Tesla battery pack?

[nebster]

Aaron,

I have a few random comments from someone who lives full time in an RV with a big inverter and a battery:

The 120V induction cooking elements are underwhelming if you are used to a regular induction or gas element at home. Think 1500W versus 4500W. Still, they work okay if you are patient. They are about 25% more efficient, which is nice but probably not an energy budget game-changer for most people.

You say you are heating with a woodstove and maybe some small electric space units. A well-insulated modern 45-foot RV with double-paned glass windows will require something on the order of 3kW average power, continuously, to hold 66F internally with an outside nighttime temperature of 38F. (In reality with sunshine the load is weighted to the nighttime, of course.) So you are going to need a nice pile of wood and maybe some blankets for your adventure, if you are not willing to burn propane, diesel, or gasoline for heat.

You mention electric water heating. Water can be heated at 400degF-gallons per 1kWh with a heater delivering 100% efficiency. Those small heaters typically are often sized for 1500W so they can be run on a typical 15A 120V residential circuit. Plan for about 1kWh per conservative, camping-style shower.

If your bus basement is reasonably insulated, and especially if you place your water heater in it, you may not ever need to run an electric blanket to keep your batteries from going below 32F. They have a huge thermal mass and will prefer to stay close to the mean internal temperature of the basement rather than swinging much.

Conversely, we know that maintaining lithium chemistries at temperatures exceeding 100F is deleterious to their health (regardless of other variables like SOC which also impact lifetime). It could be difficult to keep them cool when parked or underway on warm days on hot asphalt. This should be a real design consideration for you.

You describe being able to fit about 3kW of PV on your roof. You should expect something like 18kWh of energy from that setup on good days. The panels will derate as they warm up, and you won't get perfect tracking, and you'll not always be at the equator. You should expect you could have a whole week with effectively 0kWh. What will be your plan in these cases?

Camping in the sun is great in milder weather. When it's truly hot, the energy you get from the panels won't offset all the passive heating you'll receive in your rig. Just moving air with fans will help some, but your rig will still heat up. You and your family need to be comfortable living in that reality, or you should consider moving to the shade like the rest of us. But then, of course, you can't harvest any PV energy! What would your plan be in this case?

You say you arrived at an 8kVA (inverters are typical in VA, not W) by summing the worst-case scenario for consumption. On the one hand, I suggest you revisit that with more care and caution, and see if you really need that large a system. It is unlikely you would have all the loads simultaneously. Moreover, any big quality inverter has substantial headroom and can absorb peaks above the rated continuous power for a short while.

On the other hand, most inverters are rated in a perfect 75F environment, and you may not be able to maintain that with only fans in an enclosed space on warm days. So, oversizing may still be a good idea.

While I'm a strong believer that you should have the freedom to make your own choices and live with the consequences, I encourage you to think very carefully about using cobalt-chemistry lithium batteries in a mobile living environment. If you have any children or animals who might inhabit your space, they may not have the same ability to exit quickly in the event of a runaway. Egress in general is often a compromise in RVs. There is a reason most boaters and many of us RVers elect to use iron phosphate (LiFePO4) cells despite their higher cost, lower densities, and bigger hassle factors. Be sure you have fully assessed that tradeoff and are comfortable with the additional risk before you proceed with a large system.

Good luck!

[AaronTSchultz]

Thanks everybody for the questions and the input I really feel comfortable now knowing that what I had thought was the correct answer is actually the right answer. I need a confirmation from some people that were more knowledgeable than myself. I really want to thank ampster for walking me through a bit more detailed calculations, and walking me through the thought process. Now hopefully I won't have the Hindenburg, lol.

[flowing]

You guys havent checked out Jason's setup. All Tesla cells. Awesome setup:

https://057tech.com/solar

I would pass on them. They require cooling and heating management. Using 18650 NCO cells is dangerous and require rigid thermal; management. EV guys will not even use them. As for discharge, AGM can go to 80% DOD. However you would never ever discharge lithium that deeply.

The other thing is Tesla uses NCO cells which only have 300 to 500 cycles. The reason Tesla warrants them the way they do is because they would never allow the customer to ever fully charge them up. That is how you can get greater cycle life.

Last is economics. You claim 24 volts @ 250 wh which is 6 Kwh of storage. In a solar application 2 Kwh usable per day, 5 Kwh total usable for 2 day run time without charging. $1400 / 6 Kwh = $230 per Kwh of storage. You can get a top of the line FLA Pb battery for $175/Kwh with a 10 year warranty. Same top of the line FLA Pb battery with 6 Kwh storage is going to cost $1100. The economics do not work and with used batteries you get squat for warranty.

The best and easiest EV batteries to work with is Nissan Leaf cells. Easy to disassemble and reconfigure using simple BMS techniques top the point you do not need a BMS. .

[Ampster]

AaronTSchultz,

I found a link to a document that gives you a graph of the discharge curve of your cells. I didn't realize that the knee was so steep beginning around 3.5 volts


.https://www.google.com/url?sa=t&sour...H6agHVd2dm_tlv

[Ampster]

......

Skip the 8Kw inverter. The internal losses are killer Something like the Schneider XW+ with huge 5 & 30 minute overload capability should be enough

https://solar.schneider-electric.com/product/conext-xw-na-solar-inverter/

It's generator support works really well when programmed properly, a small inverter gen would be an unbeatable combo with it. Not sure how small it can go, I ran for a week......

That is good input regarding the Inverter.
As far as the generator is concerned, he could wrap some copper pipe around the exhaust of the generator to heat the water. I can imagine his wife's reaction when he says, "Look honey how I am heating the water without propane". That is if the noise of the generator or the smell of the diesel doesn't elicit a WTF first. LOL

[Mike90250]

...... Anyhow, Thanks for the questions, I hope I answered most of them

You are the one to be satisfied. I'm not able to endorse any of the Li packs, unless they are sold as an integrated system with UL cert. Why - product liability.

I ask the questions, and you have some good answers. Use Anderson Connectors on your slide out battery tray, so the cables don't hang up !

The thing thing is, folks new to the scene, don't know all the questions to ask. You don't need to heat to 60 ot 70F, 45F is good enough, and you sure don't want to let them BBQ in the summer with all the insulation. They will self heat a little bit, just 1 or 2% of the power is lost as heat.

I have an Induction hot plate, and can use it on sunny days , but I usually use the BBQ grill

Skip the 8Kw inverter. The internal losses are killer Something like the Schneider XW+ with huge 5 & 30 minute overload capability should be enough

https://solar.schneider-electric.com/product/conext-xw-na-solar-inverter/

It's generator support works really well when programmed properly, a small inverter gen would be an unbeatable combo with it. Not sure how small it can go, I ran for a week using a eu2000 and a autotransformer (I've got 240V loads)

[Ampster]

I am going to be using a victron 150/60 mppt that is also lithium compatable. It will also let me set the charge, float.. ect settings. Since 2400 w is 50a at 48v and this thing can do 60a at 48v. I should be fine with that. If the safe charging at 24v is about 24.6 will the safe charging limits high be 49.2v on a 48v setup? And float at 49.0 volts? And if the low is 19 would the low be 38? Please help with the voltage settings, as the amperages should not be a problem at 60 and the programming setting with victron are in voltages. Also the hybrid inverter/charger will be limited at about 12a so we won't blow fuses when plugged in at my brothers house.

As I mentioned before I like to do the math to check those voltages at the cell level.
49.2 divided by 12 equals 4.1v at the cell level. Which is a safe charging voltage.
38 divided by 12 equals close to 3volts. That is near enough to the knee of those cells that I would crank it up a bit. There is very little extra Watthours gained from 3.4 to 3.1 and i am always a little more cautious at the bottom. I would also look at when the Tesla BMS boards send a low cell message to the communications buss. Alternatively google for a discharge curve for NMC cells The cells in my pack are pretty close in voltage in the middle but vary by as much as 0.20 volts at the top and bottom. I have a mix of modules, some from the same car and others from unknown Vehicles. Perhaps your pack is more consistent. That data is where I feel I get the real value of a BMS especially if it has a logging function.

As far as charging from solar your max Watts are 2400 and you will probably never see that. You have figured out that the Amperage will be below 1C so no issues there.

[Ampster]

You tube "Morton's on the move" has a pretty good series 3 medium length videos on his install of this, with schematics which is helpful.

Did I get the reasoning right that I would just double the safety voltage for a 48v system compared to the safety voltage in a 24v system?

Yes it doubles for the pack but the cell level modules remains the same. You can verify with a voltmeter.
As far as the RV sub forum is concerned it does look.like it is primarily is 12v. A friend recently bought a used Sprinter and has asked me to help him with electrical upgrades. It piqued my interest and I have watched some of those Boondockers' videos.

[Ampster]

As far as cooking is concerned I think I understand you will be all electric. One suggestion of you haven't thought about it is induction cooktop. They are much more efficient but you do have to have pots that have some ferrous metal.

[Ampster]

So, my only criticism on the new tech of Li batteries, is:
1) many, many stories of Li batteries burning, both cell phones, laptops, Teslas and 787's Even the good designers don't always get it right.
2) the fumes while burning are extremely toxic - requiring outdoors venting. once they start, they don't stop.
3) poor cold weather performance below 40F, is bad, so you need to keep them indoors, violating #2
4) you only get 1 mistake, and then you get fireworks.

So my response is simple. Learn to distinguish between chemistries and most of your worries are moot. LFP does not catch fire therefore no fumes. As to temperature, LFP dont waste charging energy on heat. The cold weather issues can be dealt with by simple mat heaters and insulated boxes where cold is an issue.

AMost EV's have a pretty good BMS in them, their batteries mostly burn from accident damage, but there's no off switch for the fire.

Statistically per miles driven there is more probability of a fire in a gasoline powered car.

And is it easier to parallel batteries and the balancing issues, or series them for the BMS issues ?

It depends, with Tesla modules that contain 6S74P you are best to use the module BMS and some communication device to aggregate the data, so that you have flexibility in configuration. However most large Inverters would run at 48 v so you would only series two and parallel as many as you need to get the storage capacity desired. I currently used Nissan Leaf modules which have a center tap an therefore parallel very easily. The modules technically 2S2P but the center tap allows me to run BMS wires to each parallel group of cell. As far as the the rats nest of wires I guess that is a matter of choice. I have seen a lot of Off Griders pictures with exposed Romex. My BMS requires 17 20awg wires (16 cells) and one can make it neat or a rats nest. What I dont have to think about is a hydrometer, distilled water, desulfation or all the other issues when I had Lead Acid batteries. It really gets down to experience and personal preferences. To me Lithium batteries are my preferred choice but ironically both my EVs use small Lead Acid batteries to boot the electronics.That tells me they can coexist and the us vs them attitude prevalent on some of these threads is unfortunate.

[AaronTSchultz]

I separated this part of your post because I wanted to comment about the BMS. One of the things that has kept me from using used Tesla packs is the integration of the BMS. I currently own two BMSs, one for my Hybrid Inverter pack and one for a GEM NEV. Both work fine with the used Nissan Leaf modules that I have found at reasonable prices. At some point I may want to increase the pack size of my Hybrid Inverter and may go with a LFP pack using the same BMS. The alternative would be some Tesla modules. I have more confidence in the quality of Tesla Modules and am not sure I want to throw more money at Leaf Modules because the are often less than 75% of capacity. I will look into the link you provided and follow your build to see how the integration of the BMS works. At some point you may want to start a new thread so that future readers can get the benefit of this constructive thinking. There is a section for off grid RVs.

You tube "Morton's on the move" has a pretty good series 3 medium length videos on his install of this, with schematics which is helpful.

Did I get the reasoning right that I would just double the safety voltage for a 48v system compared to the safety voltage in a 24v system?

[AaronTSchultz]

Ha Ha. I hope your wife can deal with hot showers every 3 or 4 days. Rest of time they will be cold, or you have no TV. Where are all the PV panels going to go ?

So, my only criticism on the new tech of Li batteries, is:
1) many, many stories of Li batteries burning, both cell phones, laptops, Teslas and 787's Even the good designers don't always get it right.
2) the fumes while burning are extremely toxic - requiring outdoors venting. once they start, they don't stop.
3) poor cold weather performance below 40F, is bad, so you need to keep them indoors, violating #2
4) you only get 1 mistake, and then you get fireworks.

Lead acid has it's own issues, but with a 100 year old tech, there are reasonable workarounds that got developed. Not may batteries turn into a Hindenburg, even with lots of H & o2 around.

Most EV's have a pretty good BMS in them, their batteries mostly burn from accident damage, but there's no off switch for the fire.

As long as each group of parallel cells has a BMS that can shut down the charge or discharge when you exceed your setpoints you are ok. Getting a well engineered BMS that can reliably do that is a lot harder then it appears. There's lots of connection wires, sometimes ribbon cables tame that rats nest. Having a RELIABLE BMS that only triggers on real faults and is not so complex that it only has a MTBF of 10,000 hours is tough. Thermal runaway is another thing the BMS has to watch for too. That doubles the complexity

And I'll say it, because I have already trained my wife: Only use large power when you see crisp shadows on the ground.

Car batteries are generally higher energy density, LFP (LiPo4) are safer.

https://www.consumerreports.org/safety-recalls/why-lithium-ion-batteries-still-explode-and-whats-being-done-to-fix-the-problem/

But still in a bus conversion, you don't have enough space for all the panels for an all-electric household. You have to park in the sun, and then run the air conditioner.
Going to propane for heating and cooking takes a lot off the electrical demand. Any insulating you can do in the conversion process will help immensely.

And is it easier to parallel batteries and the balancing issues, or series them for the BMS issues ?

I'll ask the questions I don't have good answers for, maybe you have them.

I hear you, I tried to convince her to do a propane hot water heater, but she was set in her way. She is going to get a 7 gallon electric 120V heater that reviews said only takes 10 or 15 minutes to heat it up, and then is well insulated to keep it hot for up to 24 hours. We are going to be boondocking mostly with 100Gal fresh, so we will still use short showers of on off usage.

1. Lol I like the 787 reference I understand they don't get it right and I'm hoping that the charging/discharging tolerances are far enough took keep an easy load on the batteries, that being said hopefully we are not the 1 off story of this thing blowing up .
2. I will be mounting battery and hardware in the understorage of the bus. It will be a box of wood, insulated with 2" foam board on each side as well as a thermostatically controlled heating blanket. This will then be in the understorage with a wood floor, and above a steel floor of the bus, 2" of foam, subfloor and actual flooring. It will most likely burn down before it makes it through the steel. I am not worried about the batteries getting too hot to need fluid coolant run through them, as we will not be charging/discharging even close to the "constant charging voltage/amperage)".
3. See #2 with the heating blanket keeping the bay at roughly 60-70 F.
4. We are planning on having redundancy in the system where even if the charge controller gets reset, we will have the battery monitors with alarms and shunts to disconnect the system. We will also have the color GX monitor in the cabin of the bus showing the voltage and current in and out of the batteries, so if something starts going wrong, I will be able to see and hear it. I am also thinking of making a slide for the battery box with no stoppers to it, so I will technically be able to just drag the box out of the bus and drive the bus away to a safe distance if the apocalypse happens.

We do have BMS for each battery pack and an aftermarket BMS board that replaces the Tesla board for battery management and it can target the individual cells. There are a lot of You tube tutorials using this setup that a year or 2 in are having a lot of success, (I know that can change at any moment, lol).

We don't see us using A/C unless necessary as we will also have 2 maxxair fans in the bus where the old safety hatches used to be. This will create a nice draft and hopefully reduce the need for AC. Usually we will be parked in sun for it to be necessary to run the AC. Also trips in the summer are north, and trips in the winter, are south.

we have space for 8-300W panels as we have a flat front 40' bus. And normally we will not need even close to 8000W, but I cannot determine when the fridge will kick on and the water heater will kick on. I am just planning for that one day where we have to do all the chores, and shower, and have the fridge and AC running, as well as cooking on both the cooktop and the instant pot.... that is what I'm planning for, lol, a bit overkill, but I would rather have it and be ok boondocking than not have it.

We also just got done with sprayfoam in the bus and have about an R10 on the roof, and R12-15 on the walls. This will also be before any wood goes up for finishing work. and for heating we have a mini fire stove that puts out 35K BTU max. Saying that the temp increases quite a bit when I run a 5K BTU electric 1500W heater, I think we will be good for heat.

Lastly I don't know about the balancing with the 48V or the 24V systems. I have seen videos of people that have them in parallel and are balancing them, and series that have balancers connected to each one. I will have to research that.

I would ideally like the 48v system as I don't think a 24V system will be able to have large enough cable to run 8000W. it's like 333 Amps. I guess I could use 2-2/0 cables running to the same breaker, but that might totally fry the breaker unless it's huge.

Anyhow, Thanks for the questions, I hope I answered most of them

[Ampster]

................... I also have 4 individual BMS, 1 for each battery with a new circuit board that was attached at the re-seller 057Tech.com . I will have the high/low charging settings set on the mppt charger, and on the hybrid inverter/charger, and I will also have battery monitors with high/low shunts that can disconnect the batteries. On top of all that I am going to have a thermostatically controlled heating blanket for cold weather, and I shouldn't have any problem with heat in the batteries.

I separated this part of your post because I wanted to comment about the BMS. One of the things that has kept me from using used Tesla packs is the integration of the BMS. I currently own two BMSs, one for my Hybrid Inverter pack and one for a GEM NEV. Both work fine with the used Nissan Leaf modules that I have found at reasonable prices. At some point I may want to increase the pack size of my Hybrid Inverter and may go with a LFP pack using the same BMS. The alternative would be some Tesla modules. I have more confidence in the quality of Tesla Modules and am not sure I want to throw more money at Leaf Modules because they are often less than 75% of capacity. I will look into the link you provided and follow your build to see how the integration of the BMS works. At some point you may want to start a new thread so that future readers can get the benefit of this constructive thinking. There is a section for off grid RVs.

[Mike90250]

........ Since my wife wants everything run off electricity we are going to need an 8000W hybrid inverter charger..........

Ha Ha. I hope your wife can deal with hot showers every 3 or 4 days. Rest of time they will be cold, or you have no TV. Where are all the PV panels going to go ?

So, my only criticism on the new tech of Li batteries, is:
1) many, many stories of Li batteries burning, both cell phones, laptops, Teslas and 787's Even the good designers don't always get it right.
2) the fumes while burning are extremely toxic - requiring outdoors venting. once they start, they don't stop.
3) poor cold weather performance below 40F, is bad, so you need to keep them indoors, violating #2
4) you only get 1 mistake, and then you get fireworks.

Lead acid has it's own issues, but with a 100 year old tech, there are reasonable workarounds that got developed. Not may batteries turn into a Hindenburg, even with lots of H & o2 around.

Most EV's have a pretty good BMS in them, their batteries mostly burn from accident damage, but there's no off switch for the fire.

As long as each group of parallel cells has a BMS that can shut down the charge or discharge when you exceed your setpoints you are ok. Getting a well engineered BMS that can reliably do that is a lot harder then it appears. There's lots of connection wires, sometimes ribbon cables tame that rats nest. Having a RELIABLE BMS that only triggers on real faults and is not so complex that it only has a MTBF of 10,000 hours is tough. Thermal runaway is another thing the BMS has to watch for too. That doubles the complexity

And I'll say it, because I have already trained my wife: Only use large power when you see crisp shadows on the ground.

Car batteries are generally higher energy density, LFP (LiPo4) are safer.

https://www.consumerreports.org/safety-recalls/why-lithium-ion-batteries-still-explode-and-whats-being-done-to-fix-the-problem/

But still in a bus conversion, you don't have enough space for all the panels for an all-electric household. You have to park in the sun, and then run the air conditioner.
Going to propane for heating and cooking takes a lot off the electrical demand. Any insulating you can do in the conversion process will help immensely.

And is it easier to parallel batteries and the balancing issues, or series them for the BMS issues ?

I'll ask the questions I don't have good answers for, maybe you have them.

[AaronTSchultz]

I am going to be using a victron 150/60 mppt that is also lithium compatable. It will also let me set the charge, float.. ect settings. Since 2400 w is 50a at 48v and this thing can do 60a at 48v. I should be fine with that. If the safe charging at 24v is about 24.6 will the safe charging limits high be 49.2v on a 48v setup? And float at 49.0 volts? And if the low is 19 would the low be 38? Please help with the voltage settings, as the amperages should not be a problem at 60 and the programming setting with victron are in voltages. Also the hybrid inverter/charger will be limited at about 12a so we won't blow fuses when plugged in at my brothers house.