Lithium
Boat Batteries
Lithium boat batteries and fires are relatively common although not as frequent with LiFePO4 batteries. One of the cautionary notes is that Lithium-ion batteries have a flammable electrolyte and are at risk of what is known as thermal runaway. In certain situations, a Lithium-ion battery can suffer very rapid internal heating and once this exothermic reaction is initiated, and it is hard to extinguish or contain. The initial cause of thermal runaway is an internal short circuit. Another potential cause is very fast charging and discharging, which generates heat. Thermal runaway can also be caused by mechanical damage to a lithium-ion battery which is a result of rough handling
Maintaining lithium boat batteries within the nominal temperature range is important and effective ventilation is necessary. As lithium-ion batteries comprise a series of cells, thermal runaway will typically initiate in one cell before cascading to adjacent cells in a domino effect. This is one of the reasons that a Battery Management System (BMS) should be installed at all times. These are covered elsewhere. Many are choosing to install these however do check with your insurance company regarding installation, many marine insurance companies now have downloadable information sheets on the subject, some require installation by qualified professionals, so read the fine print about requirements.
Battery
Management Systems (BMS)
Most lithium boat batteries incorporate an integrated BMS, and some are very comprehensive and sophisticated. The term “management” is the operative word, as these systems also have built-in protection devices. These systems are able to carry a 100A continuous load with a 200A surge capacity for up to 30 seconds BUT you need to know what your particular battery specification is. The systems also have high and low voltage protection along with short-circuit protection, as well as high and low temperature protection and automatic cell balancing. BMS modules may incorporate cell balancing circuits that balance series-connected sections during charging and discharge. Pre-alarms are also used in some units to give 30 seconds notice of impending shutdown. These alarm outputs should be taken to a visual and audible alarm at the helm station, so you have some warning period in which to prepare for power loss. For example, if you were motoring and the alarm came on you could stop the engine immediately and avoid transient spike damage to the alternator. You could also flip the circuit breakers to the electronics to avoid damage. When you purchase a battery make sure they are smart batteries. Whether having integral or discrete BMS make sure you can get transmitted BMS data via Bluetooth to your phone or tablet, so you have real time monitoring of each cell. Get a copy of my book The Marine Electrical and Electronics Bible 4th Edition for all your systems knowledge requirements.
Lithium
Boat Batteries Charging
It is important to understand the variation between charging profiles. Battery charging usually takes place at 30-10% which is denoted as 0.3C to 1.0C of the battery capacity rating. The bulk charge rate of a lead acid cell can take hours whereas a Lithium battery can complete this phase in an hour which equates to a factor of 4 times quicker. Even when a lithium battery is charged at just 0.5C the charge time is still 300% quicker. A lead acid battery in the second absorption phase takes a long time, often a number if hours, where the Lithium battery can take just 15-30 minutes. This fast charge capability effectively does away with long engine run time alternator-based charging. Unlike the lead acid battery which requires float charging the Lithium one does not.
Lithium
Boat Batteries Overcharging
Overcharging is a common failure mode for Lithium boat batteries. The LiFePO4 battery cells should never be overcharged. While charging a lead acid type battery is all about 100% charging to avoid plate sulfation, these much prefer a state of charge in the range of around 50%-80%. You don’t have to charge to 100% at all. If they are experiencing regular charge and discharge cycles to 100% that is okay but the charging must cease when completely charged. This is typically 3.65V per cell or 14.6V in a 12V battery. I have heard of batteries being ruined when they are charged from an alternator during long motoring periods and where the regulator is incorrectly calibrated and the continual overcharge voltage causes great damage. Some people try and utilize a battery charger that was used for lead acid batteries, and there are risks if that charger incorporates a dead battery or de-sulfation mode. The open circuit voltage of a completely charged lead acid battery is nominally 13.1 volts. The nominal voltage level of a Lithium battery is 13.6 volts. Battery damage results when the applied charging voltage is much higher than the battery full charge voltage. It is common knowledge that a lead acid battery charge voltage is kept from exceeding 14.6-14.7 volts corrected for temperature. With a Lithium battery this is nominally 15 volts. A 100 Ah Lithium battery can be charged at 100 amps whereas a 100Ah lead acid can only be safely charged at 30 amps. It is not good practice to charge LiFePO4 batteries to 100% as this stresses the battery internal components that affects chemical and physical characteristics. This cyclic stress reduces overall cyclic life of the battery. This complete charging also results in heat generation which also degrades internal components. For maximum Lithium battery life, charge to around 80% to 90% of capacity. Make sure your charging source control matches the LiFePO4 battery specifications. Overcharging Lithium boat batteries will ruin your investment!
Storage Factors
When going into a period of storage such as the winterization period, Lithium batteries are different to AGM’s and other flooded cell chemistry batteries. As we all know, keeping a standard battery at 100% is the main aim so as to prevent plate sulfation, which is the principal downfall of deep cycle batteries. Unlike their cousins, a Lithium battery is optimally stored at around 50% of charge capacity. It gets technical but positive terminal structure becomes unstable when it suffers from electron depletion for extended time periods. When the cells are at 50% the electrons are equally distributed between positive and negative. Unlike traditional lead acid batteries which suffer from relatively high self-discharge rates, lithium batteries have a significantly lower rate. The lead acid is left on trickle or float charging to maintain the charge at 100%. This means Lithium batteries do not need to remain on chargers and given they don’t like over charging, doing so introduces some risk. A lithium battery should never be stored at 100% charge. Think about this when parked at your marina with a shore powered battery charger operating permanently.
About
Thermal Runaway
Avoid overcharging by only using the correct charging systems, fast charge regulators, and battery chargers that are suitable for use with lithium-ion batteries; many are not. This is one of the reasons that a BMS should be installed at all times. The BMS monitors and shuts off when it detects either high or low voltage limits, high or low temperature limits, and when current charge limits are exceeded, both charging and discharging. The BMS should monitor these parameters at cell level. Only install batteries that are from manufacturers with integral cell-level BMS monitoring systems.
Battery
Management System Limitations
The BMS is NOT a substitute for circuit protection such as fuses and circuit breakers as this is a common misunderstanding. What happens when a BMS automatically disconnects connected loads and charging sources. For the same reason you never open a two-battery changeover switch when charging. A transient voltage or spike is generated or what is sometimes called a load dump. When the BMS detects overcharge or over temperature condition this can occur without warning. The result can be damage to an alternator, solar or wind charging regulators, or electronics and result in loss of power and instrumentation. Consider some options which are not dissimilar to lightning protection involve surge and transient suppression devices.