Planing electric boat with 40kw battery bank

While your vision for an electric boat is exciting, I’d recommend reconsidering the use of 80 Valence U1-12RT batteries for this project. Here’s why:

BMS Complexity & Reliability Issues：Managing 80 individual 12V batteries in series/parallel configurations would require an extremely complex Battery Management System (BMS). Balancing cells, ensuring uniform discharge/charge cycles, and mitigating risks of voltage mismatches or thermal runaway across such a large bank would be challenging. Even with “gently used” batteries, aging variations and residual wear could exacerbate these issues.

Weight vs. Performance Trade-Off：A 1,200 lb battery bank (80 batteries) significantly impacts your boat’s weight distribution and efficiency. For planing speeds, this adds unnecessary drag and energy consumption. Modern marine systems (e.g., Torqeedo, Bonnen Battery) prioritize high-energy-density batteries (e.g., 1-2 large LiFePO4 packs) to minimize weight while maximizing range.

Simplified Solutions in Practice: Most electric boat builders now standardize on 1-2 battery modules (e.g., 48V or 96V systems) paired with purpose-built marine BMS. This avoids cascading failures, simplifies maintenance, and ensures compatibility with marine inverters/motor controllers. For example, a single 48V 400Ah LiFePO4 battery (~500 lbs) could deliver comparable energy with far fewer failure points.

Long-Term Viability of Second-Life Batteries: While the Valence batteries are cost-effective upfront, their remaining 50% lifecycle may degrade unpredictably under high marine loads (e.g., rapid discharge during planing). This could lead to frequent replacements, negating initial savings.

 

Super interesting to me to see the weight required force the once desired planing hull down to near displacement speeds.

@bobkart has done the legwork

The hull type is critical.

 

That’s awesome! I’ve been scratching my head too, trying to find the perfect lithium battery solution for my new electric boat. I noticed Bonnen Battery has a ton of options, so I’m thinking about reaching out to get some details. Just gotta figure out which one’s the right fit for my setup!

electric boat lithium battery, lithium marine battery | Bonnen Battery https://www.bonnenbatteries.com/applications/lithium-for-marine

 

Batteries weigh 8-9x that of gasoline for the same energy content. Weight is a huge penalty for planing hulls. Then there are the safety issues of Li-ion batteries. Just because something can be done, doesn't mean it should be done. Leave electric propulsion to submarine applications built by General Dynamics in Groton CT.

 

Funny you mention Electric Boat. They only recently switched to AGM (absorbed glass mat) lead-acid batteries, cutting edge tech from the 1970s. They've been using traditional flooded lead-acid batteries for decades. Lithium? They're lagging behind. Meanwhile, Japan’s Sōryū-class submarines have been running lithium-ion since the late 2010s.

Regarding the weight argument: electric motors are significantly more efficient than gasoline engines. Electric motors can achieve efficiencies of around 90%, whereas gasoline engines typically operate at about 35% efficiency. This means more of the energy from the power source is converted into propulsion with electric motors.

As for propulsion setups: direct-drive systems, where the motor is connected directly to the propeller without gears, are praised for their simplicity and reduced maintenance. But, they often require the motor to run at higher RPMs to hit the desired speed, hurting efficiency. Using a geared or pulley system allows the motor to run at its optimal speed while adjusting the propeller speed for better efficiency. This setup works best when you don’t need immediate torque and can use larger, slow-turning props with higher pitch for better efficiency once the boat is up to speed.

I’m the first to admit weight and cost aren’t where they should be yet with electric motors and batteries. But with older boat designs meant for 1-stroke Wisconsin 20hp engines or similar, or with modern wave-piercing slippery hulls, electric or even gas works great.

 

That comparison is not relevant. Fossil fuels engines are more efficient by weight than batteries running electric motors.

 

Okay, you have a solution (batteries) looking for a problem. Boats aren't it. My position is simply that ever since the very first commercial production of rechargeable lithium ion batteries in the '90s, humans have yet to produce a lithium chemistry battery product/system that is safe enough for distributed use in vehicles, boats, planes, or occupied spaces. After fifty years of research, we are getting to the point where we know how to, but the resulting product with it's complexity and built-in thermal runaway tolerance ends up having specs only slightly better than advanced lead battery systems, but with 20 times the up front costs and maybe 5 times the cycle life. The energy density, specific energy, and system round trip efficiency ends up only slightly better than advanced lead. On the other hand, advanced lead systems are more than 99% recyclable today. Lithium-based systems are very challenging since half of the recycling efforts so far have burned to the ground.

I suggest you go look at the hospital's battery installation. It won't be in an occupied space, and it should be physically separate from the generators, switch gear, control systems, and fuel. Normally, I'd expect to see it in a skid or container a safe distance from anything.

If you want to install such a system at your house, constructing the bunker and fire containment system for the batteries would probably cost as much as your house. So free batteries aren't looking so free anymore.

And since lithium batteries have a better lifetime through put, but arguably a worse clock-time life than advanced lead, you want to use them where they get cycled at least once per day. So emergency storage and toys are the worst places to use them.

Retail net metering usually comes with peak-rate billing schedules. Using lithium for daily peak shaving in an small, rural industrial area might make sense if there is an unused concrete bunker to put them in. Then again, it is cheaper to just double the number of solar panels. But that just pushes the problem of solar's diurnal power swings on to others.

EDIT<and so it goes - )