LFP batteries have many advantages over other chemistries, these advantages can be read on the battery conversion page. But to quickly sum them up: LFP batteries are much lighter, smaller, last longer, are safer, can be completely discharged, charge much faster and house much more energy than other non-lithium chemistries.
A big difference with other battery chemistries is that LFP batteries (and any lithium battery in general) don’t enjoy being charged to 100% and kept there all the time, instead they like to be kept around 80% for storage. Not charging and storing these batteries to their full charge will give them an even higher lifetime. Therefor, good battery management is necessary.
Fortunately, this responsibility isn’t all for the end user to worry about. Instead our *Battery Management Systems can take care of this by disabling the charger when the batteries reach a certain state of charge. If preferred it is always possible to charge the batteries to 100% simply by saying so on the control panel, this might be useful for when a long motoring haul is necessary.
Good LFP batteries have a Cycle life of at least 3500 cycles at 100% Depth of Discharge (DoD). Cycle life means the number of charges and discharges (cycles) a battery can make before losing performance. So it doesn't mean that after 3500 cycles the batteries are dead. This lifetime is much longer when 60-80% DoD is maintained.
*The BMS is a device which can talk with the batteries and get all their information, like state of charge, temperatures, cell health, current flow and voltage levels.
Every once in a while, depending on the battery usage, it’s necessary to balance LFP batteries. Batteries consist of cells, a single 12,8V battery exists of 4x 3,2V cells. These cells are connected in series to add the voltage up to the total of 12,8V.
Because the voltage curve of a LFP battery is almost completely flat, with the exception of the last few and top few percentages, it is very difficult to know for the BMS what state of charge the batteries are. The way this is measured is by counting the amount of current has flown through the cells in Ampere hours (Ah). Because these measurements have a slight error, after a while some cells might report a different state of charge than other cells, this is called battery imbalance. To solve this, the battery must be charged to 100%. When the battery reaches 100% the cell voltage will all of a sudden rise to 3,6V, with an internal resistor, the extra energy the charger is putting into this cell is dissipated so that it cannot be overcharged. Once all cells reach 3,6V the BMS knows they’re all equal again and it will reset the Ah timer and the imbalance is restored.
In a higher voltage system using LFP batteries, be it 24, 48, 96, 350, and so on… Volt, imbalance occurs between the batteries that are connected in series, just like what happens with the internal cells. This is solved the same way as described above, by charging all batteries to 100% and letting them balance for a while.
This all sounds very complicated, but this is all managed by the system and the end user won’t have to worry about anything. Once in a while the BMS will tell the charger to charge all batteries to 100% and balance them. In case of our electrical and hybrid systems the Vessel Control Unit (VCU, the brains of the system) will also automatically discharge the batteries to 80% by discharging the high voltage battery bank to the lead acid household bank. Or even better, by delivering power back to the AC grid.
This explanation is an example of how we do it in our electric system, other batteries and Battery Management Systems might do things differently.
