your discharge off at 24V, and that will keep you in the 10-25% range on discharge" Is the cut-off controlled by the by the discharge floor setting?

This is usually set in the inverter, i.e., the battery voltage at which the inverter stops using the battery.

The BMV discharge floor is the value used in computing time remaining, i.e., if you had a 100ah battery, with the discharge floor set to 50%, it would report the time remaining to 50ah.

If you set it to 20%, the "time remaining" would show the time remaining until 20% is reached.

 

[John Frum]

This is usually set in the inverter, i.e., the battery voltage at which the inverter stops using the battery.

The BMV discharge floor is the value used in computing time remaining, i.e., if you had a 100ah battery, with the discharge floor set to 50%, it would report the time remaining to 50ah.

If you set it to 20%, the "time remaining" would show the time remaining until 20% is reached.

If you have significant pure dc loads you should consider a victron smart battery protect or similar.