This is a long one. Buckle up. On my 37+, it came with 2 4Ds in the stbd settee forward of the sink. They were dead at purchase. To install and replace I had to remove the entire settee...which included removing the table. That area had been glassed in such that it made a water tight catchment so in the event of spilled acid it would be contained during all points of heel save for a full knockdown or roll.
Upon purchase I replaced the 4Ds with 3 group 27 - on the House #1 switch and 2 group 29s on house #2. They were the only available marine deep cycle batteries I could source on short notice in Brooklyn NY. I flattened copper pipe to create a bus bar witch bolted the 3 house #1 batteries in parallel. The boat also came with a Xantrex TrueCharge 40 which had evidently failed and lead to the demise of the 4Ds over winter. I replaced the original Xantrex with a larger Xantrex TrueCharge 60. It had 2 charging channels which was convienient for charging the 2 house banks. The NEW 60 amp charger didn't make it past the 350 mile delivery trip before failing as well. I promptly returned under warranty and replaced with a ProMariner ProNautic P1260. I've been very satisfied with it. My only complaint is that the remote display does not allow control, only observation. Generally this fine but in order to initiate an equalize charge you need to access the charger directly. I successfully mounted to charger to the back of the nav station drawers. The location works great accept for the formerly mentioned need to access for equalization. It has 3 channels of which I only use 2 but leaves room for an additional start battery. In 12 years of service this charger has never left me down in any way. In 2016 the NEW deep cycle marine batteries I'd originally sourced from Auto Zone began to fail. I finally had time and money to properly source batteries. I modified the previously glassed battery box under the settee to fit four 6v golf cart batteries. I got Trojan T105-RE. The RE is specifically designed for solar/wind generation storage systems. Each one has about 225Ahrs so a bank of 4 arranged (series/parallel) to produce 12v results in 450 Ahrs of storage. I bought the watering system to go with it so as to monitor water level and simplify watering when needed. I wanted batteries with flush bolt down terminals but ended up with an upright bolt terminals. This ended up requiring a 1x2 wood shim around the perimeter of the settee bench to add the required height to accommodate the terminals and bus bars. I've later added 200w of flexible solar and two 8amp Genesun MPPT controllers. I typically get ~70 Ahrs per day of charge. More information available upon request. Recall that the max charging rate of a standard lead acid battery is 25% of C. C being the AHr capacity of the battery or bank in question. So my 450AHr bank should be limited to 110 AHrs max charge capacity. 60amps is well within that limit. In your case, if you stuck with the 2 group 27s then you'd be at about 200AHrs so a 50amp charger would be your max. Its easy to think that you have no need to charge at 50amps but consider the following. Discharging your battery(bank) below 50% should be avoided to ensure maximum life span - So 100 AHrs. The max charge rate only applies until the 80% when the absorption phase begins and the rate tapers off logarithmically from 80% to100%. With a 20 amp charger you'll be at max rate for 3 hours (60AHrs) and then a tapering rate for roughly the next 3 to 4 hours. If your cruising needs are met by the 100AHr limit and 7 hour charge time then stay with what you have. If not then you'll want to step it up. On our boat we can charge rapidly enough (with the upgraded alternator - 90 Amps - and the 30 to 60 minutes of motoring on and off the hook morning and night) to be limited in our range not by the electrical supply but rather the fresh water supply. I can usually stretch the tanks 4 to 5 days with showers for 2 adults each day. As others have said, you need to consider what type of sailing/cruising you plan to do. If the setup you have has been filling the need adequately then I would be hesitant to deviate too much. In that case, my simplest and cheapest advice is to replace the charger with a ProMariner pNautic 1220. Sterling makes an exact match. Feel free to price check between the two. ProMariner 63120 ProNauticP Series 1220P - 12 Volt, 20 Amp Battery Charger https://a.co/d/amg8SKZ Sterling Power ProCharge Ultra On-Board 3-Bank All-Digital Marine Battery Charger - 90-270 Volt AC Input for Worldwide Usage - 4-Stage Charging to Protect Your Boat or RV Battery - 5 Year Warranty https://a.co/d/15sgdKj All the best, Josh Muckley S/V Sea Hawk 1989 C&C 37+ Solomons, MD
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