Just stumbled upon this DC/DC charger thread. I will be hooking up a Renogy 20A DC/DC charger to charge up my DIY 120A Lifepo4You will definitely need a "power assist" from somewhere. It's not good and here's why...
After setting the Renogy DC-DC 20A charger to a lead acid battery profile, as you recommend, it will only put < 60A of charge to your LiFePO4 battery bank over the 3 hour bulk/absorption time. That's fine if that's all you need to bring the battery up to near full capacity but if you are down 100A at the start you still require >2 hours to charge to 100%. But now your battery charger has entered "float" or "power assist" [sic] since you aren't using the correct charge profile. Because of this, your charger is trying to charge at a lesser rate with a parasitic load adding more stress to the charge cycle. Not good but probably not going to cause any fires. There are some other detrimental things shortening the life of your batteries if you use the wrong profile for your battery chemistry. I can go into those things if you would like.
In the LiFePO4 charge mode you are still charging batteries with a load but at a much more optimal voltage for the battery chemistry. You don't need a "float" for LiFePO4. They take what you give them. That's why you are protecting your alternator with a charger/isolator in the first place.
If you are charging LiFePO4 batteries then you should use a LiFePO4 charge profile for optimum charging. The charger will still "load share" (a more precise term) much like charging your cell phone or laptop while using it. It will cause less stress on the batteries this way.
So, although a lead acid battery profile will work it is not the best way to charge LiFePO4 batteries. If you are charging with a parasitic load, like I am most of the time, you want to get to ~85% and then let the batteries carry the load. That's what they are there for, not the charger. With LiFePO4 you can always charge again. They work well like that. They love to cycle through between ~30% and 85%.
S4 12v battery, with Daly BMS. After going through the Renogy manual, I was planing to set the charging profile to Lifepo4, with DIP switches 3,4 set to ON and 5 to OFF. Now can't decided what voltage to charge at. Was thinking of playing it safe and setting the charge at 14.0V or 14.2 to keep the battery topped up at around 85%-90%. That would keep the 4 cells at 3.55 max. I want to avoid over charging the battery since the Renogy keeps charging even if battery is full, albeit with low current. I do wish the Renogy would actually stop charging, but since it doesn't, the next best thing would be to keep the charge voltage under 14.6V. What do you think?