Why Are Lithium Iron-Phosphate Batteries So Great?

There has been a lot of attention around electric cars lately and the majority of the high end manufacturers are using lithium iron-phosphate (LiFePO4 or LFP) batteries.  Some boat manufacturers are choosing lithium but it is not mainstream.  Today we are going to look at why it may be time to consider lithium.

Lithium is a highly reactive alkali metal element first discovered in 1817.  The concept of a lithium-ion battery was initially introduced in the 1970's and became far more popular in the 1990's.  Lithium technology became a favourite for powering small electronics, like laptops, but soon earned a bad reputation. Those batteries used a lithium-cobalt-oxide chemistry which is prone to thermal runaway if the battery is overcharged. This was the main reason lithium was not used to create large battery banks.  In the mid 1990's a new formula emerged using lithium-iron-phosphate which has a lower energy density and is non-combustible.

Lithium iron-phosphate (LFP) battery technology is inherently safer than all other lithium battery chemistries.  The included battery monitoring system removes the source of a fault and the battery stabilizes to a safe condition immediately.  These batteries are controlled internally by electronic circuitry that keeps the cells in balance under all conditions. In comparison with typical lead acid batteries, LFP batteries are up to 70% lighter for equivalent usable battery capacity, charge faster, have a higher cycle life and an incredibly accurate state of charge indicator.  Let's take a closer look.

Depth of Discharge. LFP batteries have often been dismissed because of the high cost in comparison to AGM batteries.  This argument was based on amp-hours but if you look at usable battery capacity, i.e. range between acceptable depth of discharge and end of bulk charging, a standard AGM battery has 50% of usable capacity, a Firefly Oasis AGM battery has 60% of usable capacity, and a LFP has 80% of usable capacity.

Let’s take a look at the usable energy and cost.  We will take two Victron* batteries and compare the usable energy for a mid-size boat:

               1 X Victron Lithium = 24 VDC @ 180 Ah –This 26.4 V battery consists of 8 3.3 VDC cells connected in series for a 180 Ah rating.  The available energy is 26.4 X 180 = 4.75kWh.  The usable energy is 26.4 X 180 X 0.80 = 3.8 kWh.

               2 X Victron AGM 12V = 24 VDC @ 220 Ah - The nominal voltage of the lead-acid cell is 2.0 V/cell. Each 12 V battery consists of 6 cells connected in series with a 220 Ah rating. Connecting 2 x 12 V 220 Ah batteries in series to give 24V and 220 Ah, the available energy is 24.0 x 220 = 5.28 kWh. Usable energy is 24 x 220 x 0.50 = 2.64 kWh.

The cost for a 24 VDC 180 amp-hour Victron lithium battery is $7,400 or $51 per usable amp-hour and the cost for a 24 VDC 220 amp-hour Victron AGM battery bank (2 X 12V in series) is $1,400 or $13 per usable amp-hour. Therefore, the cost of lithium is 4 times more expensive.

Charging. A lead acid battery has three stages of charging, bulk, absorption and float.  A LFP has only essentially two, stages bulk and float. Lead acid batteries can take about 25% to 40% of capacity as a charge rate, LFP can take up to 300% of capacity. Basically, this means that the limiting factor for charging a LFP battery is the charge source and not the battery itself. For instance, a 200 amp-hour flooded lead acid battery could only take a theoretical maximum charge rate of 50 amps (25% of 200 amp-hours), an AGM battery a theoretical maximum charge rate of 80 amps (40% of 200 amp-hours). But a LFP battery could take 600 amps (300% of 200 amp-hours).

When the alternator or charger sends energy to a LFP battery, it generally charges as fast as the alternator or charger can generate output. And even if you don't manage to top off the battery to 100%, unlike a traditional lead acid battery, the LFP battery will not be damaged. LFP batteries are more efficient than traditional lead acid batteries allowing for shorter, faster charging times.  This means less generator run time.

Weight. Another key consideration is weight.  We are seeing more and more boaters adding larger battery banks to keep up with the increased loads from refrigeration, navigation equipment, entertainment systems, inverters, etc.  A 48Vdc/800Ah AGM battery weighs 1,360 kg whereas a 48Vdc/400Ah LFP battery weighs 336 kg, that is 4 times lighter. Mastervolt recently introduced an entry-level 12V LFP battery at just 30% of the weight and volume of its lead acid counterpart.

Cycles. At 80% depth of discharge, a LFP battery has 2,000 to 5,000 cycles, whereas a flooded lead acid has less then 10% of that.

LFP batteries have a higher voltage output and maintain mostly a consistent delivery of voltage power throughout discharge.  Lead acid voltage drops consistently throughout the discharge cycle. This means that sensitive electronics or electrical equipment get the benefit of steady voltage as the battery is drained allowing the equipment to run smoothly.

 Summary. LFP batteries with a top quality BMS require very little maintenance and do not need to be stored upright or in a vented compartment. LFP battery use a much cleaner technology and are safer for the environment.  They deliver higher quality performance in a safer, longer-lasting package.  Most people believe that a LFP battery will often outlive the boat.

*Calculations supplied by John Rushworth and Victron Energy Corp.

About the author: Jeff Cote is the owner of Pacific Yacht Systems, a full service shop delivering marine electrical and navigation solutions for recreational boats. Visit their website and blog for info and articles on marine electrical systems, projects and more: www.pysystems.ca.