Designing car system for road tripping/camping - 12V or 24V?

[Atom Tan]

I am designing a small LFP-based system for use while traveling in my Subaru. I have built a 640Wh 12V system to learn and now have questions.

I am a novice at solar systems. I have a decent understanding of AC & DC power. I understand batteries fairly well from years as a flashaholic. I am very comfortable working on AC systems, less experience with DC systems. Not an expert on either, but I mostly understand the boundaries of my knowledge. I get up to speed pretty quick.

I will lay out my thoughts, ask questions inline, and then summarize questions at the end. I apologize for the length, but want to be comprehensive.

Constraints and Context
My primary design constraints are interior space for components, and roof space for panels.

Cost matters, but is not the primary limiting factor.

I tend towards overkill, infusing margin to avoid problems. I am cognizant of failure modes and edge cases and try to avoid them, especially when they might hurt. I may need some help slowing my roll here.

The battery needs to fit into a single second row footwell 19” wide and 10.75” deep. Height can be up to 14". I’d prefer a shape that is squat rather than tippy to ensure stability. I have the other footwell earmarked for a water tank so it will not be available. The water tank will also approximately balance the weight of the battery with an equal-ish weight of water. I have the trunk earmarked for other things.

My roof rack is a platform with a 2” raised lip. Inside dimensions are 43” wide x 59” long. I’d prefer to keep any panels inside that, but would be willing to add 2-3" to each dimension and overhang the rack a bit.

Other parts (SCC, inverter/charger, DC-DC charger, 24V-12V stepdown) may live above the battery, or in other locations. Once I make the battery decision, I'll Tetris all of the other stuff.

Planned Loads

• Critical - 864W:
  • Dometic CFX3 75DZ fridge
    • 12/24V DC
    • Estimating a pessimistic draw of 36W/hr for 864 Wh/day
    • True worst case load around 60W/hr for 1440 Wh/day
    • I’m planning to put a logging meter on this load so that I can get a longitudinal sense of actual consumption, but that’s not helping me right now.
• Important - 1240W:
  • 4 USB QC 20W ports to charge various devices - phones, iPads, lights
    • Estimate 5 devices getting a full charge from empty in 3 hours once/day.
    • Likely to be less, probably 2-3 full charges/day.
    • Estimating 300 Wh/day for all 5 charges
  • 2 USB C PD 100W ports for laptops
    • Estimate full charge from empty in 3 hours once/day per laptop
    • Estimating 600 Wh/day, though it’ll probably often be less than that
  • 1000W AC hairdryer
    • Estimate 170 Wh/day at 10 min/day
    • I have been gently advised by my travel partner that this is non-negotiable.
  • 1000W AC electric kettle
    • Estimate 170 Wh/day at 10 min/day
    • This could be a kettle on a propane stove.
• Optional/Occasional:
  • DC Fans - USB-charged/battery powered or direct DC powered
  • USB-charged LED lights
  • Viair compressor (as needed)

So, critical load of 864 Wh/day and an important but still optional load of 1240 Wh/day, totaling 2104 Wh/day. I want to be able to run the fridge for 2-3 days without worrying about bad solar days. So, seems I need a minimum of about 1.7 kWh of battery (that’ll serve the fridge alone for 48 hours), and ideally closer to 4.2 kWh, which will serve both the fridge and the important loads for 48 hours. That puts me at 5.1 kWh of battery capacity needed.

Inverter Sizing
Planned AC loads are almost 100% resistive and top out at 1000W. I'd think I could get by with a 1000W inverter, except for efficiency losses in the inverter. 1500W would give me some headroom and account for that.

I think I want an inverter/charger to allow for charging using shore power, and, ideally, passthrough power when plugged in.

The Victron inverter/chargers are HUGE. Looking at Xantrex and Samlex but haven't dug in that deeply yet.

Batteries, capacity, and system voltage
I was initially thinking I’d do a 12V system. However:

Most 100Ah batteries have a BMS with a max continuous discharge rate of 100A, and a 1500W inverter will need to draw 150A at 12V DC at max output. (I recognize that the inverter has a peak capacity, but I’m not concerned with designing for the peak draw.)

So I either need 2 100Ah batteries in parallel, or a larger battery with at least a 150A BMS. A two battery arrangement is attractive from a fault tolerance standpoint but may not be feasible from a space standpoint. If I have to choose between fault tolerance and capacity, I’ll pick capacity/discharge.

A 1000W inverter pulling full rated 100A discharge from that one battery also seems to run it pretty hard.

That BMS discharge constraint informs the space constraint, in that 2 “normal/drop-in size" 100Ah batteries (in parallel) will not fit in the footwell. Most 100Ah 12v batteries seem to be ‎around 12-13” long x 6-7” wide.

The 12V batteries I have found with a 150-200A BMS are 200Ah+, and are also physically too big to fit in the footwell. Examples include the AmpereTime 200 Plus, at 20.5” long. Even if I could fit one, which I can't, I could not fit two, and one at 12V is only 2.5 kWh, and I’m seeking at least 4.2 kWh.

Standing a “normal” form factor battery on a small end puts one battery terminal close to the floor, so that’s out due to concerns about instability and damage to the terminal/cable. However, the BattleBorn BBGC2 or BB5024 would work as a pair side by side without that concern. Problem is that only gets me 2.5 kWh of capacity.

All of the foregoing pushes me to 24V batteries, since the max draw of a 1500W 24V inverter would only be 75A at 24V DC. I can wire the inverter with 4AWG. The Dometic and USB chargers will happily run on 24V, so that’s fine. If I end up with any significant 12V loads, I can get a 24V-12V converter.

So, 24V looks like the logical choice. However, if I’m leaving 12V behind, is there any compelling reason NOT to just jump to 48V? I’d need to add a 48V to 24V (or 12V) DC converter, but that’s not a huge deal, I don’t think. I will likely use this system inside the house when I’m not on the road, mostly as a power outage hedge, but do not plan to put this battery into a full house system. If I go that direction, I’ll probably get a stack of server rack batteries.

I know I'd need higher OCV panels if I went 48V.

I’d definitely appreciate thoughts on other battery manufacturers that might have form factors that’ll work in that footwell space. BigBattery has some interesting options but I have read enough to eliminate them.

I have found Electric Car Parts Company, who seem to have some really nice 12, 24 and 48V batteries that are all 5.12 kWh, and all 17” x 10.5”. They are expensive, but seem ideal for my use case, lacking only low temp charging protection.

Panels
I am having difficulty optimizing the roof space available. The roof rack is a platform style with a low (2”) circumferential lip. As I said above, the platform inside the lip measures 59” x 43”. Willing to add 2-3" to length and width if needed. I’d welcome suggestions for rigid panels that would make the most efficient use of that space.

I will need to consider portable panels to augment while in camp. I’m not in love with the non-rigid folding panels, as they seem unwieldy, but need to look into those more. I’m considering a suitcase style rigid panel, but they are hard to store. I like the CIGS flexible panels as a possibility.

I think I'll top out at 350W for rooftop panels and know that I will be severely under paneled vs my battery capacity. I don’t know to what degree that will be an actual problem and would welcome input. Since I’ll allow for shore charging, that will help to some extent, but don’t want to rely upon that.

Alternator Charging
My Subaru has a “smart” alternator that stops charging under various circumstances. If I were not under paneled, I’d probably skip any DC-DC charging, but it seems like this might be worth doing given that I’ll probably max out at 300W-350W on the roof. Having this ability might be very helpful.

I know I’d need a 12V->24V DC-DC charger, like the Victron Orion 12/24-10. I can’t determine whether the negative connections for the 12V input and the 24V output are on a common bus, but suspect they are.

Are there any issues using the chassis of the car as a negative ground path for a 24V house system when the car voltage will remain 12V? If my assumption above of a common bus is true, then I imagine the answer is no, but want to confirm.

Summary of Major Components

• 24V 5.1 kWh Battery
• 1500W 24V inverter/charger
• Panels
• SCC
• DC-DC charger

Question summary:

1. Am I correct that a 1000W inverter is not sufficient, and I should go with a 1500W?
2. Is it inadvisable to run a 1000W inverter hard up against the 100A continuous discharge rating of a 12V 100Ah battery?
3. If I’m being pushed to 24V battery systems due to inverter draw, is there a compelling reason to just go 48V?
4. Are Electric Car Parts Company batteries of good quality?
5. Any other battery options I should look at?
6. Any ideas for other batteries of 5.1 kWh that will fit into a 19” x 10.75” space?
7. Any recommendations for high quality 1500W 24V inverter/chargers, with priority and premium for small dimensional size.
8. Any recommendations for the maximum wattage solar panels possible with an OCV of > 28V DC, taking up a 59” x 43” area?
9. How much of a problem will my under paneling be, practically speaking?
10. Are there any issues using the chassis of the car as a ground path for a 24V house battery system when the car voltage will remain 12V?
11. What should I have asked that I did not?

 

[pharmboyjoe]

Q: Am I correct that a 1000W inverter is not sufficient? A: Eventually, YES!

I look at it like this: the wall plugs in your house are rated 15 amps @120v. That's 1800 watts right there even if you have a Unicorn* 1oo% efficient inverter. This, to me, implies you should have at least 2k inverter (to cover power loss from resistance, etc.) just to match what you're used to at home. I have "saved" money on smaller inverters only to keep needing more and more until now and I'm using a 3000 watt one on my 24v LiPo. My advice: get the biggest one you can afford now. Might just save you money in the long run!

*doesn't exist

 

[djnorth]

From my tests at home, both a typical electric kettle and hairdryer draw 1500W of power each. Accounting for inverter losses, you’ll need a 2000W inverter for them.

I fully understand the hairdryer is a non-negotiable item for some and am working on a compact 2kW system for these two items, coincidentally.

 

[pharmboyjoe]

At the risk of getting banned on my 2nd post, wouldn't it be great if we just had 1-1/2" more These guys measure 20.47 x 10.59 x 8.66 inches:

https://www.amazon.com/Ampere-Time-5-12kWh-LiFePO4-Lifetime/dp/B09BR5WJJ7/ref=sr_1_2?crid=3GF4LKS5EEPON&keywords=24v+200ah&qid=1658352700&sprefix=24v+200ah%2Caps%2C178&sr=8-2

https://www.amazon.com/LiFePO4-Battery-battery-Support-Off-Grid/dp/B08Z3TV494/ref=sr_1_4?keywords=24v+200ah&qid=1658353816&sr=8-4

 

[kenryan]

I generally agree with the 2000 watt inverter suggestions, but I also think you could get by with a 1500 watt.

I think you have greatly overestimated your loads.

Your "critical load", instead of 864 watt hours per day, might be closer to 250. Link

2 phones 10 watt hours
2 tablets 60 watt hours
2 laptops 80 watt hours
hair dryer 175 watt hours
electric kettle 175 watt hours

So, instead of 1240 watt hours for your "important loads", maybe 500 is more realistic?

That's just 750 watt hours. Add 100 for a fudge factor and you are at 850. That's a lot less than your number of 2104.

Does that make the 12 volt Battleborn battery solution more attractive? Two 100 amp hour Battleborn batteries should power a 2000 watt inverter okay.

Your charging loads refrigerator and lighting loads can bypass the inverter, so no inverter efficiency losses for them.

The only thing that I see you gaining by going to 24 volt is smaller wires from the battery to the inverter. Seems that would only be important if the inverter was a long way from the batteries.

I think your bigger problem is going to be charging, and from that perspective the bigger the battery, the better, allowing you more autonomy between shore power charges.

 

[Atom Tan]

From my tests at home, both a typical electric kettle and hairdryer draw 1500W of power each. Accounting for inverter losses, you’ll need a 2000W inverter for them.

I fully understand the hairdryer is a non-negotiable item for some and am working on a compact 2kW system for these two items, coincidentally.

I took an actual wattage measurement from each while running. They are each (barely) under 1000W sustained (hair dryer on low heat where she uses it), though I can't accurately measure inrush.

 

[Atom Tan]

At the risk of getting banned on my 2nd post, wouldn't it be great if we just had 1-1/2" more These guys measure 20.47 x 10.59 x 8.66 inches:

https://www.amazon.com/Ampere-Time-5-12kWh-LiFePO4-Lifetime/dp/B09BR5WJJ7/ref=sr_1_2?crid=3GF4LKS5EEPON&keywords=24v+200ah&qid=1658352700&sprefix=24v+200ah%2Caps%2C178&sr=8-2

https://www.amazon.com/LiFePO4-Battery-battery-Support-Off-Grid/dp/B08Z3TV494/ref=sr_1_4?keywords=24v+200ah&qid=1658353816&sr=8-4

Yeah, those AmpereTime 200Ah Pluses are attractive, but too big. Even if I were able to find the 20.47" width side to side by building a platform at height with the transmission hump and letting them creep onto that, I can't put two of them side by side, because the 10.5" front/back depth in the footwell is a hard limit.

 

[Atom Tan]

Q: Am I correct that a 1000W inverter is not sufficient? A: Eventually, YES!

I look at it like this: the wall plugs in your house are rated 15 amps @120v. That's 1800 watts right there even if you have a Unicorn* 1oo% efficient inverter. This, to me, implies you should have at least 2k inverter (to cover power loss from resistance, etc.) just to match what you're used to at home. I have "saved" money on smaller inverters only to keep needing more and more until now and I'm using a 3000 watt one on my 24v LiPo. My advice: get the biggest one you can afford now. Might just save you money in the long run!

*doesn't exist

I don't quite understand how folks deal with using an inverter that can draw more than your battery can deliver. I mean, sure, size the wire for max draw, fuse it properly, but end of the day, if you exceed that BMS limit, it's going to drop the inverter on its head when it shuts down. Is it really a problem in practice?

 

[Atom Tan]

I generally agree with the 2000 watt inverter suggestions, but I also think you could get by with a 1500 watt.

I think you have greatly overestimated your loads.

Your "critical load", instead of 864 watt hours per day, might be closer to 250. Link

2 phones 10 watt hours
2 tablets 60 watt hours
2 laptops 80 watt hours
hair dryer 175 watt hours
electric kettle 175 watt hours

So, instead of 1240 watt hours for your "important loads", maybe 500 is more realistic?

That's just 750 watt hours. Add 100 for a fudge factor and you are at 850. That's a lot less than your number of 2104.

Does that make the 12 volt Battleborn battery solution more attractive? Two 100 amp hour Battleborn batteries should power a 2000 watt inverter okay.

Your charging loads refrigerator and lighting loads can bypass the inverter, so no inverter efficiency losses for them.

The only thing that I see you gaining by going to 24 volt is smaller wires from the battery to the inverter. Seems that would only be important if the inverter was a long way from the batteries.

I think your bigger problem is going to be charging, and from that perspective the bigger the battery, the better, allowing you more autonomy between shore power charges.

I'd love to have way overestimated my critical load. I'll tell you how I got there so you can poke holes. It's a 75DZ, which is a two compartment unit, larger than the 35 you linked to, capable of running either compartment (or both) as a freezer. I hope/plan to make use of the freezer function on one compartment, so I sized with that in mind.

I took some of my draw estimates from here and used this result as my primary basis:

Ambient@<80F, Internal-1@37F, Internal-2@5F, pre-chilled): 19 hours:: (40Ahr * .8)/19 hours = 1.68Ahr/h

I expect that my car will be > 80F ambient quite a bit while parked at camp, which will increase draw to maintain temperature, and assumed that I wouldn't have both compartments completely full all the time, which will reduce efficiency. Based on that, I scaled up a bit.

That said, I take your point that I may be overly pessimistic. I just don't know how to better assess true draw on the fridge without some real world testing over time.

I can probably reduce the laptop figures a bit. Say, 60 Wh/hr per laptop over 3 hours for 360 Wh/day vs 600. Similarly, I think I could reduce the USB QC figures a bit to say, 200 Wh/day. That would get "important" loads down to 900 Wh/day for a total of 1764 Wh/day.

I agree 100% that my biggest issue is charging.

 

[45North]

Keep it simple!
Everything Automotive and RV has been available in 12v for over 100 years. Inexpensive parts and spares are available anytime anywhere. There's a rich variety of 12v gadgets available for off-grid living and camping.
Don't overthink it.

 

[camelCase]

I think you may be seriously overestimating the amount of solar watt hours you be able to achieve with such a small vehicle and minimal roof space. Given the dimensions of your roof rack, you could fit a couple of 100w panels, but fitting anything much bigger without overhanging a lot will be a struggle. Since you're mounting them flat, you also need to derate them for power calculations. Probably by 40 percent or more.

All that to say you'll probably need alternator charging more than you expect. The fact your car has a smart alternator shouldn't be a problem if you choose an appropriate DC-DC charger and wire it up correctly.

About your question regarding batteries that fit in the foot well, I just purchased one of these batteries to play with. https://www.ebay.com/itm/275386910964. I'm definitely not recommending anyone buy one as I haven't seen any reviews and I can't say yet if they are any good or not, but they were the cheapest lifepo4 batteries on eBay or Amazon that I could find that had a <32f charging cutoff. The reason I brought it up was that they are more compact with dimensions of 9.5 x 7 x 8 inches which would allow you to fit two in the space you have and in parallel or series would support 200a of current draw. Personally, I would stick with 12V for simplicity sake.

 

[Atom Tan]

I think you may be seriously overestimating the amount of solar watt hours you be able to achieve with such a small vehicle and minimal roof space. Given the dimensions of your roof rack, you could fit a couple of 100w panels, but fitting anything much bigger without overhanging a lot will be a struggle. Since you're mounting them flat, you also need to derate them for power calculations. Probably by 40 percent or more.

All that to say you'll probably need alternator charging more than you expect. The fact your car has a smart alternator shouldn't be a problem if you choose an appropriate DC-DC charger and wire it up correctly.

About your question regarding batteries that fit in the foot well, I just purchased one of these batteries to play with. https://www.ebay.com/itm/275386910964. I'm definitely not recommending anyone buy one as I haven't seen any reviews and I can't say yet if they are any good or not, but they were the cheapest lifepo4 batteries on eBay or Amazon that I could find that had a <32f charging cutoff. The reason I brought it up was that they are more compact with dimensions of 9.5 x 7 x 8 inches which would allow you to fit two in the space you have and in parallel or series would support 200a of current draw. Personally, I would stick with 12V for simplicity sake.

Yep, finding panels that maximize wattage in the space available is definitely a place where I'm looking for help. Here's a 400W that is basically 68" x 45". So, bigger than my ideal situation, but not egregiously so. No idea if it's any good, and have doubts that the voltage is high enough for a 24V system.

I think I should be able to get 300W up there, mayyyybe 350W, and that's going to be the limit. The problem I'm running into is that it's hard to shop for solar panels by dimension, since that's not a common constraint. The derating is something I hadn't thought about, and is a great point.

I am planning to have folding/temporary panels while parked, but recognize that will be a band aid more than anything.

Thanks for confirming that DC-DC charging will be a must.

 

[pharmboyjoe]

I don't quite understand how folks deal with using an inverter that can draw more than your battery can deliver. I mean, sure, size the wire for max draw, fuse it properly, but end of the day, if you exceed that BMS limit, it's going to drop the inverter on its head when it shuts down. Is it really a problem in practice?

However, a 24v lifepo4 battery is fully charged @ 29.2 volts and in theory a 100ah BMS limits you to 2,920 watts. I'd say rounding up is a good idea... and why I went with 3000 watt inverter.

 

[Atom Tan]

However, a 24v lifepo4 battery is fully charged @ 29.2 volts and in theory a 100ah BMS limits you to 2,920 watts. I'd say rounding up is a good idea... and why I went with 3000 watt inverter.

See, that's where I would have rounded DOWN to ensure that the inverter never exceeds battery capacity, because I'd be inclined to treat the least capable link in the chain as the material constraint. That said, I have no meaningful experience with inverters or how to size for them.

 

[pharmboyjoe]

Yeah, those AmpereTime 200Ah Pluses are attractive, but too big. Even if I were able to find the 20.47" width side to side by building a platform at height with the transmission hump and letting them creep onto that, I can't put two of them side by side, because the 10.5" front/back depth in the footwell is a hard limit.

They are 24v, not 12, so one of them is 5.12kw. I have 2 older 12v systems that use 12v AGM batteries and the mini fridge shuts off at low voltage way too soon. With my newer 24v system (and an Orion IP20 24/12-Volt 70 amp DC-DC Converter) it's a full 13.2 volts til the end. I can't help but think the extra "headroom" and constant voltage is better for the life of all the 12v gadgets.

 

[wholybee]

2100Wh per day seems reasonable, and is almost exactly my daily use on my boat. That is about 160Ah per day at 12V. I would stick with 12V unless you step up to a 3000W inverter. I would suggest 300Ah battery at 12V to get you through a day of rain. 200Ah is just barely one days worth of power if you run it completely flat-it could work, but doesn't leave much room. 100Ah is not enough. 350W of solar is not enough. Expect about 110Ah per day out of that much solar, on a nice sunny day, much less on a cloudy day. How much of a problem is that? You will need to run your engine every single day to charge.

So, that means you are going to use the alternator for a lot of charging. Replace your Subaru alternator with a good externally regulated alternator, and add a Wakespeed Regulator. That will allow you to charge your Lithium directly from the alternator without a DCDC charger. Then you can charge your Lithium at 100A(whatever the alternator is rated at) instead of 30A or whatever the DCDC charger is. You are 50Ah short from what you can expect from solar, so with a 100A alternator that is 30 minutes vs 1.5 hours per day to charge.

Look at Kilovault batteries. Not sure if they have one that will fit your space, but they have 200Ah and 300Ah models that have 150A or 200A discharge ratings. They are among the better drop in batteries available and much cheaper than comparable quality drop-ins.

If you do go 24V, then yes, it can (and should) share a ground with the 12V system.

 

[djnorth]

Thanks for confirming that DC-DC charging will be a must.

Targeting for 5.1kWh of battery, I would prioritize resources towards alternator charging over solar.

 

[No Idea]

Also a flashaholic and now spend my money and time learning this new hobby.

You can get 12v hairdryers about 180w or cordless hairdryers with more oomph that you can charge at lower amps. Also easy to get lower watt kettles, just longer to boil but end result the same. Even 12v kettles are available to avoid a big inverter.

All this keeps your current draw down and inverter size down or avoid inverter. It is your hairdryer and kettle that seems to be driving your voltage, size and weight requirements.

I have PD and QC3 ports and 12v sockets in my ongoing build but I could not find the PD 100w so I bought two small 200w inverters that can take 4 AC laptop chargers and they plug into the 12v sockets.

Mine are 2p4s (50Ah cells) for about 1280wh and will fit in your footwell behind a seat.

I have 6 x PD / USB combo ports and 2 x 12v sockets.

JBD BMS the TR16 capacity monitor both from Alixpress.

I can fit 20A charger, 10A SCC and 2 x 200w inverters inside the can and pull them out when needed.

Plastic ammo cans to keep weight low. Pretty cheap build.




The inverters are not much bigger than a large phone.

 

[Atom Tan]

They are 24v, not 12, so one of them is 5.12kw. I have 2 older 12v systems that use 12v AGM batteries and the mini fridge shuts off at low voltage way too soon. With my newer 24v system (and an Orion IP20 24/12-Volt 70 amp DC-DC Converter) it's a full 13.2 volts til the end. I can't help but think the extra "headroom" and constant voltage is better for the life of all the 12v gadgets.

Ah, you're right. I misread!

 

[Atom Tan]

2100Wh per day seems reasonable, and is almost exactly my daily use on my boat. That is about 160Ah per day at 12V. I would stick with 12V unless you step up to a 3000W inverter. I would suggest 300Ah battery at 12V to get you through a day of rain. 200Ah is just barely one days worth of power if you run it completely flat-it could work, but doesn't leave much room. 100Ah is not enough. 350W of solar is not enough. Expect about 110Ah per day out of that much solar, on a nice sunny day, much less on a cloudy day. How much of a problem is that? You will need to run your engine every single day to charge.

So, that means you are going to use the alternator for a lot of charging. Replace your Subaru alternator with a good externally regulated alternator, and add a Wakespeed Regulator. That will allow you to charge your Lithium directly from the alternator without a DCDC charger. Then you can charge your Lithium at 100A(whatever the alternator is rated at) instead of 30A or whatever the DCDC charger is. You are 50Ah short from what you can expect from solar, so with a 100A alternator that is 30 minutes vs 1.5 hours per day to charge.

Look at Kilovault batteries. Not sure if they have one that will fit your space, but they have 200Ah and 300Ah models that have 150A or 200A discharge ratings. They are among the better drop in batteries available and much cheaper than comparable quality drop-ins.

If you do go 24V, then yes, it can (and should) share a ground with the 12V system.

Thanks. You're right that a HO alternator would be a good choice, and the Wakespeed and Kilovault battery pointers are helpful. Those Kilovault batteries seem to have it all in terms of features, except for a form factor that meets my desires. I'm not quite ready to rethink that, but I'm filing it away.