Its time - I found a great site (altho what do I know?) re how to buy a battery https://themarinebattery.com/#B00075OSCO
My Honda 8 (yes now running fine) manual specifies 35ah and CCA270. I have a two battery system - like most 23s? I am leaning toward a gelled battery based on that - but might need to make sure I wouldnt overcharge it? I dont know if my system prevents overcharging.
I would go for around 100 amp and a reserve capacity of 60-90 minutes? I run the auto pilot alot and so far we dont use much else, like lights.
I will contact Compac too. Any advice appreciated.
Stats like CCA and reserve capacity are not important in this application and in fact may be warning signs that you're looking at an auto battery, not Marine. What you are looking for:
1. True deep cycle type battery, which conventional auto batteries are not. You will be deeply discharging it and this will kill an auto battery.
2. Flooded cell lead acid type, unless you plan to buy a charger with selectable profiles for AGM, Gel or other technologies. If you are using your outboard motor alternator to charge, this is not an option and you should stick with lead acid.
3. Size that fits your space. Probably Group 24 I would guess.
4. As much amp-hour rating as you can afford. In the 80-100 Ah range ideally. You will only be discharging it to 50%, so if your autopilot draws 2 amps you can run it for around 20 hrs etc.
Hope this helps. #1 is extremely important. Look for a true deep cycle battery. Not "starting" or "dual purpose."
Wes
You might want to think about weight in the planning of your new batteries. There are some lightweight Lithium batteries in the range you talk about, but they are pricey, with examples:
https://www.amazon.com/LiFePO4-volt-Deep-Cycle-Battery/dp/B06XX197GJ/ref=sr_1_cc_4?s=aps&ie=UTF8&qid=1510151252&sr=1-4-catcorr&keywords=lithium+battery+100ah
https://www.amazon.com/GBS-LFMP100AHX-Lithium-Battery-LiFeMnPO4-technology/dp/B01B52H5A4/ref=sr_1_cc_1?s=aps&ie=UTF8&qid=1510151252&sr=1-1-catcorr&keywords=lithium+battery+100ah
https://www.amazon.com/GreenLiFE-Battery-GL100-100AH-Lithium/dp/B01NAV9G9X/ref=sr_1_cc_3?s=aps&ie=UTF8&qid=1510151252&sr=1-3-catcorr&keywords=lithium+battery+100ah
My smaller boat I got a 55 ah lead battery (https://www.amazon.com/gp/product/B0143K8UPY/ref=oh_aui_search_detailpage?ie=UTF8&psc=1) based on a weight that I felt was reasonable and a cost that wasn't crazy (like the above examples).
Chimerakc
Just replaced the 4 year old group 24M4 batteries in my boat. I replaced #2 (my starting battery) with a marine 24M5, and the #1 with a group 31 Mastervolt AGM. I have a 50 watt solar panel connected to the #1 and the alternator on the Yanmar is rated at 60 amps output. I have had great success with the Mastervolt AGMs in the past. I have the usual compliment of instruments and a Engel fridge/freezer. I cool the fridge down low when running the motor before anchoring and then turn it off at night. This practice worked fine with the old batteries, so I expect there will be no problem with the new AGM, and it will last much longer that the standard flooded cell units. Gel cell batteries do require a different charging profile which is not compatible with flooded cell so your charging regimen gets complicated with questionable benefit.
All this is very helpful gentlemen! Yes the boat already squats so weight is something I will consider too - except not the price I just saw at that AMazon link!! Good think my heart is in good shape.
Just recently we replaced four 8D AGM batteries in a 36' Californian Trawler (twin Caterpillar 3208's).
Each battery removed weighed 165#, when new had 245Ah, and when new had about 1400 CCA.
Two Lithium banks were set, each the size of a Group 27 battery.
Each was 200Ah, with depth of discharge conversion (50% AGM = 122.5Ah, 80% Lithium = 160Ah) net amp hours only slightly reduced.
Each weighed 88#.
The total cube, 14"x14"x14" = 5kw of stored power.
Cost, 50% more than AGM replacement.
Life, 6x AGM.