LifePo4 Charging - Adjusting SOC via BMS or Charger? (Xantrex TrueCharge 20+)

[Turd Furguson]

Hi All,

An interesting link was posted here regarding SOC voltages and recommended Bulk/Absorb/Float levels for LifePo4.

I am designing a 12v, 300ah house bank for a marine system around 8x150ah cells from BLS and two of the recently recommended BMS’ from BatteryHookup. My preference is to maintain a SOC of 80-90% and only top- up the batteries to 100% whenever needed for extended periods off-grid. This capacity is overkill for everyday use & day-trips as AC shore power is available, so I can leverage the opportunity to extend cycle the life of my investment.

I currently have the charger pictured below that has adjustable Absorption/Float settings for different battery types and temperature conditions. (Statpower/Xantrex Truecharge 20+). Bulk does not appear to be adjustable with the manual stating that it delivers 20amps until the gassing voltage is achieved - "typically around 14.4v with approx 75% SOC restored". Float mode can be disabled entirely.

My question is whether you think I can adjust my SOC by flipping the switches between the absorption/float options to adjust bank capacity as needed? My theory is to maintain a lower SOC at 13.x-14.0 volts, then flip the absorption to 14.4-14.6v before a weekend away.

Alternatively, could achieve the same outcome by adjusting cutoff voltage within the BMS? This option seems plausible but likely poor practice, especially with multiple BMS units.

Thanks!

 

[electric]

I would set the Battery Type switch to AGM and then Temp switch to Hot for normal cycles and Cold for top-off cycles.

 

[Turd Furguson]

That's what I'm thinking.

What about the alternative of limiting max voltage through the BMS. Can / should this be done to control SOC?

 

[electric]

Not if you can avoid it. BMS should be the last resort to intervene in an abnormal situation. Normal cycles should be handled by the equipment other than BMS.

 

[John Frum]

Not if you can avoid it. BMS should be the last resort to intervene in an abnormal situation. Normal cycles should be handled by the equipment other than BMS.

I do test low voltage cutoff if not outrageously low.

 

[electric]

You could also try Gel/Hot with lower voltages, but your absorb stage will probably take forever and might not be practical depending on how mobile you are and how short/long your AC grid connections are.
There is honestly not that much difference in battery life as long as you stay a bit below 14.6V and even that is not critical as long your charger has a float stage, so it doesn't sit at max voltage for many hours every day. Temperature has more effect on battery life, so pay attention to that if you want it to last longer.

 

[Turd Furguson]

Very informative - Thanks all!