Hi all it's been a while since posting on the Tugnuts forum. In the last month I received two Tugnut PM's on this thread subject. One was from Paul so I thought I would answer using his thread. Questions using Deep cycle marine Lead Acid flooded batteries 12V batteries are becoming fewer and fewer because (1) There are not that many to choose from if any (2) Today the battery replacement is converting to LiFePO4 batteries. Expensive and it requires a lot of thought to do it correct and meet or exceed ABYC recommendations.
After two years of owning and cruising with the factory installed maintenance free Centennial LA batteries in my C26 Cutwater. I decided to replace the batteries with Northstar AGM Group 27 DP with 90AH rating. These batteries met the requirements of the engines CCA and had a decent AH rating for a Group 27 battery. They were expensive. The plan was they would last longer than a standard flooded or maintenance free LA Battery that were significantly less in cost. We did not use the Cutwater as a dock queen. We cruised and anchored a lot. The battery charger installed by Fluid motion was not sized correctly IMO 20 amps to charge (3) battery banks, I did not have solar and I did not have a generator. The two 90AH AGM batteries (180 AH -50% usable = 90AH of usable stored AH's ) I found that it was not enough. After 12 to 14 hours on the hook the house bank was depleted below 50% with battery voltages at 12.0V to 12.2V. I then installed a cross-over switch to combine the thruster battery. (270 AH-50% = 135AH of usable stored AH's) Better, this helped by giving me almost 24 hours before I needed to leave an anchorage needing to start the engine to charge the battery banks. The Volvo alternator was hi-output IMO 165A but it doesn't charge close to that rate unless the engine is turning at least 2000 rpm. The results after two years of cruising the expensive AGM batteries were compromised. Usable but not capable of returning to 100% charge. Without having the ability to charge to 100% the usable AH was reduced by close to 20%. The batteries did not fail but would not provide the much needed AH I needed for the way we used the boat. I continued to use the batteries and purchased a 2200W Honda to use when on the hook to help with power management.
I sold the Cutwater C26 and purchased a 34 Mainship pilot. After purchasing it I replaced all the batteries and rewired the complete battery install. I followed the ABYC recommendations and realized how much had changed in the recommendations from the build date in 2002 to 2021. It was an extensive job. I replaced all the batteries in the boat with Deka East Penn flooded Group 31 DP 100AH batteries. (4) house (2) Thruster. Engines heavy duty group 31 950 CCA flooded Deka East Penn. I also installed 50 amp battery charger, 3000 W inverter, 600 W solar 100V/50A solar charger. I was not going to purchase expensive batteries this time. I learned that more AH and better charging capacity would be better than installing expensive AGM batteries.
Results, we cruised for 14 months while doing the Loop. I was able to stay on the hook for up to 5 days and maintain the LA battery bank and never went below 60% charge. The Northstar AGM batteries in 2018 cost with tax just over $1300.00 . Deka ( labeled has Duracell ) same battery made by East Penn. price $132.00 each with core (2021) Six, group 31 DP flooded 105AH cost just under $ 800.00. Two, 950CCA Group 31 flooded under $300.00 I installed a Cross over switch in this installation that combined the thruster bank with house which gave me (630AH-50%=315AH ) along with solar and larger battery charger helped maintain the batteries. The batteries were battery load tested and internal resistance checked in May of 2025 showing a loss of 10% . I could have easily used theses batteries for at least another year with better performance than I was getting out of the AGM's purchased in the past. Even with this result I decided to replace house and thruster banks with (2) LiFePO4 Epoch 460AH (ea.) batteries. (920 AH - 10%= 830AH of usable ) Game changer!!
LiFePO4 is a big project! It was a fun Project to do. It is an expensive project if the ABYC recommendation are followed. They should be. This is for a different thread.
The DP flooded LA batteries IMO will provide good service and the cost is much less. Think of it this way. The best serviceable life you may get out of AGM batteries is 5 to 6 years if they are maintained and used as per manufactures recommendation. Comparing apples to apples (2025 prices). Duracell AGM 800CCA 110AH = $269.00ea with core exchange. Duracell Flooded LA 700 CCA 100AH =$159.00ea, Heavy duty Duracell start batteries 950CCA = 169.00.ea. Paul's application (4) new group 31 DP 110AH house, thruster and engine AGM= $1076.00+tax. Or house and thruster (3)DP Group 31 flooded 700CCA ,100AH = $480.00+ (1) Engine Heavy duty 950CCA flooded LA $169.00 = 650.00 + tax Replace the LA with new fresh batteries in 3 years when still good but 10%+/- loss in capacity. My choice is new batteries more often for a couple hundred bucks more. AGM that will loose performance over 3 years too. Ready for full replacement at 5 to 6 years.
Opinion is based only on my experience. Everyone has opinions.
