Getting ready to upgrade, does this make sense

[ericfx1984]

Please see signature for CURRENT system specs (batteries are currently 4 years old)

Here is the plan (I think)

Pull the existing batteries out of service...

Install 2 or 3 of these in parallel https://www.ebay.com/itm/373490407212

THEN

install a small 750 watt inverter attached to my old FLA GC2 batteries. From there I will add a 20-30 amp MPPT(less than $100) and a spare 290 watt (40v VOC) panel i have laying around to charge the FLA battery bank

the plan is to run a 250w cattle heater off this...

I still need to figure out how to shut the inverter off when the voltage is too low... BUT my guess is that the water heater would only run from 32f and lower... and since the water tank is 275 gallons and under my skirted RV(1/2" thick foam board) if SHOULD retain heat very well and the water tank SHOULD help to heat up the under belly of the RV a bit too...

The other issues I run into is IF I run the FLA down to 50%, i would need to put 215ah back into them... the panel and an MPPT charge controller MIGHT deliver 24 amps... 290w / 12 = 24amps .... that means I would need full sun for around 11 hours... not going to happen in the winter...

Is there a simple way that I can charge from my new Lithium battery bank, into my old FLA bank... maybe a DC to DC charger? I know it would not be all that efficient but just thinking out loud here... I guess one way to do it would be to have a small 20-30 amp charger pulled into the 3000 watt inverter, and then use that to charge the FLA battery bank...

But of course that brings up another question how do I make sure that the battery bank is not depleted below 50%? I mean I could certainly monitor this since I'm home working all day and shut it off during the day but in the evening that's not so easy

I suppose one way to look at it is it is a resistive load I could have some type of timer set up so that it only can run a certain amount of time during no light hours... It doesn't seem that there's a particularly easy way to achieve this

Thoughts?

 

[HRTKD]

That LiFePO4 battery is "low temp" only in that it won't allow a charge below 32° F. So what happens when the temperature doesn't get above 32° F for days? The battery won't get charged. You'll eventually go into a low voltage disconnect and now you're really in trouble because it won't charge or discharge.

How many amps does your tank heater draw?

I insulated the crap out of my 120 gallon fresh water tank. Good enough that I don't run the tank heater unless I'm also running the generator for other things. I also have a skirt on the trailer. I've camped down to -8° F without freezing the fresh water tank.