Tesla modules in boat

3272 Views 80 Replies 10 Participants Last post by remy_martian, 2 mo ago

Ohno

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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reiderM

· #2 · 3 mo ago

For what it's worth, if the modules catch on fire, you're not going to stop the fire from spreading regardless. When a factory EV catches on fire, it's often just allowed to burn out on its own since lithium batteries supply their own oxygen which makes extinguishing them exceptionally difficult. The battery box vent just prevents the battery box from becoming a pressure cooker and possibly exploding in the event of a fire (not ideal!). So whether the box is steel, aluminum, plastic, or otherwise, you're in for a very bad time if the modules catch.

An ounce of prevention is worth a pound of cure. There are only two ways which will cause the batteries to light: physical damage to the cells (eg. a puncture), and overcharging above 4.2v. Lithium batteries can also thermal runaway above about 70 C, but I have a hard time seeing a scenario in which your modules would get anywhere near that threshold given the discharge and charge rate you mentioned.

With all that being said, you certainly don't have to stick to steel. Many folks have used aluminum without issue.

What are you using for BMS/charger/fusing/contactors etc.?

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reiderM

· #5 · 3 mo ago

Ohno said:
Thanks very much for your thoughts. I would happily give up the idea of building a steel box if it won't provide enough, or any, added safety. That said, I have to believe that a battery fire could be contained in a steel box if it was heavy enough but that it's not worth the expense and weight for EV manufacturers to do that.

Probably the safest thing would be to use LiFePO4 batteries. They are heavy and stupidly expensive though.

I have questions about what BMS, charger, contactors, fuses and switches I need at the batteries. So far my plan is to use an Orion BMS, a fuse on the negative cable of each battery box, and maybe a power cut off on each box? The contactor is inside the outboard motor cowling along with the controller and motor. Those are the basics at this point. In addition there are various gauges, motor/controller coolant system, DC-DC converter, etc.

Here again any ideas you have about this stuff would be much appreciated. For instance: My two battery boxes are 4' apart. Each has a 72v battery. The boxes are connected by a conduit. My plan is to connect those two batteries in series. Can I use one Orion BMS for this set up? (Note: I edited my original post I switched "parallel" and "series")

Yeah, I mean if the metal is thick enough at a certain point of course it will be a non issue. A foot of steel will definitely stop the spread of a battery fire inside (and you can probably ditch the blowoff valve at that point too)! I like the idea @kennybobby mentioned with the nested boxes and a vent, if you really want to be safe. But rest assured that many people have li-ion systems with aluminum siding in the mm-range that have run for ages without issue. Tesla modules also have cell level fusing which is nice in case of a cell failure etc. Everyone's risk tolerance is of course different, so ultimately it's up to you. Li-ion batteries when managed properly are quite safe (see the fire rate of EVs versus ICE vehicles).

So you'll have a 144v system? Or a 72v system? 6s would be 144v, 3s2p would be 72v. Keep in mind that a BMS like the Orion cannot manage separate paralleled strings. I'll plug the SimpBMS here as I am the retailer for it in North America (so I may be a little biased!) as it can manage paralleled or series modules without issue and is a good bit cheaper and simpler than the Orion for Tesla modules. If you are interested in that just shoot me an email at [email protected] and I'll send you all the docs on that etc.

The BMS should control at least one contactor separately as well to disconnect the battery from any load in the event of a battery failure - this is a key aspect of safe battery operation. The BMS should also control charging to prevent overcharging.

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reiderM

· #10 · 3 mo ago

Yep, all documentation is there. Plus I can do a better price ($20 less) off eBay since eBay's fees are quite high for sellers.

Not necessarily a contactor in each one, but you should definitely have a fuse between paralleled strings. IMO it's better to leave modules permanently paralleled when there are cell-level fuses (as is the case with Tesla modules) but the opinions on this vary. I can give you more details on the pros/cons of that if you're interested

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reiderM

· #12 · 3 mo ago

Good eye! Yep, 13' whaler from sometime around 1970 - can't remember the exact year. Had a real old 40 horse on it and put on a new Mercury 25 and it's a great lake boat as long as it isn't too choppy. Incredible that it's held up for 50+ years, still all original wood trim and everything.

I've worked with that module vendor on eBay a good bit, once you're all set to order I can actually invoice you for the modules and the SimpBMS on a single invoice, and the modules will just ship from EV Racing's location in California instead of mine.

Yes, a fuse between the parallel strings, and then a larger fuse between the paralleled strings and the load. In my build with paralleled modules, I did a 400a fuse from the negative of each series string to a shared B- bus, and then the positive of each string leads to a single 800a fuse to the main B+ bus.

A shutoff switch in my opinion isn't really necessary. The only reason you'd want a shutoff switch is if your contactor(s) weld shut, which indicates a more serious problem that shouldn't happen in normal operation. Even if that did happen, since your system voltage is only ~72v, it's not super dangerous to just unbolt connections like it is with live 400+v connections. It also can't hurt, but if you're going for simplicity, then the manual shutoff is unnecessary and doesn't add much, if anything, to the safety profile.

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reiderM

· #15 · 3 mo ago

Yes but there are also downsides. What if the contactor between paralleled strings fails to close during charging, possibly due to something as simple as a loose connection? And then, when reconnected, there is a multi-volt delta between string voltages? THAT will cause an internal issue. A better option IMO if safety is the sole pursuit and cost and complexity can be disregarded, would be having an independent breaker device between the strings, that opens the circuit between strings if certain conditions are met (eg. voltage delta between strings, temp above 50c, etc.). For a perfectly safe setup, redundancy is always key, and a BMS is just one level of protection. But the real world often dictates limitations on what can be used unfortunately.

While batteries can have spontaneous internal issues, it's exceptionally rare. See the fire rate overall for electric vehicles, it's quite low. I am very curious as to what caused the fire on @oudevolvo's car, I dug through his website and couldn't find much info on the fire directly. As for what the OEMs have dealt with: the chevy bolt fires that have happened due to a serious fault in the manufacturing process have still only affected a limited number of vehicles.

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