[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.

 

[reiderM]

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.?

 

[Ohno]

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.?

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")

 

[kennybobby]

From what i remember, the 2 boeing 787 screamliner APU starter battery fires were contained in aluminum boxes; their "fix" was to build a vault to hold this 36V 8s lithium pack with a vent tube to exit the airplane outer skin in the event of another fire. There are other lithium packs for other functions in the electronics bay, but they are not in any vault and they don't have the same 4-level bms, and probably aren't held on a full-voltage trickle charge at all times during a flight.

i think you could build an aluminum battery holder and be fine with a good bms and attention to not over-charging. And a good fire and smoke alarm system to let you know if you need to get out quick.

 

[reiderM]

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.

 

[Ohno]

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.

I am leaning toward making a thin wall (.090"?) aluminum box with a welded aluminum frame made of whatever shape extrusions are needed to hold the batteries.

The set up is 3s2p. Since I have two battery boxes I suppose I need a contactor in each one connected to the BMS?

I am definitely interested in the SimpBMS. I see a link to the documentation on eBay. I suppose that's the same documentation you're talking about?

 

[Ohno]

From what i remember, the 2 boeing 787 screamliner APU starter battery fires were contained in aluminum boxes; their "fix" was to build a vault to hold this 36V 8s lithium pack with a vent tube to exit the airplane outer skin in the event of another fire. There are other lithium packs for other functions in the electronics bay, but they are not in any vault and they don't have the same 4-level bms, and probably aren't held on a full-voltage trickle charge at all times during a flight.

i think you could build an aluminum battery holder and be fine with a good bms and attention to not over-charging. And a good fire and smoke alarm system to let you know if you need to get out quick.

What sort of duty cycle would you have on the electric motor operation or its purpose? Once the wind is up a catamaran would haul ass against any electric motor prop drive wouldn't it?

I expect to run the motor at around 200amps depending on what the 'sweet spot' ends up being for speed vs power consumption. This is a powerboat so no sails are involved!

 

[electro wrks]

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).

In one of Tesla's original patents, it states that their battery fires can burn at 850 C (1562 F). As aluminum melts at ~660 C (~1220 F), it might be a poor choice as a material if you don't want your battery box to potentially burn through and melt in a fire. Stainless steel melts at ~ 2X the temperature of aluminum. It might make a better choice, particularly in a marine environment.

 

[Ohno]

In one of Tesla's original patents, it states that their battery fires can burn at 850 C (1562 F). As aluminum melts at ~660 C (~1220 F), it might be a poor choice as a material if you don't want your battery box to potentially burn through and melt in a fire. Stainless steel melts at ~ 2X the temperature of aluminum. It might make a better choice, particularly in a marine environment.

That 850C is a really interesting number.
I wonder if there is an example of a battery box containing a battery fire?

 

[reiderM]

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

 

[Ohno]

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

i'd be happy to leave off the contactors. The less unnecessary electrical stuff on a boat the better I say.

So, a fuse between paralleled strings. Also a fuse to protect the battery to controller cables? I'm also wondering if I need some kind of manual shut off at the batteries?

The SimpBMS looks like the way to go. I'll contact you about that as I get all of my ducks in a row.

Do you have any thought about a reputable source for Tesla modules? I'm looking at these - TESTED Gen 2 Tesla Model X S 85 90 Battery Module 24V 250Ah 5.3 kWh w/ Warranty | eBay

That looks like a 13' Whaler in your photo. Classic!

 

[reiderM]

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.

 

[electro wrks]

I wonder if there is an example of a battery box containing a battery fire?

This fire was with Tesla modules. Take heed of the posters message and battery box design:

My car caught fire as well. It was while parked with highest cell at 4,1V.
It is because I used vent valves and a 2mm stainless steel box which kept the fire inside to a large extent so there is at least a car to rebuild
Will share some further details on my rebuild blog later Electric Volvo Amazon wagon from 1967 and converted to 100% electric

I used different ones, but I use these vent valves nowadays Vent valve for battery box - EVcreate

I would say the use of aluminum, plastic, or wood as the primary battery enclosure material with Tesla modules is a real bad idea. That said, As I recall, some Teslas, maybe Ford and Rivian may have Aluminum in the lower portion of their under vehicle battery boxes - but always steel on top, next to the passenger area. Is this to keep a fire down and away from the passenger area, melting through the aluminum bottom first? If you are serious about doing this right, you want to think long and hard about these safety issues.

 

[kennybobby]

Contactors between parallel packs are used to isolate them in the event one side has an internal issue. See Wolftronix threads about integrating the BMS with the contactors.

 

[reiderM]

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.

 

[kennybobby]

In a catamaran with packs in each hull, should the connecting wires be de-energized when the vehicle is Off, or will they be Hot all the time?

If using used modules from a wrecked car, then just assume that you will have a fire--are you wanting to save the vehicle or just save your life? Is it just a run-about type of boat or a live-aboard?

An EV has safety aspects to protect first responders in the event of a crash that a Boat doesn't necessarily need, just depends upon your level of risk tolerance.

Cool boat build

 

[oudevolvo]

Still need to do a writeup on my root cause analysis on my fire.
Regarding battery boxes: the fact that I used 2mm stainless steel battery boxes with vent valves are probably why there is a car to rebuild.
It contained the fire of three Tesla modules until there was nothing to burn inside anymore.
And the front tires still held air.
Thought on the root cause I’ll share on my rebuild blog Electric Volvo Amazon wagon from 1967 and converted to 100% electric but still want to test some hypothesis.

 

[Ohno]

Still need to do a writeup on my root cause analysis on my fire.
Regarding battery boxes: the fact that I used 2mm stainless steel battery boxes with vent valves are probably why there is a car to rebuild.
It contained the fire of three Tesla modules until there was nothing to burn inside anymore.
And the front tires still held air.
Thought on the root cause I’ll share on my rebuild blog Electric Volvo Amazon wagon from 1967 and converted to 100% electric but still want to test some hypothesis.

Thanks for responding. It looks like you did a fantastic job in building your car. I am sure like many people I would love to read about the cause of your fire.
Have you shared anywhere photos of your battery box?

 

[Ohno]

In a catamaran with packs in each hull, should the connecting wires be de-energized when the vehicle is Off, or will they be Hot all the time?

If using used modules from a wrecked car, then just assume that you will have a fire--are you wanting to save the vehicle or just save your life? Is it just a run-about type of boat or a live-aboard?

An EV has safety aspects to protect first responders in the event of a crash that a Boat doesn't necessarily need, just depends upon your level of risk tolerance

I don't yet know enough about this stuff to say if there is any value in de-energizing the wires between battery boxes. It seems like the power will be on whenever there are people onboard.

This is a runabout. Really if the boat is consumed in flames there is no good place to go. That's why my OP - should I build a battery box that contains a fire? I guess "how safe is safe enough?" is a question that only I can answer in this case. I mean, I don't wear a parachute in small planes even though they occasionally fall out of the sky.

 

[oudevolvo]

Have you shared anywhere photos of your battery box?

Thanks, yes, in this blogpost https://www.oudevolvo.nl/en/blog/2018/09/27/stainless-steel-battery-boxes-for-tesla-modules/

And when I stopped blogging updates can be found on Instagram @oudevolvo eg around

http://instagr.am/p/CNErkAKH7hG/
And on Facebook OldVolvo