Comparing AGM and Lithium RV Battery Systems

Comment added August 10, 2017; see the notes at the bottom of this post, in particular #5.

A couple of years ago, I considered swapping my AGM batteries for Lithium (LiFePO4). I looked again in January 2017 and I again decided against doing that.

What's the problem? It's simply dollars and sense.

As can be seen above, running both 200AH lithium and AGM battery systems to 80% depth of discharge (DoD) the cost of the AGMs is sufficiently less. So, while the lithium batteries are more "elegant" from an engineering perspective they may not provide more benefits and at a higher cost. Read on for the details.

Here's the background:

Battery life: Lithium 2000 cycles at 80% DoD. AGM 700 cycles at 80% DoD.

Cost for 200 AH: Lithium with BMS = $1,939. AGM = $450.

As can be seen above, here is the arithmetic:

AGM: 2100 cycles at 80% DoD requires (3) sets of batteries, or 3 x $450 = $1,350.
Lithium: 2000 cycles at 80% DoD requires (1) set of batteries at $1,939.

Conclusion: use AGM batteries, install a better battery monitor and run the batteries to 80% DoD. Compared to Lithium, save $589 while getting about the same performance and none of the low temperature headaches.

Other Considerations: 

1. AGMs weigh more than LiFEPO4 batteries, so if I needed more than 200AH of battery capacity (more than 10 hours @ 16.8A) then I should re-evaluate an alternative to the 200AH of AGMs I have. 
2. Installing lithium ion batteries will also require additional electronics, including a charger and an Energy Management System, at additional cost.  My AGM system includes the necessary electronics, I added a digital Volt/Ammeter, so all I have to do is replace the batteries at the required time.
3. AGM batteries can be charged at below 32F. LiFEPO4 batteries have to be heated to be charged at 32F and below. In my case (AGM), that means no heaters and no wasted electrical energy warming up lithium ion batteries prior to charging. In my case, that makes installations simpler. I can keep the 200AH of AGM batteries in the outside compartment. I had decided that if I chose lithium ion batteries that I would install them inside the coach. I would have had to give up valuable real estate (square footage) to do that. 
4. Because Lithium ion batteries weigh less than AGM batteries, if I really needed 400AH or so, I'd look at the volume and weight differences. But that is not currently an issue for me. 
5. Over on social media, putting info about the relative merits of AGM batteries versus Lithium (LiFePO4) usually causes a bit of a stir. Here is my response to one social media rebuttal:  "I agree about the "light duty", but that also changes the cycles for AGMs and what' the point of buying a lot of capacity not to use it? What is missing in the chart you provided is remaining capacity and that does make a difference. The charts for the AGMs I'm using indicate about 60% capacity remaining after 80% DoD and 700 cycles. It is a known AGM characteristic that capacity does gradually decline, and by 700 cycles capacity decrease to 50-60% is usual. That certainly can have an impact on [battery] selection. Specifics may vary from manufacture to manufacturer. I used the table of the AGM battery manufacturer in my coach, and it might be accurate or not. I also based cost on the actual cost of the batteries (AGM's in my coach and the current price of the LiFePO4's I was considering). The lithiums don't include installation, which would definitely not be drop in. One other issue to beware of in AGMs is full discharge. The battery numbers vary based upon "relative" DoD. In other words, some battery specs go all the way down to 10.5 volts, which is a dead AGM battery. Other charts use relative terms in which the 0% AGM charge is 11.66V or so, which is actually about 20% DoD. Everything I've seen indicates that AGMs when fully charged generally have about 80-100% capacity which gradually diminishes. Lithium batteries also experience capacity loss, although that doesn't seem to become significant (below 80%) until about 400- 500 cycles. I'm sure there are installers who have better data based upon dozens or hundreds of installations. On the other hand, they might not want to provide data that kills the golden goose.