Looks like I need to replace at least one of two cranking batteries on my boat. Have always used lead acid but it seems like I’m always replacing one or both. Decided to switch to AGM. Their cost is very high now … $400 + for one Optim which means over $800 for two batteries.
Thinking of buying just one and running it together with one lead acid. For those of you who have more practical experience on batteries than I, would this cause any problems with alternator, etc? Also considering buying two automotive rated AGMs at 750 cranking amps in lieu of the 1000 recommended. With both batteries in parallel I don’t see a problem doing that.
Can anyone comment?
If the AGM's are that high now, you might consider the Lithium batteries, they are within a hundred dollars of that price and they supposedly have a much longer life span, (more recharge cycles) plus they are light weight considering.. However, I do know that Lithium and Lead Acids don't play well together, they use a different type charger for both, so I wouldn't mix them, unless your electrical circuits are separated and you use the appropriate charger for both.. sorry i don't know if AGM and LA will play well together...
As far as stacking multiple batteries together rather than one bigger one, that is fine as long as you have room and the extra weight isn't an issue. The CCA may not be as high, but the Amp-hour rating should be double... so they will last a lot longer... may not spin as fast though... Should have paid more attention in that class that taught about this... Later Jeff
I have wanted to replace my crank/ accessory battery also. Looked at Lithium, but every company I talk to say that Lithium is NOT suitable for a crank battery. AGM's are nice because they are sealed, no worry about spilling acid if they get tipped over. So I have replaced my crank with an AGM ( WalMart Everstart, Series H7, $170. 00) then a 12V 6AH Lithium to power my depth finders. Will see how it works. Using AGM's for the trolling motor also. Would like to replace those with Lithium, b u u u u t will keep thinking that !!
Thanks for your replies. My boat already has two cranking batteries that provide all power for boat and accessories. Decided to replace both with AGMs and use the existing good LA as a backup for truck. Bought NAPA Automotive AGMs. Can’t be any worse than the LAs. I do think it’s not good to mix battery types. I think I slept thru the battery subjects, too!
(04-21-2022, 08:24 PM)trueblue2 Wrote: [ -> ]Thanks for your replies. My boat already has two cranking batteries that provide all power for boat and accessories. Decided to replace both with AGMs and use the existing good LA as a backup for truck. Bought NAPA Automotive AGMs. Can’t be any worse than the LAs. I do think it’s not good to mix battery types. I think I slept thru the battery subjects, too!
I think you'll see much better results having matched AGM's especially if you are using them to run a 24V trolling motor. Just make sure your charger is set to the AGM setting.
+1 AGM--Optima is really proud of their pricing. I used to buy the Sears Die Hard wet cell lead acid for any of my vehicles and finally purchased their AGM marine dual deep cycle/start in 2007 for the boat. After 15yrs that AGM is still holding a full charge. I added another AGM to my boat last year. It's another Die Hard marine. Advanced Auto Parts now has the Die Hard battery bought out from Sears which I purchased just under $200. Just recently had to replace the wet cell batt in my Caddy Escalade also purchased an AGM for it. I'm slowly transforming all my vehicles to AGM. On the average the wet cell only has been giving me 3-5yrs use and the price has gone up.
Okay boys and girls, todays lesson is 2-fold. 
Part one is why you should not use an automotive battery to supply electrical power to your boat. Your homework assignment is to read this article: Marine Battery vs. Car Battery: What Are the Differences?.
Pay special attention to this quote from that lesson: “Keep the above differences in mind, and avoid the temptation to choose an automotive battery instead of a marine battery for your boat. Using the right one means you’ll be safe and sound out on the lake, all season long.”
Part two is about how AGMs differ from Lead Acid. Your second homework assignment is to read this article: Marine Battery Selection: Lead Acid vs AGM, Deep Cycle vs Cranking.
Section #4 is very good advice.
And for what it is worth, I have about 18 years experience with running marine batteries to start my outboards and run the boat accessories and a separate set of marine batteries to run my electric trolling motors (both 12V & 24V motors). I have always used standard marine lead acid batteries to crank with and marine AGM batteries to run the trolling motors. AGMs are better suited to the deep discharges associated with running electric trolling motors. My marine lead acid batteries used to start my outboards and run the rest of the boat have lasted anywhere from 5 years to 7 years. I wouldn’t consider using automotive batteries for marine applications for the reasons pointed out in the two articles listed above. My recommendation is to use the marine products, but in the end, its your money and your choice. Good luck and tight lines to all y’all. 