The Li-Ion chemistry chosen for this bank is called Lithium (Li) Ferro/Iron (Fe) Phosphate (PO4), LiFePO4 or LFP for short. There are a few variations on this chemistry such as LiFeMnPO4 & LiFeYPO4 but the end result is still essentially an LFP bank that has the same inherently safe characteristics. For this build I chose four 400Ah Winston LFP prismatic cells, and the bank is set up in a 4S or 4 series cells configuration.
What the heck is 4S?
4S just means four 3.2V cells in series to make a 12V nominal pack. The pack/bank is really closer to a 13.3V pack as the resting & nominally loaded cruising voltage of these cells is around 13.2V – 13.35V.
If you want to do other than a 4S configuration the cells are typically connected in parallel first and then in series. Parallel first is done so that you only need to monitor 4 cell voltages for a 12V nominal pack. The parallel cells stay naturally balanced which means a less expensive BMS can often be used. If you were to choose series-first you would require more cell level monitoring. The nomenclature system you will most often see for a 12V nominal house bank is as follows;
4S = Four Series Cells
2P4S = Two Parallel Cells / Four Series Cells
3P4S = Three Parallel Cells/ Four Series Cells
4P4S = Four Parallel Cells/ Four Series Cells
etc. etc..
There are many ways to configure LiFePO4 Cells in series or in parallel/series. When possible, I tend to prefer the simplicity of a 4S configuration, if that cell size works for the desired Ah capacity. 4S requires less overall connections and less work when doing cell balancing. Some would argue that if a cell is ruined with a 2P4S bank you could re-wire it and use the remaining cells. At sea? While anything is possible, I personally prefer the simplicity of a reserve or start/reserve lead acid bank as opposed to a complete LFP re-configuration at sea. A lead acid reserve bank or start/reserve allows you to take the LFP bank off-line while addressing any potential issue. Rewiring & re-balancing an LFP bank at sea is not a task you’d ideally want to perform.
LiFePo4 Battery Bank Type Definitions:
Factory Integrated Lithium-Ion Battery – A lithium battery designed to work as a factory integrated system including the charge sources
System Integrated Lithium-Ion Battery – A lithium battery system with the capability of system interaction/communication with external charge sources, vessel loads, alarms or safety systems.
Drop-In Lithium-Ion Battery – A self-contained lithium battery, with or without an integral BMS and contactor/s, which lacks external system communication with charge sources, vessel loads, alarms or safety systems.
Examples:
Factory Integrated Lithium-Ion Systems – Victron & Mastervolt are about as close as it gets because they sell both the charge sources and the LFP batteries as a factory integrated system.
System Integrated Lithium-Ion System – Lithionics ex: Genasun & well executed DIY – Designed to work with third party products and can communicate with them.
Drop-In Lithium-Ion – If the internal BMS is sealed, and the battery can’t tell an external charge source when to stop charging or a load to stop discharging, then this is considered a drop-in battery. Whether or not a drop-in batteries are well suited for your use will depend upon many factors.
There are three basic options of getting LFP on your boat, with DIY being the least expensive, and most technical. The categories are:
DIY Builds:
This is a real cost saver but is not for the faint of heart or the limited skill DIY’er. In a DIY build you source the cells, confirm the cells are well matched for Ah capacity, choose all the components, choose the BMS, design the system schematic, choose the high voltage cut and low voltage cut relays, main contactor, wire and assemble everything, balance the pack and chose chargers, solar or alternator regulators that can be programmed to suit LFP. A well executed DIY build, and there are many of them out there, is a a time consuming project. If you have the gumption to forge ahead, it is method that can save over 50% of the cost of a factory made bank.
Marine Specific Factory Made LFP Systems:
Lithionics/OPE-Li3, Victron & Mastervolt all build LiFePO4 batteries for marine specific applications. These systems are well engineered, well executed yet also at or near the top end of the pricing spectrum. You do however tend to “get what you pay for“. If you want LFP and don’t have the ability to DIY, of these three companies I can certainly recommend the Lithioncs/OPE Li3 system. The Lithionics/OPE Li3 battery system at the top of the list for us and checks every box for a marine specific application. I personally have the most experience with Lithionics, Genasun (now defunct) and Mastervolt marine LFP systems.
