The Official Magazine of the Bluewater Cruising Association

SWIFT Gets New Lithium Batteries

Melanie and Morgan Finley

Moody 44
October 3rd, 2022

As we sail into our third year of offshore cruising, we find ourselves reflecting on the places we’ve been, the experiences we’ve had, and the people we’ve met. It gives us perspective on where we want to go next. We also reflect on the boat, how she’s standing up to the rigors of offshore sailing and the rigors of providing a full-time home for a family of four. With safety in mind, upgrades, and just plain maintenance, looking after the boat is one of the more time consuming and costly aspects of this lifestyle. Our latest upgrade: changing our house bank batteries to lithium.


We were 600 miles from Panama, 80 miles southeast of Kingston, Jamaica, when we discovered that our fancy 1.5-year-old AGM (absorbed glass mat) house batteries had completely failed. The battery cases were warped and bulging, and the bank voltage was jumping erratically between 12.0 and 12.2 volts. After two hours of chopping towards Jamaica, we had isolated the best one of the three batteries and disconnected the others, but decided to turn back towards Panama. Four days later, we arrived in Bocas del Toro, Panama, running on a spare battery strapped to the bed frame and with nearly empty fuel tanks from repetitious engine charging to supplement our solar system.

The failure of our house batteries was looking like a major hit to our cruising budget, and to understate things, a major frustration especially given that the batteries were relatively new. However, we first put the disappointment behind us to celebrate landfall after a difficult passage, then started to wonder if this was an opportunity to try lithium batteries. Aside from two unexpected months locked down in a UK marina, we have been traveling for the last two years on our Moody 44 monohull sloop Swift, and we are always looking at potential safety and lifestyle upgrades. Lithium batteries as an energy storage solution is a hot topic amongst cruising boats.  We had read the glossy brochures, but we didn’t have any personal experience or many firsthand accounts. We wondered if these batteries really warranted the hype. Faster charging, deeper and more frequent discharge potential, better voltage consistency regardless of state of charge, and lighter weight – these were some of the of the potential benefits of a lithium system and they all sounded like wins to us.

We were quite proud of the energy system we had set up in Greece six months into our trip, with a goal of being wholly reliant on the sun for our energy needs. At that time, we increased our solar array from 350 watts to 700 watts of mono-crystalline panels installed on a custom-built arch at the back of the boat. We also swapped out our 300Ah of generic sealed lead acid batteries for those 510Ah of expensive AGMs that failed en route to Panama. Regarding our energy needs on Swift, we have a fridge, a separate freezer, an inverter, a DC water-maker, fans, lights, and many other devices running on 12 volts (we have our two girls with us, so at times we also require extensive device charging to keep everyone happy).

While it is a common exercise to calculate an energy budget for your boat, we tend to measure our energy requirements as the deficit between solar charging cycles. We have a Victron BMV-712 Battery Monitor to monitor both state of charge and our real-time consumption versus charge rate. Our average nightly deficit at anchor, prior to installing lithium, was 85 to 100Ah. For passages with the addition of the autopilot and navigation, it ranged from 125 to 150Ah. With the 700 watts of solar, we could often reach an indicated 100% state of charge on our AGM batteries while at anchor, but not on passage. We were very keen to see how the lithium would compare.

The new lithium batteries finally arrive in Bocas Town, Panama


We reached out to several lithium battery suppliers, and we had a very positive response from Dragonfly Energy, the supplier for Battle Born batteries based in Nevada. They assemble their Lithium Iron Phosphate (LiFePO4) batteries in-house, and each comes complete with an internal battery management system (BMS). We ordered four 100Ah BB10012/H batteries to replace our defunct 3 x 170Ah AGMs. Dragonfly shipped everything promptly to a freight forwarder in Florida. Unfortunately, things slowed down after that, but eventually our batteries and supplies arrived in Bocas Town and it was finally time to get started with the install!

The batteries arrived as “drop-in” replacements, suitable for most house battery configurations. We took the opportunity to redo our battery box and reorganized all our house battery connections. We invested in new battery cables, added appropriate fuses where necessary, and installed a second shut-off switch right at the house bank. Our Victron solar MPPT charger needed reconfiguring to best support the new batteries, but this was done easily through the Bluetooth interface. The biggest change to our system was that we had to install a DC-to-DC charger to protect the engine alternator on our single alternator system – more on this later. We don’t have AC charging on our boat, so no changes or reconfiguring were required for the AC system.

Hot work in Panama prepping for the new batteries

Battery Review

Less Weight

An immediately noticeable difference between the new lithium and the old AGMs was the weight. Our four new batteries came in at just over 100 pounds total. The dead, fused-together AGM batteries were an ugly block weighing over 300 pounds, presenting us with a very difficult and potentially dangerous challenge for removal. Fortunately, we were able to hoist them out the back hatch and onto the dock using the boom – good riddance!

Improved Battery Life Span

Lead acid batteries do not live long when subjected to regular or prolonged deep discharges (e.g., greater than 50%). They also require frequent, ideally daily, recharging to 100%. Our AGM supplier provided an estimated battery life expectancy of only 300 cycles for 100% discharge, or 1000 cycles if drawn down by 40%. Given the not-insubstantial cost, we were very concerned about damaging the AGMs and reducing their life expectancy, so we vowed never to bring them below 50%. In comparison, Dragonfly provides an amazing estimated 3000 to 5000 cycles for 100% discharge! So, although we have reduced our overall Ah rating from 510 (AGM) to 400 (lithium), we have increased our theoretical usable Ah by switching battery technologies.

Reduced Battery Anxiety

Assuming 50% usability for the AGMs, and a conservative 80% for the lithium, the available Ah increased from 255Ah to 320Ah. We have never used 100% of our lithium bank, but we have used 75% and been stuck at more than 50% used for several consecutive days. Although we have only had the lithium batteries for less than 6 months, so far there has been no noticeable change in performance. In comparison, we were so careful with those AGMs, and the deepest discharge ever was only down by 36% (still more than 50% full). We also rarely ended the day at less than 100%, but still they failed after only 1.5 years. It sure sounds like another win for the lithium. With our lead acid battery system, we suffered from a constant nagging battery anxiety. It’s sad but true. Time will tell, but there is certainly less battery anxiety with the new batteries!

More Consistent Voltage

Another improvement has been the voltage consistency of the lithium batteries. When I say voltage consistency, I’m referring to the voltage available at the battery, as related to the state of charge. For the AGM batteries, at 100% state of charge the batteries were around 12.9 volts, but by 80% they were 12.5 volts and by 50% only 12.1 volts. For our lithium batteries, if the state of charge is greater than 50%, we see around 13.2 volts and for 25% to 50% state of charge, it is still over 13.0 volts. There is also less voltage drop when a draw is applied to the system. Most of our electronics, particularly the fridge, freezer and watermaker, love this consistency and work better at the higher available voltage.

Better Charge Efficiency

Decreased overall weight, improved voltage consistency, increased available Ah, improved battery longevity and reduced battery anxiety are all wins for lithium. But, in my opinion, what makes lithium such a game changer for cruising boats is the improved charge performance. LiFePO4 batteries have low internal resistance in comparison to lead acid batteries and this means that lithium will charge faster. Lithium also has a better charge profile, which is linear up to approximately the last 1% in comparison to the curved charge profile for lead acid, where charging slows noticeably as they fill. Good solar and fast charging lithium is an amazing combination. Reprograming our solar charger for lithium increased our solar charge efficiency by 30%.

New LiFePO4 batteries ready to go

Our best charge rate from our solar for the AGMs was around 35A, but for the lithium we regularly see 50A. Living our self-sufficient and often remote lifestyle, we can’t stress enough that the faster you can put Ah back into your battery bank the better. Whether from solar or some other charge source, we have seen a vast improvement in charge efficiency and charge rate after switching to lithium.

Risk of Overheating Alternator

We made one mistake when switching our system over to lithium. We have a single 115A alternator on Swift. It is our only other means of battery charging, but the low internal resistance of LiFePO4 can cause alternators like ours to overheat. There are three ways of approaching this problem: hope for the best; install an external temperature sensing regulator; or, install a DC-to-DC charger from the lead acid starter battery to the lithium house bank. We chose the latter option and installed a Victron Orion DC-to-DC charger. The Orion is programmable for charging lithium, but only to a maximum charge rate of 30A. When solar is insufficient for charging our batteries, this can mean long engine run times, which we really need to avoid. We suspect that a fast-charging external regulator would have been a better solution, but have not tried one. For now, we have installed a bypass switch to charge the lithium bank directly from our alternator, and in by-pass mode we have seen a big improvement in charge efficiency. However, we have also seen high alternator temperatures, so we only use this option for short duration charging.


Some final words on switching to a lithium house bank. It is important to understand that lithium batteries are only the energy storage component of a larger electrical system. Before making the switch, review your electrical system, assess your energy needs, and consider your charging options. Be prepared to incur costs beyond the batteries themselves, e.g., possibly new battery leads, a smart alternator regulator, or a new AC charger. Solar and lithium make an amazing combination, but for those days when the sun isn’t shining, you will need an alternative charge source that is lithium compatible and makes the most of the lithium’s rapid charge potential. The initial cost of the batteries will also be higher than lead acid batteries. However, if you factor in the differences in battery lifespan and consider the potential reduction in engine or AC charging due to the fast-charging characteristics of lithium, a lithium bank should be cheaper in the long run.


While working on some of this article, we were drifting along under spinnaker between the Marquesas and the Tuamotus. The skies were clear, the batteries were full, and I was feeling no battery anxiety, although I was feeling some kind of awkward obligation to eat bananas, as the entire stalk rapidly ripened behind me. I reflected on having done this same route with my family when I was growing up. Back then we didn’t have roller furling, electronic charts, AIS, or even a GPS. We now take those “upgrades” for granted, and while a boat could do without, we wouldn’t want to. Is the comparison between the newer lithium battery technology and the traditional lead acid analogous with those other innovative changes to cruising? Perhaps. Regardless, the switch to lithium is, as far as we are concerned, a definite upgrade and should be considered by any boater looking to improve their cruising experience.

If you have questions or want to know more about our system on Swift, feel free to contact us through email or message us on Instagram @sailing.swift. We’ll do our best to respond!

700 watts of solar charging our new lithium batteries


  1. Devin J. says:

    Great article – thanks for the info. Due to change out my battery bank as well and this info will be a big help. Cheers!

    1. Anonymous says:

      Thanks Devin and good luck with the battery change!

  2. Rod+Morris says:

    Perfect timing….thank you. I am just going through the process of changing to lithium as my aging AGMs are done. It raised several questions, mainly about the alternator charging rate. Your article was really helpful.
    Cheers, Rod

  3. Anonymous says:

    Good article. We are also looking at making the jump to lithium.

  4. Siggi Kemmler says:

    Thanks for communicating your research and findings. Best info from a family who has experienced both types of energy storage. Hope to cross paths on a future voyage! Stay safe!

Leave A Comment

Your email address will not be published. Required fields are marked *