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Tesla modules in boat

3271 Views 80 Replies 10 Participants Last post by  remy_martian
I am converting a 25 hp gas outboard motor to an 18kW electric PMAC motor for use in a hydrofoil assisted power catamaran. Converting an outboard to electric is something I've done a couple of times but this time I'd like to make my battery safer. My last electric boat is a little lacking in the safety department - Electric Foiling Catamaran from Recycled Parts.

This time I'd like to use 6 Tesla Gen2 XS modules. 3 modules will go in a box on each side of the boat. This is a 72v system so I'm thinking that the 3 modules in each box will be wired in series then the two boxes combined parallel.

What I'm hoping you very knowledgable people here can help me with is battery box design. My thought is to make a watertight (within the limits of the IP67 rated vents) steel box out of .050" - .080" thick sheet that fits fairly close to the size of the 3 modules. That steel box would have a battery box vent Vent valve for battery box - EVcreate
and a layer of furnace insulation on the outside. The steel boxes would be mounted inside of composite boxes that are used as seats and will also hold BMS, contactors, fuses, emergency shutoffs, etc.

This is a carbon/honeycomb boat and I'm fighting to keep the weight down so that I can drag around those heavy batteries without too much performance loss. It kills me to think of 100 lbs of steel boxes onboard but I'm not seeing a way to mitigate the fire risk without something like that. It sounds like aluminum's melting temperature is not up to the job.

Any thoughts? Am I overthinking this? I'm not pulling a lot of current, maybe 200 amps continuous. I think that's like .5C. Charging C rate would be substantially lower.
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In the case of a boat, I'd cut a square hole in the bottom of the boat then seal a stainless steel battery box with a plastic bottom to that hole - in the event of a fire, the flaming batteries get delivered to Davey Jones locker.
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The wands for height control are brilliant...I'm now wondering if there's a mechanical wave height averager that could be introduced using adjustable-valving shock absorbers (or even corn starch dampers, lol).
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Safety requirement, not merely best practice. It used to be "best practice", which is nowadays unacceptable for occupant and first responder safety in roadgoing and competition vehicles. There are too many conversions being done these days to where someone will wreck and electrocute a responder....then we are done as hobbyists as we either get regulated to death or outright banned. There's no nicespeak on this - if there's more than 60V on ANY wire, it needs to be able to reliably get shut off in the closed box that put the voltage out.

"Wetted area" increase...yuck. But you're the boating genius here.

Meanwhile, yes, you could use a flight controller's pitch, yaw, and roll sensor to move the surfaces. Instead of your wands, use the four motors and props from a quadcopter, each placed in each of four corners of your boat that are actually flying in air (a couple of feet above water so they don't dunk) to servo the position in the air, then run those down with a control rod/wire to a trim tab on the foil control surfaces. The trim tab acts as a force multiplier and is usually how autopilot servos move control surfaces on aircraft. This should only pay attention to the gyro on the flight controller and will not care about wave heights and should not require a single line of code to be written.
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You can always put the battery box in its own dinghy and tow it.

Use fiber rope for the tow and it self disconnects in a fire 😂
LFP is double the weight.

You can either "diode-OR" the power connections, which wastes power, or do a power selector switch with contactors to drop the submerged battery from the circuit. Yes, the bms gets complicated - you may need two. Or you could use a pair of manual disconnects. In emergency power, should your inverter care what the BMS thinks? Arguably, you'd want to ignore the bms and toast the pack by overdischarging it if you HAD to get her to land.

In theory, if you used lead bolts for the bus bar connections, or a low melt alloy, it could self disconnect at the drop passively from the heat, maybe suspend the pack by those connections? But - that same system could strand you.

This is also not without risk - you'll make a lot of hydrogen with the electrically live, burning, cells dropping into seawater (explosion risk), though dropping the battery case in might not melt the case but won't cool the cells so it'll keep burning. In rough (anything but glass) seas, the vented case will still flood and sink.

It's a silly thought-exercise, which is ok. Creativity is a muscle that needs exercise.

Your main concern, imo, should be keeping the vessel afloat and sharks starved, not maintaining propulsion, so my vote is jettison a burning pack (this is unlikely to happen, but...can) and maybe do it so the jettisoned battery case "icebergs" to where you can maybe salvage it after the fireworks are done and also not be labeled an eco-terrorist for its sinking by West Coast hippies who hate fossilfuels and also hate hydro dams, wind turbines, solar, and "toxic" lithium batteries - build a pedaled hydrofoil to make the granola munchers happy 😂.
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A titanium box would be beauty in several respects (including, let it burn) - you seem to be super-resourceful in aerospace scrounging...I suspect an ex-Boeing dude 🤨
It's a chemical fire, doesn't need O2, so it will continue to burn under water until it gets cooled sufficiently.

Anything above 160 (some say 200) degrees is begging for thermal runaway which gets stuff all flamey.

If you're the better, or within a magnitude or two, (guessing) conductor to seawater, yes you could become a conductor. iirc, we're pretty much seawater once dry skin is compromised.

I'd think EPA would frown on lithium battery disposal at sea if it can be foreseeably avoided. Again, guessing. Not good karma to plan for it.

A fire is very unlikely. If you have one, the smoke will be seen by other boaters and you can get a tow. Your primary job should be to protect occupants, which includes keeping the ship from becoming ballast and includes not using ABS, Urethane, PVC, or other toxic plastics near or in the battery box.

Protecting an asset (like half the pack) is nice, but having been in a couple of engine outs in aircraft I was piloting, the last thing in priority is saving the ship, not the first -- saving the ship as a priority is what invites Mr. Death over for tea.

Dropping it into water will hydrolyze seawater into hydrogen and oxygen and it will continue burning since it does not need oxygen to sustain the lithium fire. It will explode because of the hydrolized H2 and O2 plus the burn...maybe a few times. You don't want it anywhere near the boat, or be tethered to it, imo.

Mount it on an angle onto compressed car coil springs, with an aluminum bolt keeping it compressed 🤓
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If only the guy knew what he was talking about vs a regurge from engineering.

The vehicle does not handle a thermal runaway, it prevents one.
Other than the Bolt series of charging problems on badly made cells, how many lipo4 packs caught fire just doing normal operation last year out of how many installs?
LG Chem....other manufacturers have also caught fire for the same reason, but GM had the "problem" of having sold 150,000 cars, unlike the others who were selling compliance cars.

LiFePO4 is twice as heavy as NMC. A lame step up from Pb, imo, and a nonstarter for anyone who wants the full benefits of EV performance.
From the 100.03 regulations( Regulation No. 100-03: Approval of Vehicles with Electric Power Train.) referred to in the video:

"6.12.1. Under vehicle operation including the operation with a failure, the vehicle occupants shall
not be exposed to any hazardous environment caused by emissions from REESS. [Rechargeable Electrical Energy Storage System]"

This is just common sense stuff, people. Why somebody be so superficial and unimaginative to think the engineer in the video was referring to anything else??!!

As for as relying solely on prevention to deal with battery thermal runaways, let's recall the quote the great popular philosopher, Mike Tyson:
There is no safety system.

The marketing monkey was briefed by engineering that they had a prevention system for thermal runaway, he paraphrased that dangerous tidbit in an interview, and now some of you are running around thinking there's PFM in play (Pure...Magic) from a components assembler.

The reality is that a lithium fire starts when there is positive thermal feedback for an exothermic chemical reaction.

The ONLY ways to stop it are to cool the adjacent cells to below the threshhold temperature, or to poison the chemical reaction. The latter is impossible without destroying energy density of the cells.

Thermal management means isolating the cells from each other thermally. Few DIYers understand this, many DGAF.

As far as aircraft and marine systems, a planned jettison is RELIABLE. Vents merely delay a burndown and are there to prevent the sealed battery box from overpressuring and becoming a bomb.

I can't believe you quoted a dumbass punching bag on this site 🤦‍♂️
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Further your own ignorance - it's freedom of choice
The point of using a lithium pack is energy density. Energy per unit volume. Yes, you can put one cell at each of your friends' and relatives' houses to let them burn out safely, but other aspects get complicated really fast and weight goes up unless you have this kind of money for your project to use super-expotic materials and alloys (weight and volume still go up, though):
Product Rectangle Font Circle Screenshot

When reading such articles (for which you should cite the source, not personal recall, so the context and methods can be reviewed), you need to be able to distinguish science and engineering - they have very different objectives. Most science whiz-bang stuff is ridiculously impractical, yet interesting. "It can be shown..." and "further work may lead to..." are science favorites to keep the playtime money flowing in.

SpaceX sends a rocket up for ~1/1,000th that cost.

Imagine the lunar city you could have set up with the Artemis money and using SpaceX for launch staging of your Lunar Quonset huts in earth orbit.
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Good idea. The nozzle design would be very simple to design vs one that's for a SSTO, in that it always will operate at, and after a few minutes, below, sea level.

The problem is: battery pack caught fire on a vessel that's out to sea.

Constraint: don't use passengers or self to chum for sharks
Your pistol catches fire while you are on your sailboat because you bought cellulose grips on sale on eBay. The steel starts glowing red...

Do you:

1) Throw it overboard
2) Hold onto it because you have good health insurance and real men can take 3rd degree burns without wincing
3) Toss it on the deck, relying on the chambers to isolate the other rounds from going off and shooting holes in the bottom of the boat
Tesla cells have vents that release hot gases (cave men gave these hot gases the name "fire"), so there is no pressure...they go off like little bottle rockets, and once they cutting torch their was through the passenger compartment floor, they launch into the cabin and fly around in it until the chemical reaction stops.

Once the cooling system fails, each cell heats its neighbor, lighting it off. Significantly less than boiling water temperatures will light them off, so do the math with a neighboring cell at several thousand degrees, about 2 or 3 mm away from the cell that hasn't lit yet.

The job is to ensure they don't light off.

The job is to isolate the cells/modules if they do

The job is to give occupants time to egress before the car melts.

The job in a boat is to egress the boat into water with sharks, Great Whites (yes, they live in the Pacific Ocean, off the Left Coast), in it...oh wait, let's keep the people on the floating boat and get rid of the worthless burning battery.
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