AGM vs. Lithium when keeping a high 80% DoD

[jdy2kgt]

Last year I purchased a 24’ electric pontoon boat (Torqeedo 4.0 – 48V) and converted it from being charged with an extension cord/ProMarine chargers to being charged by 440W of solar through a Victron 100/20 MPPT. The boat was set up with 4x12V SLA Everstart 100AH batteries. There were already several years old but I kept them for the first year just to see how everything worked out. I also have a fifth 12V battery (SLA) that is the house battery for the boat’s lights, radio, fish finder, etc. For that I have the Victron 48-12 9A DC/DC converter that keeps it charged. As you can see in my attachment’s I put MBRF’s on everything (battery terminals as well as a triple block that the MPPT, DC/DC and motor connect to. I also have a 300A on/off blue sea switch for the motor wire coming off that block. Okay so now that I’ve filled you on what I have, let me get to my questions that I’m hoping you can help me with. I’m looking at replacing the batteries and obviously looking at Lithium (LifePO4) from various companies but I’m also still thinking about AGM’s. Cost is the main reason, but I’m also wondering other than weight would I truly see the advantages of the Lithium in my setup. For example – last year on average we used the boat once a week (so it had 6 days to charge in between). Normally were would cruise around using only 500-800Ws (I also have the Victron 712 shunt). The lowest DoD I saw was 80%. And there were several times we went out, anchored and were charged back to 100% before we even went back to dock. So if a typical AGM is listed at 1500 cycles 75% DoD wouldn’t the math mean that these batteries would last 1500 weeks or more on my typical usage (aka 28 years!). And that’s where my question is. What am I missing here? Or am I right that the way we use our boat we really wouldn’t see the advantage of Lithium because we are only cruising speed and getting charged constantly by solar. Or am I missing something like AGM’s naturally degrade with in “x” years even if you use them conservatively and stay well above 50% DoD. I should also mention that I live in central Virginia. We might get below freezing a handful of times during the off season but that's it. Normally I take the SLA's off the boat and store them in my garage, but if I did go to lithium couldn't I just set the MPPT not charge the batteries below a certain temperature to prevent any damage?

And my second question is much shorter. The AIC rating of the blue sea MRBF’s is 2000 on a 48V system. If I switch to lithium do I need to replace everything or just install a T-fuse in between the battery and the triple MRBF? Is there anything else in my system that would need to be upgraded?

Thank you,

Jim

 

[time2roll]

If the Lithium is only cycling the top 20% the battery might be too large. Also the AGM is slow charging in the last 10% so it can often be difficult to obtain full charge from solar often enough to avoid sulfating. The number of cycles is not always practical in the real world. Lithium otoh can be short charged every day and may live a bit longer for it.

 

[jdy2kgt]

If the Lithium is only cycling the top 20% the battery might be too large. Also the AGM is slow charging in the last 10% so it can often be difficult to obtain full charge from solar often enough to avoid sulfating. The number of cycles is not always practical in the real world. Lithium otoh can be short charged every day and may live a bit longer for it.

Just to be clear I don't currently have lithium I have SLAs.

 

[Hedges]

So if a typical AGM is listed at 1500 cycles 75% DoD wouldn’t the math mean that these batteries would last 1500 weeks or more on my typical usage (aka 28 years!). ... Or am I missing something like AGM’s naturally degrade with in “x” years even if you use them conservatively and stay well above 50% DoD.

That sounds like quite a premium quality AGM.
Here's the one I use:

http://sunxtender.com/pdfs/Sun_Xtender_Battery_Technical_Manual.pdf

I expect 700 cycles to 70% DoD, or 10 years, whichever comes first.

I have a 100 Ah 48V bank, about 40% of original capacity remaining after 13 years. It just sat around, occasionally getting recharged.
I have a 400 Ah bank now 2 years old. Mostly kept at float, has been cycled maybe 10 times.

Should be good for cold/freezing weather. Reduced output under those conditions, but holds up so long as not left discharged to freeze. Charger needs temperature sensor on battery.

I've read 4000A short-circuit current for 100 Ah AGM. For lithium, based on 0.17 milli-ohm per cell, I expect 20,000A (although actual may be less due to some chemistry or ion mobility effect.)

LiFePO4 is getting to be cheaper in purchase price than AGM. More complexity though.
LiFePO4 is about 1/5 the cost for same cycle life (assuming cycled daily.)