Absorbtion time lifepo4 bank

Newenough

Solar Enthusiast
Will has said 14.2v for absorbtion. My MPPT(s) also have the ability to set the Absorbtion Time as well. Currently I have it set for 45 min (I must set a time) but not sure about that. The system is 300ah 8s5p (24v). Charge rate is .2C.
 

gnubie

Photon Sorcerer
The reason for the timer is to account for there being a load across the battery. If the charge controller relies on current only it will never detect end of charge because it will see the load's draw.

Absorption charging is more of a lead acid thing. Lead acid batteries quickly come up to 60% or so charge but then the charge current falls off and the climb to full charge is a slow one. The voltage is raised and current monitored to bring the battery to fully charged much more rapidly at the expense of splitting the water in the cell. Sealed batteries have recombiners to recover the water but only at a low rate. Excessive time in absorption charging will ruin a sealed battery.

The important factors for a lithium cell is to not raise its voltage above the maximum spec and to keep charging current within the spec too. Leaving a lithium battery at the typical lead acid absorption charging voltage won't cause it a problem as the current will taper down to 0 once the battery is charged and unlike lead acid the lifepo4 battery does not have a midpoint where it suddenly becomes harder to charge.

Victron's more recent firmware no longer monitors charge current to see if a lifepo4 is charged instead just leaving the battery at the higher charge voltage for a number of hours (4?) before dropping down to float.
 

Johan

The law of conservation of misery.
Will has said 14.2v for absorbtion. My MPPT(s) also have the ability to set the Absorbtion Time as well. Currently I have it set for 45 min (I must set a time) but not sure about that. The system is 300ah 8s5p (24v). Charge rate is .2C.

[1] suggests that 4 hours may be way too long for LFP, shortening cycle life. At the relatively low 0.2C charge rate, my first guess would be that you do not need any absorption at all because it's already nearly full. 14.2V for the bank = 3.55VPC (Volt Per Cell). You could even go a bit lower to 3.50VPC* in order to "stay out of the knee" for longer cycle life.

*however [1]: "[...] any termination voltage below at least 3.5V/cell should be considered as inadequate if the installation experiences incomplete charge cycles."

From [2]: "Absorption times with lithium iron phosphate batteries are typically in the 30-40 minutes range in most situations when charging with high-current sources, and much less if the battery is being charged at low current. If a time-based termination is going to be implemented, then the absorption time should be determined experimentally by monitoring the current taper."

From [1]: "Maximum Peak Charging Voltage: 14.2V to 14.6V (no absorption at all – STOP once this voltage is attained)"

References
[1] https://marinehowto.com/lifepo4-batteries-on-boats/
[2] http://nordkyndesign.com/practical-characteristics-of-lithium-iron-phosphate-battery-cells/
 
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gnubie

Photon Sorcerer
The current rapidly falls off with lifepo4 so charging them at high current at nominal absorption voltages isn't really possible. Victron has the time limit to account for not being able to monitor the load current separately from battery current. The current default time limit is 2 hours for lifepo4. Of course all this is configurable should someone wish to not have that elevated charging stage at all.
 
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