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eddies cat3

The Marshall Cat Boat shown above is equipped with electric propulsion and powered by Mastervolt Lithium Tech batteries.

So this week I’m going to share an increasingly common question and my answers to them regarding lithium tech batteries.


I have a 44′ sailboat from the 80’s with two Group 8D batteries for the house bank. I am thinking about making some system changes and improvements after they apparently discharged last winter while she was in storage although I followed pretty much the same charging protocol as I have for the last 7 years. So I will be replacing them.

The batteries are located on a shelf under the sole in the galley area where they block access to the forward elements of the auxiliary engine. There is no hold-down or box to contain them. The house bank powers the vessel’s autopilot, radar, VHF, depth sounder/speedometer, SSB, nav lights, and bilge pumps. I am switching the lighting to LEDs gradually which is lightening the DC load. The alternator charges a different battery that powers only the engine starter motor and the windlass. The refrigerator is powered by a fourth separate battery that charges from an echo charge from the Freedom Marine 15 charger/inverter.

As the years go by, I find the task of moving the Group 8Ds to be more and more unwelcome, not to mention how I feel the next day. As I thought about the situation, it occurred to me to look into lithium ion options to see if I could improve the installation and, not incidentally, lighten the work a little.

I was formerly certified by ABYC in marine electrical systems (I let it expire in 2017 due to changes in my situation) so I am aware of many of the important considerations involved in the system. I have read your pieces in various publications over the years and Powerboater’s Guide to Eectrical Systems. I am writing to ask if you have any experiences or information about lithium ion installations in marine settings that might be relevant to this project. I know that there have been many developments and improvement in the technology in the past decade. If you have ideas or recommendations that you can share, it would be very helpful in my planning process.

I hope that you are enjoying some boating this summer. Thank you very much for your time and attention.

Jonathan ”

Jonothan, nice to hear from you. Regarding your thoughts about replacing your 8 D’s with lithium tech I’d be cautious. The first consideration is whether the choice of replacement comes matched with a proper “BMS” acronym for battery management system. This really is an imperative to ensure safety. If the answer is yes it does, then decisions will need to be made about some of your existing equipment. Can your current battery charging system be programmed to match the requirements of you battery manufacturer? Lithium batteries have a phenomenally high recharge acceptance rate which is great because it means that theoretically the batteries can be recharged more quickly. The risk here is overheating the alternator. Installations I’ve seen have added forced air cooling hoses aimed directly at the alternator and thermal sensors that turn off the alternator at a prescribed temperature. Voltage regulation and the currently installed battery charger(s) may also be an issue. Are they programmable? If so to what extent? down to 0.1 volt of precision is important to some lithium battery manufacturers.

Finally, understanding that currently used lithium technology (LiFePO4) has demonstrated a very low likelihood of  thermal runaway issues is important. That said, some consideration should be made for where and how these batteries get installed on board. In the event of a thermal runaway situation will the batteries simply burn a hole in the bottom of the battery compartment and fall into the sea as most all of the catamaran rigs I’ve seen will? Sounds scary, but way better than burning to the waterline for sure. Remember that currently available fire extinguishing agents will NOT be any help in the event of a runaway. Cold water is the only thing that will reverse this thermal issue.

Carefully consider cycle life and cost of new charging components before you make any final decisions on using lithium tech. You might just discover that AGM tech is a better choice with lower risk.



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