I recently swapped out the AGMs for Lithiums. I got 2 battleborn 12V x 50 ah for the trolling motor, and have not used more than 10% charge on an 8 hour trip. I also got a Relion 100 ah for cranking / boat accessories. I did a ton of research. I am not a pro tournament angler. I made my decision on the price point of the batteries. Battleborns have a 10 year warranty, they were $575 each and weigh 17 lbs, about 1/2 size of a group 31. The Relion is group 31 size at 30 lbs, was $1300 w/ 5 year warranty. The relion is a great battery. There is lots of discussion about ah. all i'm gonna say is when a AGM or lead acid 100 ah battery drops below 50% charge (or so) , it is no longer dishcharging 12V, so it is probably not usable beyond that without charging. lithiums discharge 12v until they are 90-95% depleted. so if only 50 % of the 100 AH is usable, then the equvilant lithium is a 50AH. So I got into 3 lithiums for less than $2500. took 210 lbs out, replaced with 60 lbs. my hole shot is unreal now.
Forgot to mention the charger. All 3 batteries charge to 100% with the stock Dual Pro charger which came in my Ranger. i did not have to get a new charger, or have this charger updated with the "lithium profile". that is an option, not sure what it costs, but it can be done if needed. The relions and battleborns are "drop in" replacements. so they are designed to charge on a standard charger. once the batteries are charged, I unplug the charger. Lithiums do not need to be trickle charged as they don't discharge while they sit. My batteries are good for 3k-5k cycles. BTW a cycle is 1 full charge. so if you use the battery and it depletes only 10% and you charge it. that is .1 cycles.
So I figure I will not need to buy new batteries ever, based on how often I fish and charge the batteries! lol.
if I told you something you already know, didn't mean too. just sharing. :)