Battery Basics
Lead Acid Batteries
Lead acid batteries work by submerging a lead dioxide plate (positive pole) and a lead plate (negative pole) into a sulfuric acid electrolyte, this creates a chemical reaction that produces current. AGM batteries use a fiberglass mat to absorb the electrolyte, making them spill-proof and more resistant to vibration. AGM batteries also have faster charging, higher discharge rates, and better performance. This make AGM the choice for building a lead acid house battery bank.
You can connect the batteries in series to double the voltage and keep the same amperage, or in parallel to keep the voltage and double the amperage. Each setup may have a place on a vessel but it is important to know the difference to avoid damaging the batteries or what is connected to them. While you can get a 24v or 48v lead acid battery, it more cost effective to connect 12v batteries in series to get the needed voltage depending on the base voltage of a particular boat. Some boats have 12v systems, some have 24v and others may have 48v.d
The capacity of a battery is indicated as amp hours (AH). A battery that has 100 AH rating can deliver 1 amp for 100 hours. Divide the total AH rating of the battery by the total amps being used and this is the time you have before the battery fails, 2 amps is 50 hours, 50 amp is 2 hours. This is used to determine the size of your house battery bank if you don't want to run a generator overnight. Some equipment, like a dual voltage refrigerator, turns off the appliance when in 12v power mode, the voltage drops below a preset voltage, usually 11.5 volts, to keep from damaging the batteries.
To determine how many amp hours are needed for a house battery bank, add together the maximum amps of all items that will be drawing from the batteries. This information can be obtained from manufacturer information, product labels or by searching the internet. If unable to get the specific information on the items installed, search for like items on the internet and use those values. After totaling the amp draw per hour multiply that by the hours you want the batteries to power that draw and that is the amp hours used in that time frame. Lead acid batteries are able to be discharged to 50% before needing to charge, making it necessary to find the time the batteries can run the load before passing the recovery threshold.
When calculating amp draw some items do not run 100% of the time. Refrigerator compressors only run when temperatures rise above the thermostat setting, we randomly show them at 45% and 40% for two units or a single unit with dual compressors. Lights, fans and vacuflush systems are not likely to be run 100% of the time once engines or generators are stopped for the night if at all. The most common items that run continuously are anchor lights, sonar for depth alarms, and an MFD for anchor alarms.
In this example, running 12v items for 8 hours during the night uses 65.6 amps. To determine how long the house battery bank will run at this load, divide the AH of the battery bank by the total projected amps used. An 800 AH battery bank will run for 12.2 hours before it is 100% drained. Since lead acid batteries need to be charged when they are at 50% capacity, divide the run time by 2 for a total of 6 hours until the battery needs to be charged again.
In some cases, with the right AGM Deep Cycle, they can discharge to 80% or even 100%. In order to do this, the batteries need to be ultra high quality. Along with these ultra high quality batteries, a robust battery management system (BMS) will need to be used to ensure the batteries are properly discharged, equalized and charged. This will significantly increase the cost of a battery system if trying to maintain a budget friendly project.
Using this information makes it easier to customize battery setup to meet the needs of the systems on board a vessel. On Viatori we have three categories of batteries based on the amperage we need, starting batteries, accessory batteries and house batteries. The accessory batteries power the bow thruster and the windlass.
Once the battery banks are planned out they need to be charged. There are two ways that can be set up to charge. A charger specifically for AGM deep cycle batteries for when the vessel is on shore power or generator. AGM batteries have three charging phases, bulk, absorption, and float. AGM chargers supply between 14 to 15 volts. A standard automotive charger can supply up to 17 volts which can overcharge and damage an AGM battery. To charge when not plugged into shore power is done by the generator, or the alternators while the engines are running. Installing a charger connected to the 120 system is fairly straight forward, using the manufactures install instruction to connect to the batteries then plug into a 120 outlet. Using the alternators requires some additional set up as they need to be connected to battery isolators. The isolators allow for a single alternator to charge up to three different battery banks per alternator. This is the easiest way to have the alternators charge the batteries, these can be a little finicky so use a quality proven product.
From this point the battery and charging system starts getting more complicated and exponentially more expensive. Complicated systems are necessary to get the power needed, but it makes keeping the batteries healthy, charged and maintained much easier. To see our vessel details visit our electrical refit page for diagrams and parts list.
Lithium (LiFePO4) Batteries
LiFePO4 stands for Lithium Iron Phosphate, the main components of modern lithium batteries. In place of lead dioxide, lead and sulfuric acid to create a chemical reaction, the LiFePO4 uses lithium salts (lithium carbonate) for the electrolyte, graphite (negative pole) and lithium iron phosphate (positive pole). These batteries operate the same as an AGM with a few exceptions. LiFePO4 batteries usually have a built in BMS, can discharge 100% before recharging, and have a longer lifespan. The main drawback is the cost per battery and that they can be heat sensitive. Precautions when using alternators to charge LiFePO4 needs to be taken as the high current draw can damage both the alternator and the batteries.
Most of these batteries have an internal BMS, but an external BMS, a DC to DC charger to limit current from the alternators and other batteries is required to maintain them. The cost of these batteries along with the additional equipment to manage a LiFePO4 battery bank may not be beneficial for cruising yacht that has a large generator on board. If you want to get rid of the generator, extend the time between charging, then a consultation with a marine electrician specializing in combining LiFePO4 and AGM batteries is worthwhile.
Outside of these considerations, the battery banks are designed the same as an AGM bank. You still need to know your amp draw, you still need to know the system voltage, and you still need to decide how long you want to be on battery power. The main things that changes when calculating the output is the depth of discharge (DoD) of the batteries. The extended life of LiFePO4 batteries over AGM Deep Cycle batteries may offset the initial set up after a few years.