We installed our new Lithium Iron Phosphate (LiFePO4 or LFP) batteries in the spring of 2015. It's now late 2015, over six years later. What do we think of the upgrade?
At this point we're getting close to financial break-even over buying replacement AGM batteries. The LFP system cost a bit less than $5000 while a set of three Lifeline AGMs would have been $1800. If we had stayed with AGMs, we would have done the first replacement in March 2015. My experience is that lead-acid cells last 4-5 years. Let's say that we took good care of them and they lasted 5 years, so we would have done a second replacement in March 2020. That's $3600 so far.
Cost is only one of the factors. Weight is another, but I can't say that we've detected any difference in Lux's performance under way, so scratch that one. The big factor is the amount of current we can pull from the house bank. We regularly run the microwave and the airconditioner off battery power. Both consume a lot of power - 90A and 120A from the 12v bank, respectively.
The depth of discharge is the other major factor, which means that we can run the airconditioner for several hours from the batteries. This summer was particularly hot and humid, with the heat index around 100 F in the middle of the day for a week or more at a time. On one week-long cruise, we ran the aircon for the entire trip. There was little wind, so we did have the output of one engine's alternator to supplement the batteries. We were with a group cruise that went marina-to-marina, so we had shore power for nights and to recharge the batteries.
Yes, we also have a NextGen 3.5KW generator that can charge the batteries and run the aircon. But for the most part, it wasn't necessary to use it for the above week-long trip.
What about the LFP bank's current capacity? The bank has a total capacity of 540Ah, so 80% consumption would be 432Ah. Well, we experimented with that by consuming 400Ah at one point during the summer. The system voltage had dropped to 11.8V, still well above the low voltage threshold. This capacity means that we can run the aircon for three hours at 100% duty cycle on batteries alone.
Success! We would have never been able to handle the aircon without the genset on AGM batteries.
Would we do it again? Yes! In fact, I've learned of some new cells that are about the same physical dimensions and have 50% more capacity, so we could now build a bank with a capacity of 840Ah to fit in the same space. If I could have a do-over, I would use these cells and install high-output alternators on each engine with WakeSpeed controllers and not install a genset. However, as I noted in my Lithium Battery Engineering article, if you're not comfortable building your own system, go with a pre-designed system from one of the established vendors (BattleBorn, Genasun, Mastervolt, Relion, Victron).
