The Li chemistry chosen for this bank is called lithium ferrous phosphate, LiFePO4 or LFP for short. There are a few variations on this chemistry such as LiFeMnPO4 & LiFeYPO4 but the end result is still an LFP bank and has the same inherently safe Li characteristics.
For this build I chose four 400Ah Winston LFP prismatic cells, and the bank is set up in a 4S 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 - 13.4V.
If you want to do other than a 4S configuration the cells are ideally connected in parallel first then in series. This is done so that you only need to monitor 4 cell voltages. The parallel cells stay balanced and the BMS only has to monitor 4 cells for a 12V nominal system. if you choose series first you will require a lot more monitoring. The nomenclature system you will most often see for 12V house banks is as follows.
4S = Four Series
2P4S = Two Parallel / Four Series
3P4S = Three Parallel / Four Series
4P4S = Four Parallel / Four Series
There are many ways to configure LiFePO4 Cells in series or in parallel/series. I tend to prefer the simplicity of a 4S configuration when that size bank/cells works. It requires less overall connections and less work when doing cell balancing etc.... Some argue that if a cell is ruined with a 2P4S bank you could re-wire it and use the remaining cells.. I much prefer the simplicity of a redundant lead acid reserve bank. Rewiring an LFP bank at sea is not a task you want to undertake...
As prismatic cells get larger the concern over vibration tends to grow. The most common reported failures for "unknown odd reasons" has by far been in 700Ah and larger cells. At least one manufacturer, Sinopoly, suggests not using cells larger than 200Ah on a boat. Their competitors don't claim this and still hold that all their cells pass the same testing.
What I do know is that none of the commercially available prismatic cells from the likes of Winston, Voltronix, CALB, Sinopoly, Hi-Power, GBS etc. are tested to any specific industry standardized or Mil-Spec vibration testing. All vibration testing is done in-house to their own in-house standards.
Getting a straight answer on vibration testing, from any of the Chinese manufacturers, is like walking up hill on ice with leather sole shoes. I have tried and their info is non-existent on this subject.
There are three basic options of getting LFP on your boat, with DIY being the least expensive, and most technical. The categories are:
This is a real cost saver but is NOT for the faint of heart or the limited skill DIY'er.. It is not recommended for the average boater. In the DIY build you source the cells, all the components, choose the BMS, choose the high voltage cut and low voltage cut relays main contactors, wire and assemble everything, balance the pack and chose chargers, solar or alternator regulators to suit LFP charging. It is a very time consuming project.
Marine Specific LFP Systems:
Lithionics/Bruce Schwab, Mastervolt & Victron build LFP systems for marine specific applications. These systems are well engineered, well executed yet also expensive. If you want LFP and don't have ththe systems Bruce sells, and the Li3 Lithionics system is quite beautiful.
I really even hate to mention this type of Li battery because it has so many issues, when installed on a boat, that I find it tough to swallow making mention of them. Some of these systems lack a BMS altogether and others only have a single relay that disconnects the bank on low or high voltage. If it seems to good to be true, it probably is! Caveat emptor on Li Drop-In's. It a Li battery does not have any sort of external communications capability, it is a "drop in".
I URGE STRONG CAUTION ON ANY "DROP-IN" LFP BATTERY!!!!
When in doubt go with the Ocean Planet Energy Li3 Lithium, Mastervolt or Victron. All of these companies have well thought out marine specific systems.
EDIT 10-27-2015 - Recently Genasun decided to leave the LFP marine market, when good inventory runs out. This was reportedly due issues in the supply of the CALB LFP cells not meeting Genasun specifications for consistency. According to Bruce Schwab,the Genasun factory representative, the Coulombic efficiencies of the cells, even with measured identical capacities, were not matching well enough to meet Genasun's very high standards.. This can make for a balancing nightmare.
Bruce Schwab the owner of Ocean Planet Energy worked closely with Lithionics to develop a new LFP system, suitable for cruising boats, and to fill the hole Genasun left, and it is now finally shipping. For more info on the Ocean Planet Energy Li3 Lithium Batteries see Bruce's web site.
The Cells: The cells were very well packed and got here in great shape.