Battery Selection
A goal is to have no diesel power at all on board. In removing the old diesel electric refrigeration will replace the original engine driven refrigeration. Having an electric auxiliary and no genset requires a significant investment in batteries and planning for battery sizing and to meet charging needs of lower voltage batteries.
The electric auxilliary selected requires a 48 volt battery bank with a recommended capacity of 7-14 kWh. The 48v battery bank would be charged directly by the solar array and wind generator plus some regen when underway. Shore power is expected to be rarely used (the boat will be kept on a mooring) but an AC charger will also be installed. The 48V bank can drive a solar controller to charge a 24 volt battery for refrigeration and other large loads such as air conditioning, heat, water maker, windlass, autopilot, radar and inverters. This can also drive a second solar controller to charge a 12V house battery.
Battery Requirements
Electric auxilliary auxilliary require a 48V battery bank (or higher voltage for larger motors). Solar panels of at least 58V are required to properly charge such a bank, unless panels can be put in series. A discussion of solar, wind, and shore charging can be found elsewhere.
Some high power loads are better served by a 24V battery bank, rather than a 12V bank. These include refrigeration, inverters (all but very small inverters), autopilot, radar, and windlass. Using 24V allows smaller wire diameters to be used with less loss in the wires.
Some loads can only be run off a 12V battery. These include most marine electronics, radio, cabin lighting, and navigation lights.
With the need for battery banks at three voltages, three appropriately sized battery banks are the best solution. An alternate is to use DC-DC converters but finding marine or at least water resistant versions of these in high capacities is difficult. Failure of a DC-DC converter could make important systems inopperable at critical times.
The 48V battery bank needs to be very large. The 24V battery bank or single battery can be smaller. The 12V battery can be still smaller. The latter two can be charged from the 48V battery bank when that bank has high state of charge.
Battery Chemistries
The battery chemistries in use for marine battery banks are lead acid and lithium ion.
Lead Acid Batteries
Lead acid comes in three varieties, wet cell, gel, and AGM (absorbed glass mat).
Wet cells provide the greatest capacity per cost. They provide lower volume and weight density than gel or AGM. Wet cells require maintenance and don't last long. Their longevity is greatly reduced with a small number of deep discharges (down to 20-30% of capacity) and can be ruined with a single full discharge.
In order to avoid severely reducing longevity it is recommended that lead acid batteries never by discharged below 40% of capacity, and normally be operated in the 60-100% or 50-100% capacity range. This reduces effective capacity to 40% to 50% of rated capacity.
Gel and AGM have similar characteristics, with AGM being more commonly used today. Both gel and AGM are maintenance free. The volume and weight are slightly better per capacity than wet cell. They are longer lasting. They are also somewhat more tolerant of occasional deep discharge.
A gel or AGM battery can last a factor or two or more longer than a wet cell bettery. The price of gel and AGM have dropped to the point where, considering their longevity, it makes very little sense to buy lead acid batteries.
The Concord Lifeline is a line of high quality 12V AGM batteries, often considered the gold standard. The Concord Lifeline size 8D battery provides the best volume, weight, and cost density within this line. Capacity is 255 Ah (3 kWh). Useful capacity is about 1.5 kWh. Weight is 156 lbs. Volume is 1.13 cuft. Footprint is 1.6 sqft. Cost varies but is generally about $700.
Lithium Ion Batteries
Lithium ion (Li-ion) batteries have until recently been very costly. They are still costly but with prices steadily and significantly dropping in recent years. Where lithium ion batteries excel is in smaller volume and much less weight per capacity, and much greater longevity. Longevity in a renewable energy charged system is further improved since Li-ion batteries need not be fully charged promptly after being discharged to avoid degredation as is the case for lead acid. Despite the high initial cost, the longevity makes Li-ion batteries more cost effective over time.
For marine use, lithium phosphate (LiPO4) batteries are used. Despite misinformation on this topic, LiPO4 batteries cannot catch fire. Lithium cobalt (LiCo) batteries used in cell phones and some older laptops and most lithium ion batteries used in automotive applications (LMO, LMC, NMC, NCA) can catch fire, which is likely the basis for the misinformation. In automotive applications the more energy dense but flamable batteries are protected by cases designed to contain any fire and isolate a fire to damaged modules.
Lithium ion battery cells are extremely sensitive to overcharging and discharge below 20%. These cells are always sold with an integrated battery management system (BMS) except for large industrial or utility use where separate cells and separate BMS may be used. The BMS prevents discharge below 20%, slows charging significantly at above 80-90% state of charge, stops charging entirely somewhat below 100% capacity, and provides thermal protection. The battery ratings state the useful capacity of the battery, which may be 75% of cell capacity or less.
A battery with excellent density and cost per capacity is the Lithium Battery Power (LBP) 24V 75Ah battery. This battery has a rated capacity and useful capacity is 1.8 kWh. Weight is 27 lbs. Volume is 0.42 cuft. Footprint is 0.59 sqft. Cost is $1,300. These batteries can be expected to last 2-5 times as long, or longer, than an AGM battery yielding a lower long term cost.
Comparing AGM and Lithium Ion
The following table gives the approximate specifications for a 48V battery bank with a target useful capacity of about 7 kWh. The prices are as of 2019. Note that lead acid useful capacity is half of the rated capacity due to the need to limit depth of discharge to avoid excessive battery degredation.
| 48V 7 kWh Battery Bank Comparison | ||
| Specification | AGM | Litium Ion |
| Batteries used | 4 Concord Lifeline 8D | 4 LBP 24V 75Ah |
| Useful Capacity | 6.1 kWh | 7.2 kWh |
| Volume | 4.5 cuft | 1.7 cuft |
| Footprint | 6.3 sqft | 2.4 sqft |
| Weight | 624 lbs | 108 lbs |
| Cost | $2,800 | $5,200 |
| Expected Lifespan | 3-5 years (up to 8) | 10-15 year (up to 25) |
| Cost per year | $560 - $930 | $350 - $520 |
In the past the tradeoff would be the amount of space and the weight of an AGM battery bank vs the high cost of a lithium ion battery bank. With significant reduction in price and significant improvement in longevity of lithium ion batteries, the long term cost of lithium ion is now as good or better than AGM. At this point there is no good reason to buy AGM batteries.
Batteries Considered
Some of the manufacturers of lithium ion batteries with an established reputation include Renogy, Battle Born, Lithium Battery Power (LBP), and GreenLife. There are numerous others with little established reputation or poor reputation. Some less expensive batteries are not designed for a marine environment and are targeted at solar installation, RV, or EV conversions. Battle Born is not cost competitive and so not listed in the table below.
| Lithium Ion Battery Comparison (December 2019 pricing) | ||||||||
| Battery | Cost | Cost/Capacity | Bank Cost | Bank Capacity | Volume per kWh | Footprint per kWh | Weight per kWh | Comments |
| Renogy | ||||||||
| 12V 100Ah | $800 | $667/kWh | (8) $6,400 | 9.6 kWh | 0.30 cuft | 0.37 sqft | 23.3 lb | Very low cost. Average weight. Marine use. Good reputation. |
| AIMS Power | ||||||||
| 12V 200Ah | $1,799 | $708/kWh | (4) $7,196 | 9.6 kWh | 0.43 cuft | 0.59 sqft | 28.8 lb | Low cost. Heavy. Big. Not specifically for marine use. No product returns and questionable warranty. |
| Green Life | ||||||||
| 12V 100Ah | $780 | $650/kWh | (8) $6,240 | 9.6 kWh | 0.37 cuft | 0.51 sqft | 25.8 lb | Lowest cost. A bit heavy and big. Not specifically for marine use. Fixed output voltage so can't use voltage for SoC and therefore can't use battery protectors. |
| Lithium Battery Power (LBP) | ||||||||
| 24V 75Ah | $1,300 | $722/kWh | (6) $7,800 | 10.8 kWh | 0.23 cuft | 0.33 sqft | 15.0 lb | Low cost. Smallest and lightest by significant margin. Marine use. Can't use 12V battery balancer. |
| 12V 100Ah | $899 | $749/kWh | (8) $7,192 | 9.6 kWh | 0.35 cuft | 0.49 sqft | 26.7 lb | Average cost. A bit heavy and big. Marine use. |
The table above considers some of the best batteries from each manufacturer for use in creating a 48V battery bank of about 10 kWh as of 2020. Costs per capacity range for about $650-$750 per kWh. For the 12V batteries, either 3 or 6 battery balancers can be used (at $65 each for Victron).
The smallest, lightest bank and one of the most cost effective is the LBP 24V 75 Ah battery. Support from LBP is excellent where some others were completely unresponsive to questions about their products. LBP also offers a quantity discount that puts the price among the most competitive marine battery.
As of 2025 Battle Born have become more cost competitive. Renogy and Lithium Battery Power remain competitive. AIMS Power and Green Life seem to have disappeared. There are new manufacturers on the market. Cheap batteries from China offer low prices but may be using low quality cells. Cost per kWh for name brand batteries is now about $500/kWh.
Planned Battery Upgrade
The old batteries, water heater, and previous owner poorly constructed plywood and pine support structure have been removed. There is room aft for all three banks and larger capacities than initially planned. The "Battery Banks" table includes the batteries that have been ordered and delivered, but not yet installed.
A 10% discount was applied by Lithium Battery Power for quantity order. Shipping added to the cost.
| Battery Banks | |||||
| Voltage | Number | Battery Used | Total Capacity | Total Cost | |
| Ah | kWh | ||||
| 48V | 8 | LBP 24V 75Ah | 300 | 14.4 | $10,400 |
| 24V | 2 | LBP 24V 75Ah | 150 | 3.6 | $2,600 |
| 12V | 2 | LBP 12V 100Ah | 200 | 2.4 | $1,800 |
| Total Cost | $14,800 | ||||
Prices before discount rounded up to nearest $100.
Original Batteries
The original batteries were a Concord Lifeline 12 volt 125 Ah battery, weighing an estimated 74 lbs, with a separate Interstate 12 volt starter battery. The Interstate battery looks to be group 27 and likely weighs about 40-50 lbs with about 60 Ah. This is a total of about 120 lbs with starter battery and about 1.5 kWh of house bank rated capacity and 0.75 kWh of usable house bank capacity.
48V, 24V, and 12V Battery Banks
No battery balancer is needed for the LBP batteries connected in series. Some current from an near fully charged battery is leaked so as to allow another battery in series to balance. This is a feature of their BMS. If the sun is shining and batteries approach fully charged the entire set of batteries will be gradually balanced.
Due to the light weight and small footprint and volume of the LBP 24V 75Ah battery this will be used for the 48V battery bank. The same battery will be used for the 24V bank. The 12V battery bank will be source from LBP as well and will be the LBP 12V 100 Ah battery.
Battery Installation and Weight Savings
The entire set of battery banks will be installed all at once along with a 48V charger. While the solar panel install is delayed, the 120VAC charger will be the sole means of charging.
A separate order for charge controllers was placed with Morningstar Corp. Bus bars, primary fuses, chargers, and battery protectors will be installed forward of the motor. The existing electrical panel will have to be replaced and panels rewired (and simplified) separating 24V and 12V loads on the panel.
Ultimately 8 24V batteries will be used for the 48V battery bank, with a total weight of 216 lbs. Two more 24V battery and two 12V battery adds another 118 lbs. This is 334 lb replacing the existing 120 lbs of lead acid batteries and the diesel engine weighing about 625 lbs. This is a reduction of about 400 lbs plus the weight of the diesel fuel in the tank (about 240 lbs) plus the weight of the diesel tank itself (about 40 bls) minus the weight of the electric auxilliary (70 lb). A total of about 600 lbs will be lost.
Details on the planned battery install can be found on the Battery Installation Plan web page.