DC DC Charger to mix chemistries?

[ItsMaak]

Hi All,

First time poster and very happy to see a forum still running.

I'm likely going to upgrade my 2 x 100Ah AMGs to Lithium in the near future. There's still a lot of life left in the AMGs and I'm trying to work out a way to use them for longer stays off grid and want to know if anyone has used a DCDC charger to mix chemistries?

My Renogy 30A DC DC with MPPT will charge the house battery when the vehicle battery is >12V, cutting out if it drops to 11.5V. Could you connect the two extra AMGs to the DC DC input and use the ignition wire to trigger charging the Lithiums off the AMGs. The Renogy will also charge the vehicle battery when connected to solar and the house battery is in float charge. So ideally the system would run seamlessly once connected, charging the Lithiums off the AMGs down to 11.5V then topping the AMGs up with solar and the Lithium's full. I imagine I'd need to limit the charge current to prevent too much voltage droop from the AMGs but I can live with that.

Has anyone tried this?

cheers

Mark

 

[grizzzman]

I run a hybrid system in my trailer. I would not recommend that you pull the AGM's down to 11.5 volts as you will damage them in short order. Once in a while would be ok. I would run them any lower then 12.2 volts during discharge. The lithiums will do most the heavy lifting anyways. I would suggest using the DC to DC charging from the LFP to the AGM's is this a RV system?

 

[ItsMaak]

Yes this is an RV (camper trailer) - most of the trips it would be lithium on the trailer and AGMs left at home which is why I want the lithium set up as the House battery. Then for the longer trips, take the AGMs in a battery box, unplug the DCDC charger from the tow vehicle and plug into the AGMs.

So you have this setup all the time? Just with the lithium on the “vehicle” side and AGM on the house battery side?

I appreciate the concern about deep discharge on the AGMs - it’s unlikely to be more than a couple of times a year but it I want to do it more often I may look at a cutoff relay. I don’t think the 11.5v is something that can be set on the DCDC.

Cheers

 

[grizzzman]

I do have a DC to DC 40 Amp charger but it is only a backup charging source. Most of the time solar takes care of charging. During use the lead acid and Lithium are tied together. When not in use I turn off the lithium side and allow the stock converter to maintain the lead acid bank.