This is a common question right now as the idea of switching from lead acid batteries to lithium batteries for starting outboard motors in boats becomes more popular. The cost of LiFePO4 batteries has dropped to the point that the average consumer can at least consider it as an option. And the performance and longevity benefits are well known and very attractive.
So how do you know if a lithium cranking battery will work for your boat? Unfortunately, a definitive answer is still hard to get. But here are four things to investigate and consider that will lead you to an answer:
1. Check with the boat/motor manufacturer - Some manufacturers, such as Yamaha, do not support lithium starting batteries at all and will tell you that using them will cause your boat to throw error codes and possibly shut down. This is because the electronics in the boat are not programmed to deal with the higher voltage of a lithium battery. The charging system is also usually not programmed to provide a CC/CV charging profile compatible with lithium batteries. So the first step is to check with your manufacturer for any official stance on support for lithium batteries.
2. Find specifications for the charging system - If you cannot get an official stance from the manufacturer, the next step is to look at the charging system in the boat. Find out if the charging system has programmable settings or a lithium mode to handle lithium starting batteries. If not, that doesn't mean it can't work but you will have to make some modifications. Usually, a regulator installed between the alternator and the battery will work to make peace between the two. Sometimes a higher output alternator may be necessary. Other times, with older boats, replacing the charger with a newer model that is compatible with lithium will be the answer.
3. Find someone who has a similar setup - If you aren't comfortable with making modifications to your boat, the next step would be to look for someone else who has already tried using a lithium battery with the same boat/motor that you have. Use internet forums and Facebook groups to ask for anyone with experience with your specific combo. You can also reach out to boat mechanics, marinas, and other boating industry professionals to see if anyone has hands on knowledge.
4. Try it out yourself - The only surefire way to know if it will work is to strap on a lithium battery and see what happens. LiFePO4 batteries are very stable and will not catch fire or explode. There is a slight risk of damage to the alternator but most of the time the electronics in the boat will protect components from damage and will warn you with error codes or shutdown systems completely. But if you have an older boat without a lot of electronics and protections, keep an eye on the temperature of the alternator and monitor the electrical output of the charging system, alternator, and battery with a power meter during testing.
10 years from now this will all be a distant memory and new boats will be sold from the factory with lithium starting batteries. But for now, it takes some research, perseverance, and ingenuity to enjoy the benefits of a lithium cranking battery in your boat.