Check out the Firefly 31 batteries made by Caterpillar. Read some reviews and hit up some user reviews. These batteries behave much like a Lithium battery and can be taken down to 80 %, some folks report lower. They were designed to be used and abused as Caterpillar was having problems with batteries in their big equipment that sat unused for months then wouldn't start. And vibration was also an issue. So they design and built this new type of battery which doesn't require a complete charge and still keeps going on like the Ever Ready battery:
https://www.emarineinc.com/Firefly-Oasis-12V-Group-31-Battery
This is from an article written by Jeff Cote, a systems design engineer and owner of Pacific Yacht Systems:
"Carbon Foam Batteries. The Firefly Oasis carbon foam AGM battery was developed by Caterpillar to replace the FLA batteries that they were using in their work vehicles. They found that the old battery technology could not, consistently, withstand constant vibration, inconsistent charging or long periods without charging. This new technology is based on the sealed valve-regulated design of a conventional lead acid battery but the negative plate has been replaced by a light-weight, sulfation-resistant, microcell material. One of the biggest benefits of this technology, for boaters, is that the battery can be left in a partial state of discharge (i.e. extended cruising, sitting on a trailer or at a dock without power) and it will not lose any permanent capacity. You simply have to charge the battery and it will return to 100% of its original capacity
If you are looking for the maximum capacity with the smallest footprint that weighs the least, then Lithium batteries (LFP) batteries may be the best choice for you. They are by far the most upfront-cost option but if you use your boat a lot and plan on keeping it for a long time, they will pay for themselves and offer the best value in the long term. A unique characteristic of LFP batteries is that they don’t go through the three stages of charging: bulk, absorption and float. The battery is bulk charged until full (almost no absorption stage required), at which time float voltage and currents are mandatory to prevent over-charging. Note there are very few chargers or alternator regulators that are capable of safely and efficiently charging LFP batteries. If you are designing or implementing a LFP battery system, you will need to choose charging components that are compatible with this technology.
Cost Per Cycle Calculation. This year at the Vancouver International Boat Show, we featured a battery display in our booth that compared the total usable amp hours, weight and cost of FLA, AGM and Carbon Foam batteries. If you are trying to spend the least amount of money possible to replace your battery bank, it would seem to make sense to go with FLA batteries but if you do the math, you can see the value in AGM and Carbon Foam.
Adding Batteries Means Upgrading Your Charger. Another challenge we see is that boaters increase the size of the battery bank but do not increase the size of the charger. One of the most common causes for a battery to fail prematurely is overcharging or undercharging. When sizing a charger, the general rule of thumb is that the charge rate should be at least 10 percent of the total amp hours of the battery bank. For instance, a 400Ah deep-cycle battery bank would require a 40A charger. With large battery banks (800Ah or more at 12VDC), it is worth considering adding a large charger or even a second charger, in parallel, to ensure that the batteries charge at the right rate of charge to prolong battery life.
For FLA, AGM and Carbon Foam AGM batteries, we recommend a smart three-stage charger. These smart chargers detect voltage and resistance from the battery prior to charging and determine which stage to charge, either bulk, absorption or float. They can be left for months at a time and will reliably monitor the state of your batteries. As a reminder, you will also have to change the settings on your charger and your battery monitor to reflect the new battery chemistry.
When a boat has no space for additional batteries, we often recommend upgrading from FLA to AGM. More usable battery capacity with AGMs can translate to increased battery capacity if you keep the same number of batteries or alternatively you can reduce the battery bank size and still have the same amount of usable capacity. If you have been contemplating replacing your battery bank, these new batteries just make sense. Although the upfront cost is higher, the value proposition is unbeatable because of the combination of increased depth of discharge and faster charging.
About the author: Jeff Cote is a systems design engineer and owner of Pacific Yacht Systems, a full service shop delivering marine electrical and navigation solutions for recreational boats. Visit their website and blog for info and articles on marine electrical systems, projects and more:
www.pysystems.ca