Keeping LIFEPO4 at 70%SoC

Wellydex · Nov 3, 2020

I run a 24V 200ah system for my travel trailer. My MPPT is a Victron 35A. I have 1kw of solar and run a compressor fridge constantly. I have set the absorption voltage to 27V and float to 25.8 and find that the system settles at about 70% SoC after a few days. A day before I head away I reset the absorption voltage to 28V which brings it up to 100% soc which is where I leave it while I am away. I then reset to 27 on return home. I figure that this leaves the batteries at a more Gentle 70% during long term light cycling due to the fridge. Does anyone see any problems with this. Thanks

snoobler · Nov 3, 2020

From where are you getting the 70%?

RCinFLA · Nov 4, 2020

A float of 25.8v is only 3.225v per cell (if balanced). That would be slightly less then about 20% state of charge.

70% SOC would be a float of 26.6v. 50% SOC would be 26.4v. Critical to have reasonably accurate voltage reading at float.

If you have resistor dump balancing in your BMS, 27v absorb level may not provide much balancing action as they typically do not engage until greater then 3.4v on a cell. Running loads from such a low state of charge and having little balancing could get you into trouble with weakest cell tripping BMS low voltage disconnect.

As a side note, pretty much all rechargable batteries have some surface charge. It is created between battery layers and is somewhat like an electrolytic capacitor. For LFP batteries, the surface charge energy storage is very low and dissipates quickly with light load. But it can increase the appearent float voltage of a cell by about 0.15v. If you float for a long time without any discharge current and run at a low float voltage setting you can end up with very low SOC on battery. Most hybrid bi-directional inverters will have some short periods of discharge current during float charging that will prevent surface charge build up.

Wellydex · Nov 4, 2020

snoobler said:
From where are you getting the 70%?

I have both a victron 712 and an electrodacus sbms0 so I am pretty confident that both my voltages and SoC are accurate

Wellydex · Nov 5, 2020

RCinFLA said:
A float of 25.8v is only 3.225v per cell (if balanced). That would be slightly less then about 20% state of charge.

70% SOC would be a float of 26.6v. 50% SOC would be 26.4v. Critical to have reasonably accurate voltage reading at float.

If you have resistor dump balancing in your BMS, 27v absorb level may not provide much balancing action as they typically do not engage until greater then 3.4v on a cell. Running loads from such a low state of charge and having little balancing could get you into trouble with weakest cell tripping BMS low voltage disconnect.

As a side note, pretty much all rechargable batteries have some surface charge. It is created between battery layers and is somewhat like an electrolytic capacitor. For LFP batteries, the surface charge energy storage is very low and dissipates quickly with light load. But it can increase the appearent float voltage of a cell by about 0.15v. If you float for a long time without any discharge current and run at a low float voltage setting you can end up with very low SOC on battery. Most hybrid bi-directional inverters will have some short periods of discharge current during float charging that will prevent surface charge build up.

Thanks for the above, I probably didn’t explain my scenario very well. As I only charge from solar, it re bulks every day and I want to achieve a cycle between 70% and 60%ish as the fridge discharges overnight. I don’t actually want any float hence setting it so low. I use it every couple of weeks and the reset to 28 absorb and 26.4 float to get back to 100% and allow the float to keep the solar sustaining the load during the day. This ensures it has 3 days where the balancing is working although very rarely more tha 10mV out of balance which is the threshold for the sbms to start balancing.

Keeping LIFEPO4 at 70%SoC

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