A Second Look at Lithium Marine Batteries
Advances in LiFePO4 technology have brought safety and practicality to the lithium battery sector for marine applications
Until recently, my most memorable experience with lithium batteries was when the battery in my MS Surface laptop computer decided to head into a runaway charging loop, become super-heated, and begin steadily and rapidly increasing its physical volume so much that it was bursting free of the computer’s case. In the instant, as fast as I could, I disconnected the charger and threw the computer into my backyard, well away from my house. The battery did not burst into flames, but the experience shook me, and I never again left a laptop with a lithium battery to charge unattended.
Scary? You bet. But the fact is the computer battery in question employed lithium-ion technology, in contrast to the current marine lithium batteries which are almost exclusively lithium iron-phosphate based, a technology that has proven to be much safer and robust. So, understanding this, I thought it might be useful to review how LiFePO4 stacks up against lead-acid batteries.
In general, at their current state of development, LiFePO4 batteries are superior in several ways for marine applications than either lithium-ion or lead-acid batteries. Lithium iron phosphate batteries are far less prone to combustion and thermal runaway than lithium-ion batteries, making them safer, by a wide margin, for marine applications. And the much longer life cycle of LiFePO4 batteries means they will last outlast lithium-ion by a factor of at least three to four and lead-acid by an average factor of eight.
There are some (minor) differences between flooded lead-acid batteries and absorbed glass mat (AGM) batteries, with some AGMs being somewhat more power-dense than “standard” flooded lead-acid batteries. However, for our purposes here, we can treat all lead-acid batteries as being in the same technology group. Which means we can summarize the advantages of LiFePO4 batteries vs lead-acid as follows:
LiFePO4 batteries can store about twice as much energy as lead-acid batteries, per unit physical volume overall. In other words, if a group 24 lead-acid battery typically stores 65 amp-hours of energy, a LiFePO4 battery of the same physical dimensions can be constructed to store 130 amp-hours of energy.
However, impressive as that might be, it’s not the entire story. The voltage in a battery drops as it discharges its stored energy. And when a lead-acid battery is more than 50% discharged, its voltage will drop below a usable level, that is, at 50% discharge, a 12-volt lead-acid battery will no longer be able to supply current at more than 11.9 volts. Consequently, it’s common practice to size lead-acid battery banks at twice the anticipated maximum energy draw over the time between charging. So, if you figure you’ll need 50 amp-hours of power between charges, you need a 100 amp-hour battery.
In contrast, an LiFePO4 battery can be discharged as much as 85% of total capacity before its voltage drops to below usable values. Which means that, effectively, a 100 amp-hr LiFePO4 battery will deliver as much usable power over time as a 170 amp-hr flooded lead-acid battery. Which means that either a) you can use a lower-rated LiFePO4 (for the same delivery of power as the lead-acid unit) or b) you can have 70% more usable power available in a LiFePO4 battery as in a lead-acid battery rated for the same number of amp-hours.
Lastly, a LiFePO4 battery will weigh only about 50% that of a flooded lead-acid battery of the same amp-hour rating. Yes, that’s right, 50%. Whereas a group 24 12vdc flooded lead-acid battery weighs about 50 to 55 lbs, a LiFePO4 battery of equal amp-hr rating weighs about 27 lbs, plus or minus a pound or two.
The importance of this weight differential in favor of LiFePO4 is inversely proportional to a yacht’s size. The smaller the boat, the greater the importance of the weight saved.
For example, I am in the midst of refurbishing and upgrading a 27-foot twin-outboard cuddy-cabin walkaround I acquired last year. The boat is fitted with two 12v AGM starting batteries, two more dual-purpose AGMs for ship’s service, and a fifth battery that was dedicated to the small A/C unit for the cabin. That’s a total battery weight of some 300 lbs placed right aft, just forward of the outboard well. No wonder the boat at rest is trimmed down by her stern by what appears to be 2 to 3 inches (judged by the location of her static load waterline in relation to her stern cockpit scuppers).
I am working to reduce the stern static trim by a couple of inches to bring her actual load waterline back to where her designed waterline was originally marked out. And my intention is to do this by replacing her current battery banks with a single 130 amp-hr LiFePO4 dual purpose (1,000 CCA) starting battery and two 100 amp-hr LiFePO4 deep cycle house batteries. The reasoning behind the reduction in rated capacity is the subject for another discussion, but the change will result in a total battery weight reduction of more than 200 lbs. Which will bring the stern up the necessary amount.
To be sure, until very recently, I’ve personally been in concert with the advice that Chuck Husick, sailor, engineer and one-time president of Chris-Craft used to give on the topic of boat batteries:
Buy two or three hefty warrantied lead-acid batteries from a major supplier, then swap them out for new ones every other season or so, under the pro rata exchange. For that will yield the highest unit of power delivery per dollar outlay you will find, notwithstanding the existence of more sophisticated battery technology…
These days, however, with the major lead-acid battery suppliers limiting their warranties to one or one and a half years for marine applications, and with the significant advances in lithium battery technology in the form of LiFePO4, not to mention the extension of warranties to ten or more years by lithium battery manufacturers, it seems the old wisdom is just that — old and outdated. And even I have to admit that.
— Phil Friedman
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