Replacing AGM Batteries with Lithium Ion on a Boat


Last Updated on May 15, 2021 by Amy

After three and a half years of cruising, our AGM batteries weren’t holding a charge like they used to.  We did our research and decided to upgrade our house bank to Lithium-ion (LiFePO4) batteries.

Our AGM Battery Set-Up

Battery Setup from the Factory

Here’s what our Helia 44 came with from the Fountaine Pajot factory:

  • 600 Ah of AGM (lead-acid) batteries located in the port engine room.
  • Victron Multiplus compact Inverter-charger running at 2000W/80A located in the starboard engine room.
  • Stock alternators for Volvo D2-55 engines putting out a constant 14.4V and equipped with “dumb” regulators.
  • An Onan 12kVa Onan generator.

Our Outfitting Additions

We added 400 Ah of AGM batteries, bringing our total house bank up to 1000 Ah.  In addition, we installed dedicated start batteries for the generator and both engines (so three start batteries total).  Read more about our battery upgrades in Palmetto.

At the Maimi Boat Show, we purchased and installed 1000W of solar panels with Genasun MPPT controllers that were not programmable.  To that, we added a Philippi Monitoring System.  Read more about our solar panels and the install in Palmetto.

gensun solar controllers on a sailboat
Genasun solar controllers.
AGM generator start battery for sailboat
Generator start battery.
AGM engine start batteries
Engine Start Batteries.
Philippi Monitoring System.
sailing catamaran with solar panels
Solar panels.
Old house bank of AGM batteries.

Reasons to Switch to Lithium Boat Batteries

More usable hours

  • AGM batteries have a 50% depth of discharge per charge cycle
  • LiFePO4 batteries have 80% depth of discharge per charge cycle
  • 1000 Ah AGM bank = 500 usable Ah
  • 1000 Ah LiFePO4 bank = 800 usable Ah

Ease of Charging

It’s recommended to charge your batteries up to 100% on a regular basis.  AGM batteries have three charging modes: bulk, absorption, and float.   That means that the amount of power charging into the batteries changes as the batteries fill up.  Bulk means that the maximum current is going into the batteries.  Absorption drops down the amount of charge in order to keep the batteries at a bulk voltage which can vary depending on battery manufacturer.  That means that the amount of power going into the batteries is being throttled back.  Float means that your batteries are full.  For us to go from 50% to full through bulk and absorption took almost 12 hours of charging.

Read more about charging profiles for lead acid batteries.

Our generator is huge.  When running a generator, it’s important to put a load on the output.  When we ran our generator, we would always turn on air conditioning units in addition to whatever other power we were using (like our watermaker) to have a decent load on the generator.  Basically, we were wasting generator power while filling our batteries at a slow rate.

Lithium-ion batteries also have an absorption mode, but this taper charge is significantly shorter.  This means that they can accept more current coming in consistently until they are almost all the way in float mode.

Longer Lifespan = Cheaper per Life Cycle

With our heavy use of full-time cruising, our batteries lasted us a little over 3 years.  Towards the end, we were starting to have to charge them every day.  We expect these Lithium-ion batteries will last for 10 years.  While the batteries themselves are more expensive, when you factor in the lifespan and calculate a cost per lifecycle basis, Lithium-ions make more sense.

Now, that’s assuming your boat is already set up for Lithium-ion batteries.  As you can see, we’ve had to incur a one-time cost of changing a lot of our additional components out, but next time we (or in all likelihood, Starry Horizons’ next owners) need to change batteries, it’s an easy swap.


The Lithium-ion batteries themselves are lighter than the equivalent AGM batteries, by twice as much per Amp hour or one third as much by usable amp-hour.  This means for the same amount of usable amp-hours you have fewer batteries, and the batteries you have are lighter.

Our New Lithium-Ion Batteries

House Battery Bank

Since we’re in Australia, we found a company called Outback Marine that offers a very unique Lithium Battery set up.  In total, our new house bank has a total of 720 Ah, or 576 usable Ah.  The entire unit is a modular setup with 4 “minibanks” making up the entire housebank.  Each “minibank” made up of 4 180Ah 3.2V CALB LiFePO4 cells wired in series to make a 180Ah 12.8V bank.  There are then 4 of those minibanks wired in parallel to make up the full 720Ah house bank.   Having a modular setup is a huge advantage because if one of the  minibanks fails, we can rewire things, removing the minibank from the whole house bank, and still have a useable house bank at a lower Ah capacity until we can fix the problem.  Cell level charge balancing keeps all cells at an equal level, and there is monitoring for overvoltage and undervoltage conditions.  Also included is a temperature sensor.

Lithiums are quite sensitive to operating temperature and being in a hot environment, such as an engine room, can significantly reduce their lifespan.  So while we were reworking our electrical setup, we moved the new lithium battery bank to the main salon, under the couch.

Start Batteries

We kept the AGM start battery for the generator.  However, we decided to swap up our engine start battery set up.  Instead of having a dedicated start battery for each engine, we now have one AGM start battery for both engines.  This change was made to accommodate the Outback Distribution Control Board as it is designed to work with one start battery.  But it allows for the solving of a potentially big problem with lithium batteries and we’ll talk about that in a bit.

System Controls

We installed a new Outback Battery Management System (BMS) which monitors house bank and start batteries.  A BMS is essential to the proper functioning of the lithium bank.  If the voltage gets too high or too low, the BMS will take the bank offline to prevent damage.  One feature of the Outback BMS is that we can control cross charging remotely.

The Distribution Control Board (DCB) is the central board for all wiring and perhaps the largest reason we decided to go with an Outback Marine system.  It includes isolation solenoids for the house bank and the start batteries, cross charging, and emergency start.  This allows us to bypass any issues with the start batteries and start engines from the house bank if needed.

It also provides for a seamless swap of charge going from the house bank to the start battery, which addresses one of our major concerns about a swap over to Lithiums.  What would happen to other electrical systems on the boat if the Lithium House bank went offline?  If the entire electrical set up is done properly, that shouldn’t happen.  But we live on a boat.  Things definitely can and do go wrong.  Say for example the engines were on and the alternators were working when the BMS took the house bank offline.  The alternators would be pumping out current with nowhere for it to go.  That will very quickly damage alternators or some other type of charging source.  And you’ve quickly gone from having a problem with your batteries to causing significant damage to other expensive parts of the boat.  Add being in remote locations in the mix and you’ve got a recipe for disaster.

So having the DCB automatically take any current flowing and putting it into the start battery is a massive fail-safe and will prevent significant damage.  Now we will get the alarm that something is wrong and have time to take charging sources offline, fix the problem and get up and running again.  Huge peace of mind.


While upgrading the batteries we took the opportunity to upgrade our inverter-charger to a Victron 3000W/120A unit.  We installed the new inverter-charger in the port forward guest closet to minimize the amount of wire from the inverter-charger to DCB under the couch.  This inverter-charger enables us to charge the batteries faster, and also pulls a significant load down on the generator.  We no longer have to run an air-conditioning while running the generator, provided the batteries are in bulk charging mode.

Solar Panels and Controllers

Solar panels are all the same.  We had to change the solar controllers to Victron MPPT controllers, which are programmable and programmed to Lithium-ion.


The factory-provided Mitsubishi alternators were not compatible with the Lithium-ion charging profile.  We upgraded to two 120A Balmar 6-series alternators with a shared Balmar smart regulator.  While installing, we wired in switches to turn off the smart regulator and use the internal Balmar regulator as a backup if necessary.

New solar controllers for our Lithium-ion batteries.
New digital battery management system.
Distribution control board.
alternators for lithium boat batteries
New alternators.
Smart regulator for the new alternators.
Inside the smart regulator.
lithium-ion boat batteries - house bank
New house bank batteries, engine start battery, and DCB.
lithium boat batteries - house bank
Lithium-ion house bank batteries.
New Victron inverter-charger.

Contractors Used

We did this work while at The Boat Works in Coomera, Australia.  Brent from Cat Impi recommended Jack from Outback Marine.  Jack designed the system, and Jim from Odyssey Marine installed it.  Jack and Jim were both great to work with.  Jim was very hands-on, and when we had a few issues, he made sure to come out and fix them for us.

Jack from Outback Marine.
Jim from Odyssey Marine.


While motoring around Moreton Bay we got a voltage alarm on our Volvo engines.  Volvo is not super helpful with troubleshooting, but we were able to figure out that the voltage drop in the wires running from the alternators to the house bank was too big (drop to big = wires too small).   Jim came out to Scarborough Marina to run new wires in and that solved the problem.

Our autopilot compass was located in the guest closet where the inverter-charger is now located.  We did notice some interference, so we moved the compass into the cabinet behind the tv in the main salon.

When we moved the battery bank forward, even though it is lighter than the AGMs, the weight being moved forward and Starry Horizons became a bit heavier in the bow than we liked.  We had to shuffle things around to put more weight in the back of the boat.

Project Costs

Doing a swap to Lithium batteries is not a cheap project.  The economics of doing a straight AGM for Lithium swap is starting to get pretty attractive with the dropping cost of lithium batteries.  But unfortunately, we had to/chose to upgrade quite a lot of other systems as well in order to build a lithium battery set up that we were comfortable cruising within remote corners of the world.  This dramatically increased the cost of the project.  Plus, we chose to hire qualified people to design and help install the system.  David has learned quite a lot about working on boats over the last several years, but for something as critical as our boat’s electrical system, we chose to go with experts.

Below you can see exactly how much we spent on various parts of this project.

Final Thoughts

It’s been a month since we fixed the cable run from the alternators to the house bank and left Scarborough Marina.  So far, so good.  When we run our watermaker every five days, the house bank batteries are fully topped off by the time our water tank is full (roughly five hours).  Our diesel usage for the generator should drop by over half, plus the decrease in hours on the generator will increase the lifespan.

One last word of advice.  We are very happy with our new set up, but doing it in Australia cost us an extra 10%.  This is because, in Australia, you only receive GST reimbursement for projects completed in the previous 30 days of leaving the country.  We could have saved a lot of money by doing the project in New Zealand instead where you are GST exempt for your entire stay.

Watch the Video


Watch Gone with the Wynns talk about AGM versus Lithium-ion.

Cat Impi has two videos on their Lithium-ion set up, also done by Jack from Outback Marine.

This website has some very in-depth discussions on the Electrical Design for Marine Lithium Battery Banks.

Pin it!


  1. Hey, Amy Thanks for sharing this amazing article, I found it very interesting and the details you’ve mentioned are one of it’s kind, so for motivation keep up the good work.

  2. Best write up and video on the subject. Do you have a wiring diagram of the system. There will be fuses, shunts and other details like this that are helpful to see in the design of the system. This will answer a lot of detailed questions about how best to do this kind of install. It was really a great example of a professional installation. Thanks mate

    1. Hi Daniel. I’m afraid I’m not sure exactly which “circuitry” you’re referring to. The output voltage is still 12v and 12v is 12v, whether it comes from lead acid or lithium. The major differences between lead acid and lithium are in the charging capabilities and their depth of discharge levels.

      If you want to clarify a bit more what you’re interested in, I can try to give a more detailed answer.


  3. Very Good Review And I Like You Candor About The Battery Manufacturers And Like You Said I Would Go For Battle Borne For The Warranty, The Low Temp Cutoff, And The Customer Service! Great Info Thank You!

  4. Hi,

    Nice write up. We are starting to research LFP batteries. Our six 183Ah Dekas are starting to act up and the are only 20 months old. We are going to rewire them into a starburst pattern instead of the current daisy chain. We have 1040W of solar through two Midnite 150 controllers, and the rest of our electrical is the same as yours, pretty much. We are a 220V boat. We have the 11KW Onan gen and agree it is too big. Our Cristec chargers are lithium compatible. Hopefully our current batteries will last till New Zealand, which will not be until 2020.

    Cheers, Rick
    SV Airborne
    French Polynesia

  5. Can we get a video update of your conversion. What you learned since, changes you would make, reliability, monitoring, etc.
    Do you think eliminating a Genset is really practical with a Lithium setup?

    1. Hi Bruce. We will add it to the list of things to film but it’ll be a while. We’ve only had the batteries for about 8 months now. We’ve discussed some changes in our recent videos. A blog post might be more likely.

      We would not get rid of our genset, although we would prefer a smaller one.

  6. In preparation for returning to the seas I started tinkering with solar power at home. Careful consideration is needed for the cost to value to benefit breakdown. It truly is an amazing system, well thought out, planned with precision and very neatly implemented. Please share a cost table for the components and installation breakdown, it is a deciding factor to consider. Many thanks in advance, so far your most helpful learning video!

  7. Great video! If you were putting this exact system including two high output alternators and a solar array that can top off your battery bank daily into a brand new Helia, would you forgo the dedicated Onan generator for lower weight and maintenance? Why or why not?

    1. We would prefer a smaller generator, but we would still have a generator. We run two things off the generator: our watermaker and air conditioning. While we don’t use the air conditioning much ourselves, we have used it a majority of the time that our parents were onboard (including a record heat wave in the Sydney area this summer that was a high of 117 F). The watermaker is a CruiseRO high output 30 gph watermaker. We are not stingy on our water, and as it is I could take 5 hours to fill 3/4 of the tank. We’re happy with a high output watermaker.

  8. I know this is a sensitive subject however a cost breakdown would be extremely helpful to a lot of your followers who are making plans of their own. The Space Between did a DIY version which honestly may be more attainable for us however it would be nice to compare costs.

Leave a Reply

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

This site uses Akismet to reduce spam. Learn how your comment data is processed.