LiFePo4 battery charging for maximum cycle life with solar panels

Question

The problem is charging lithium iron batteries for "maximum cycles" with a solar charge controller wile using loads.

I have a PowerSonic 45 amp hour LiFePo4 battery for my Caravan house battery. I am powering it with a 100 watt solar panel and a pulse width modulation "PWM" charge controller.

My loads are a small multi-voltage refrigerator that draws about 2.5 amps at 13.5V with appx 50% duty cycle, a Sony car stereo, some LED lights and other small 5 VOLT USB loads, phone and tablet charging.

Currently I am using a factory programmed PWM charge controller that should limit the bulk charging voltage to 14.2 volts but it doesn't and the voltage rises to 16 volts where the battery management system takes over and cuts the voltage to battery bank to 13.7V. The load low voltage cut out is set to 12.1V, the maximum solar output is theoretically 7.1 amperes at 14 volts but because it's not a maximum power point controller realistically it's approximately 5.6 amperes maximum which is well below the recommended 20 ampere maximum charge rate.

I have ordered an EP solar Landstar programmable PWM charge controller for the solar panel and plan to set the bulk charging rate at 14.1 volts and the trickle charge rate to 13.3 volts and the load low voltage cut out to 12.5 volts.

I have also ordered a buck boost converter and plan to connect it to the Caravans auxiliary output set to 13.7 volts at 8 amperes for supplemental charging while driving. I hope this will give me a charge profile to extend the maximum charge cycles for my LiFePo4 battery's application.

It's my understanding that deep discharging and charging too high reduces the cycle lifespan of lithium iron batteries.

What bulk-charge and float-trickle charge voltages should the charge controller and buck-boost be set to for maximum cycle life?

Answer 1

I Have two LiFePO4 8s battery=packs (Headway 40152 - LiFePO4 cells) and I undercharge them with my charger. I bought my charger with a default 29,2 volt and re-set the voltage to 28,5 volts.

Per cell will this give: 29,2 / 8 = 3,65 volt per cell (Default for LiFePO4)

and after: 28,5 / 8 = 3,56 volt per cell (Undercharging LiFePO4 for more cycle-life)

You can set the voltage with the blue block with copper screw closed to the Power out. Just connect multimeter to the charging plug and turn the screw until the multimeter gives 28,5 volts instead of 29,2 volts.

This undercharging will cost almost nothing of the total capacity, because of the voltage drop and climb characteristics of LiFePO4. I think it will cost only 1% or less of the capacity of the cell or battery. Check it in a discharge table of a LiFePO4 cell. Very easy to Google to find a discharge curve of a LiFePO4 cell. Example: 40125 discharge curve.

An other advantage for lowering the max. charging voltage is that It's more unlikely that you get damage through unbalance.

Answer 2

Nowadays, a few years beyond. I think 3,40 volt per cell for LiFePO4 is best eco/longlife full charge! So for longlife and Eco charging: 12 volt = 4s = 4x 3,40volt = 13,6 volt. 24 volt = 8s = 8x 3,40volt = 27,2 volt. 48 volt = 16s = 16x 3,40volt = 54,4 volt.

Discharge End voltage per cell: 3,10 volt. Best way to charge is solar! High amps, inverter. An other big pro is in these voltages (3,40volt-3,10volt) you will not have too much in-balance between the cells in the pack!

My Headway packs 40152s are still going strong and I bought a new 8s 280Ah pack for offgrid use. Most important is: Take measurements of your pack regulairy. Even daily if you are new to this. Never think of of "full = good" that is a bad idea if you want to enter the world of batterypower. Although LiFePO4 is more easy on you, than Li-Ion ever will be. LFP is the new solar future, longlife, green, reliable and environmentally friendly!