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Discussion Starter #1
Vehicle Info
1975 Volvo C304 TGB 1314
17ft long 6ft wide roughly 7ft tall
6x6 with portal axles and locking differentials
factory weight is about 6400lbs
all up weight with camping rig finished will be at or around 10k lbs



Hello Guys! I need some help with planning this build. I have been back and forth on which small diesel engine to put in this bad boy, but I think the future is in electric I am just trying to determine if the future is now!

I have a good amount of skills and access to most tools including cnc mills and lathes. I want to make it as least complicated as possible because traveling the country and eventually the world would there are not much for replacement parts im sure.

I would love to get a 200 mile range (more would be great) I think 200 miles would at least get me to a different scenery until I could charge the batteries by solar/wind/diesel generator.

I dont need much for performance it does need to pick itself up some offroad trails. I would like the vehicle to reach 65mph and cruise there comfortably

Money is always an issue. I dont quite have a budget yet as I am still in the planning and feasibility stage. Lets talk dollars after we determine its possibility.

as of right now I have zero parts except the rolling chassis.


Where do I start?
 

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Very cool project. :)

One thing to consider from the beginning is the battery pack. It needs to have enough capacity, it needs to fit into the vehicle, and you need to be able to buy it.

While not big by RV standards, this is still a massive box to drive electrically, with poor aerodynamics and six off-road truck tires. If you can estimate how much energy it will take to move, you can estimate your energy storage requirement... and it is going to be huge.

For instance, if you need one kilowatt-hour to move one mile, your 200 mile range will require 200 kWh of battery. That's double the largest Tesla pack, and more than the largest pack in the recently announced Rivian R1T pickup. That's a lot of battery to buy, a lot of space taken up in the vehicle, a lot of weight that can't be used for camper equipment, and a lot of energy to replace when you get to somewhere that you can recharge.
 

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I don't know if comfortably cruising at 65 mph is a realistic expectation. A stock C304 probably can't reach that speed. Although the limitation of the stock vehicle may just be engine power, making the electric version more powerful may not be practical, and cruising at that speed in a vehicle with the aerodynamics of a barn will consume a lot of energy.

Wikipedia lists the top speed as 62 mph (100 km/h). There are better sources, of course, but that gives an idea of what is reasonable in these vehicles.
Volvo C303
(a C304 is a 6X6 variant of a C303)
 

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I would love to get a 200 mile range (more would be great) I think 200 miles would at least get me to a different scenery until I could charge the batteries by solar/wind/diesel generator.
This raises the issue of how you would charge the vehicle. That's a little different for RVs than for most vehicles, because RV owners normally want to go different places than the average EV (that is, to remote places or natural areas, not cities). Another difference is that many RV owners stay at campgrounds with services (including power), while others stay in places with no services at all... and power service is significant to recharging.

The idea of an electric RV comes up frequently. These are some of the previous discussions, which might provide some possible answers, or raise more questions. They cover a range of vehicle types and sizes, which should be kept in mind for context. Some are also quite old, so information about available components (particularly batteries) are out of date.
Common themes include:
  • the high energy consumption of a large vehicle
  • plans for solar charging, usually with unreasonable expectations
  • need for a much larger motor compared to typical conversions, or multiple motors
  • the RV never actually gets built or converted
There have also been several discussions of hybrid (usually diesel-electric) RV ideas, but if you're not planning a hybrid these would probably not be very helpful, so I left out most discussions of potential project vehicles with short electric-only range. I also found a few more discussions which appeared to contain no useful information, so I left them out.
 

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Since you have the troop carrier - not the ambulance or radio unit with the taller body - there isn't much headroom. One set of specifications that I found online say that there is 1250 mm (49") of floor-to-ceiling headroom in the cargo area. Do you plan to add a pop-top (which appears to be the popular choice), or a raised roof? I would want the taller roof, but that would increase drag (increasing the battery capacity requirement or decreasing range).

So, for the "where do I start" question... I think you start by understanding what you need (such as ceiling height), then what that means for energy and power requirements, so that can be followed by design choices to meet those requirements.
 

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Discussion Starter #6
Thank you so much Brian for your help with researching. I think after looking at what I would actually need (or think I need) for battery capacity (somewhere in the 300-400kwh range) and at $100/kwh price which I think is actually on the low end. I am looking at 30-40k in just batteries. Its not quite a feasible option at this point. I planned on spending around 15k on the electric conversion. I think a diesel/electric hybrid would possibly work for me though and get me through the next decade until power storage devices can reach a better point.

I would use multiple motors either directly attached to the differentials or somehow directly attached to each wheel.

I do plan on either building a new box completely or modifying the box I have to accommodate the headroom needed for a comfortable place to stay. I plan to widen, heighten, and lengthen the box so a new box might be easiest since this body is built from aluminum. (however the body unbolts and leaves a lot of easy points to add in pieces)

How feasible are the hybrid builds for bigger vehicles? Some people say that converting mechanical into electric then back to mechanical seems to be inefficient but ive also ready that with the phenomenal efficiency ratings of the electric motors now day (im not sure the generation how efficient it is) that it can make sense to go this route.

What are your thoughts there?
 

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Pure battery-electric RV feasibility

I think after looking at what I would actually need (or think I need) for battery capacity (somewhere in the 300-400kwh range) and at $100/kwh price which I think is actually on the low end. I am looking at 30-40k in just batteries. Its not quite a feasible option at this point. I planned on spending around 15k on the electric conversion.
I agree that pure battery-electric is not economically reasonable. It has been done with even larger vehicles, but only by companies that have business objectives and the money to pursue them, and normally in applications that are better suited to an EV.
 

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Multiple motors

I would use multiple motors either directly attached to the differentials or somehow directly attached to each wheel.
I also like the idea of three or six motors both to avoid the need for a very large motor, and to allow full-time 4WD without inter-axle differentials.

Motors are heavy to mount directly to the axle final drive housings, but it is done with buses and trucks, and it would make packaging enough battery between the frame rails easier. I think that a way to do six motors would be to build de Dion axles with the portal drop boxes on the ends, and pairs of motors mounted to the frame at each axle line. You could also replace the centre section of each axle with a pair of reduction gearboxes and motors connected to them, but to that much fabrication and still have the mass of the motors (and gearboxes) riding on the axles doesn't seem like the best compromise... although that's what some production and planned-for-production heavy bus and truck electric axles do; the ZF AVE is even a portal design (designed to be used inverted for low-floor buses, but also used upright for the Mercedes Urban eTruck).
 

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RV coach body

I do plan on either building a new box completely or modifying the box I have to accommodate the headroom needed for a comfortable place to stay. I plan to widen, heighten, and lengthen the box so a new box might be easiest since this body is built from aluminum. (however the body unbolts and leaves a lot of easy points to add in pieces)?
I'm not sure about the off-road plans, but in tight spaces a wider body might be problematic. Bolting on a taller roof and keeping the current width might be a more functional option.

On the other hand, the C304 has sloped side walls above the beltline, so replacing them with a box which is the same width as stock at the floor, but with vertical walls, might make a more usable interior space without hanging out the sides further. I suspect that sloping the walls in had a purpose, such as being less likely to hit stuff when tilting on rough and narrow roads/trails.

I hate to suggest the weight and complication of slide-out sections, but moderate slide-outs may be the way to get good liveability without more width in travel mode. I do have some thoughts on slide-outs, if you're interested...
 

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... I think a diesel/electric hybrid would possibly work for me though and get me through the next decade until power storage devices can reach a better point.

...

How feasible are the hybrid builds for bigger vehicles? Some people say that converting mechanical into electric then back to mechanical seems to be inefficient but ive also ready that with the phenomenal efficiency ratings of the electric motors now day (im not sure the generation how efficient it is) that it can make sense to go this route.

What are your thoughts there?
Hybrid design certainly works for large vehicles; 25-ton hybrid urban transit buses are common.

There seems to be a common assumption that a hybrid should be a series design. While that has some advantages including an effectively infinite choice of gear ratios (engine to road speed ratio) and freedom of engine/generator location, an RV spends most of its time on a highway in a narrow speed range, so the most efficient is going to be a parallel configuration. Even at 85% efficiency per conversion, converting from mechanical power to electrical and back again saps a third of the power produced by the engine. Parallel also avoids the need for a generator which can handle the full output of the engine.

It looks like the Volvo has no rear inter-axle differential, so there's some appeal to breaking the mechanical connection between them (for less scrubbing and thus less energy use). It also has part-time front drive, since there is no centre differential, so if the original mechanical driveline is kept it can't drive the front axle on the highway. To me, that leads to something like this approach:
  • driving the leading rear axle with the engine and mechanical transmission (not using the stock transfer gear to drive the trailing rear axle)
  • driving the front and trailing rear axles only electrically (with one motor per axle or one motor per wheel)
  • adding a motor-generator at the engine (or between engine and transmission, or connected to the PTO)
  • regeneratively braking whenever possible
  • using the motors on the front and trailing rear axles only when required for traction, or when using battery power, or when braking
With the leading and trailing rear axles not mechanically connected, the trailing rear axle could be converted to steering (using steering knuckles as with the front axle) and steered in proportion to the front to allow tighter turns. Of course that might be expensive: I don't know what a C304 front axle costs, and then there's the steering gear. It could be a later enhancement.

A large (but not hundreds of kilowatt-hours) battery would allow
  • capture of energy in regenerative braking (by all axles)
  • power boost for acceleration (by driving front and trailing axles)
  • lots of energy for use while camped (can charge using motor-generator connected to engine)
  • plug-in charging to displace some fuel consumption
As an alternative, the most minimal hybrid would be a mild parallel hybrid, with a single motor-generator used for regenerative braking and for a bit of acceleration or hill-climbing boost and a battery sized more like a plug-in hybrid car than an EV. Current production mild hybrids typically run at only 48 volts, but the 200+ volts of a common non-plug-in hybrid might be a good match for a system that easily produces 120 V AC power when camped. These are usually connected by a belt drive to the engine, and replace both the alternator (in combination with a DC-to-DC converter) and the starter motor. In my motorhome I would like just enough to limit speed on mountain grade descents without resorting to engine or friction braking, and a large battery for power while camped; a mild hybrid system would do that.
 

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Discussion Starter #11
The only issue with that idea is then I have the expense of an electrice drivetrain and a mechanical one.

The current state of the project is motorless so it will need a new motor and transmission as of its current state otherwise keeping the factory motor intact could have been an option.

I would prefer to go with an all electric drivetrain so in the future I can utilize upgraded battery technology to gain higher ranges and eventually downsize the generator needed... and also I wont have the double expense of an electric drivetrain and a mechanical one.

from what ive read and calculated I will need at least 1kwh / mile to move this thing fully loaded so a 100kwh batter would give me around 100 miles. What would I need for a generator wise to produce enough electricity to keep be moving forward... This particular RV wouldnt be used to cruise long highways at 70mph it would be back roads with frequent stops at 35-55mph most likely on the dirt.

What are your thoughts? I would love to make this work as a hybrid but if not my next option is a small mercedes diesel or maybe the new cummins 2.8 repower crate motor.
 

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The only issue with that idea is then I have the expense of an electrice drivetrain and a mechanical one.

The current state of the project is motorless so it will need a new motor and transmission as of its current state otherwise keeping the factory motor intact could have been an option.

I would prefer to go with an all electric drivetrain so in the future I can utilize upgraded battery technology to gain higher ranges and eventually downsize the generator needed... and also I wont have the double expense of an electric drivetrain and a mechanical one.
Yes, that's true of any hybrid; you need something to handle the engine output. In a series hybrid you have the same transmission as a straight EV (with one motor), but you need a generator on the engine capable of taking the entire engine output, which is not cheap or small.

Even with no engine (and no transmission?) to start with, an engine and transmission (presumably not the same as the Volvo originals) would be less expensive than an electric powertrain... but I realize that's not the purpose of the project. ;)

from what ive read and calculated I will need at least 1kwh / mile to move this thing fully loaded...
That's roughly what I'm thinking, on the basis that both rolling resistance and aero drag will be about three times as great as for a large modern car.

What would I need for a generator wise to produce enough electricity to keep be moving forward... This particular RV wouldnt be used to cruise long highways at 70mph it would be back roads with frequent stops at 35-55mph most likely on the dirt.
If you take either the series hybrid approach, or a parallel system with enough electric power to make up the difference between a small engine and whatever is needed at maximum power demand, then the engine can be sized for not much more than average power demand. There are very few production series hybrid light vehicles; the BMW i3 REX does have only a small engine, while the Honda Accord has nearly the same engine power as the non-hybrid. The BMW i3 REX shows that a small and hard-working scooter engine is not an efficient way to power a car.

As an example, if you're using 1 kWh per mile to run at 60 mph, you're using 60 kW average. That's not much less than the original engine, and if you use any less engine you won't be able to sustain that speed (or a lower speed while climbing), but you're saying that's okay. Perhaps an engine that can produce 30 kW efficiently (rather than at peak output) would keep up suitably.

I think I might pick a Prius engine - the 2ZR-FXE produces anything from about 13 kW to 30 kW while staying within 5% of its impressive peak efficiency (of 220 grams of fuel per kWh of output) - but it's gasoline, not diesel. You might get away with an off-highway equipment (e.g. tractor) diesel of some sort; among road vehicles available in North America the engine from a VW car or the Chevrolet Cruze are the only reasonably small diesels that comes to mind, although of course there are smaller engines from other markets.

In a series hybrid the engine needs a generator; if they are directly linked (not through a gear, belt, or chain drive) then the generator turns at engine speeds and needs to handle the maximum engine output torque.

I would love to make this work as a hybrid but if not my next option is a small mercedes diesel or maybe the new cummins 2.8 repower crate motor.
This is a great example of what the Cummins 2.8 crate motor package is intended for. I don't know what it costs, or if a used Duramax 2.8 or Powerstroke 3.2 might be cheaper (and presumably cheaper to maintain than anything from Mercedes or BMW).
 

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Discussion Starter #13
Thank you Brian for helping me through this.

My concerns right now are

What can I expect to pay for a full electric drivetrain with a 100kwh battery for drive on all 3 axles.

I am also concerned with having the electric motors directly driving the axles and not having a second speed gearing for more efficient cruising speeds.

What would the downside of running one bigger electric motor through either a transmission and transfer case or possibly even just a 2 speed transfer case to drive all 3 axles?

I can find many different options nearby for a medium to large diesel generator. I am looking at 10kwh-20kwh generators. I wouldnt need to sustain forward movement if I had a 100mile ish range I can go and park for a while.

I plan to have a lot of solar and now I have even thought about building a separate small trailer with extra batteries and a solar array that I can deploy at a "campsite"

Thoughts?
 

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Gearing and power distribution

I am also concerned with having the electric motors directly driving the axles and not having a second speed gearing for more efficient cruising speeds.
With suitable AC (permanent magnet or induction) motors and enough voltage, that's not a problem - it's how production EVs are configured. If you try to get away with brushed DC motors or half the voltage a typical modern EV, I agree that the speed range will be an issue. With three axles to drive, multiple ratios mean either multiple transmissions, or a single central motor and keeping something like the stock system of transfer cases.

A compromise solution might be to use single-ratio electric drive to one or two of the axles only at low speed, and to disconnect (or just idle) those ones at higher speed. Apparently at least a couple of production hybrids or EVs leave the secondary axle's motor connected but unpowered at high speed for efficiency.

What would the downside of running one bigger electric motor through either a transmission and transfer case or possibly even just a 2 speed transfer case to drive all 3 axles?
Most transfer cases are unable to drive just one output, or to drive both outputs with differential between them, in low gear; they have "2 Hi", "4 Hi" and "4 Lo" modes, but not "2 Lo" or a full-time "4 Lo". That's true of the Volvo's transfer case - it has no centre differential. If you only want low gear for traditional "4 Lo" conditions, that's okay; if you want two ratios for both high-speed road use ("2 Hi") and low-speed road use ("2 Lo"), it's hard to find one which will be suitable without modification.

The C304 has no inter-axle differentials, so with one motor
  1. you can't drive the front axle on the highway, and
  2. the tandem rear axles scrub due to turns or any tire size mismatch
With a single motor, you can either live with these limitations, or upgrade. The transfer case (assuming that you still have the stock one) could be replaced by one with a centre differential. The simple power take-off on the leading rear axle (which drives the trailing rear axle) could be replaced by an assembly with an inter-axle differential (also called a power divider), but I don't know what's available in a suitable size and I doubt there's anything like this designed to work with the Volvo C304 axle.
 

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Discussion Starter #15
so if I were to do 3 motors on 3 axles and wanted to go 55mph (maybe a bit faster) Which motors would you use? to move 10k lbs? and where is the best place to get 100kwh worth of batteries?

I need to start looking at price values so I don't keep wasting your time if it does not make sense to me at this point in time...

If I had $15k to spend on this electric drivetrain with 100kwh of battery would I be able to pull it off? Just talking parts here. I plan to fabricate and install it all myself or with the help of a few friends.
 

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Hi tmoney

The cheapest (and best) batteries are from a production EV

I would suggest that the best way to get what you want would be to buy three crashed EV's
Something like three Leafs
That will give you three power units and about 75 kwh of batteries

Don't know what you would have to pay for a crashed Leaf - guess about $3000
 

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Discussion Starter #17
Hey Duncan!

I will definitely try to look around and see what I can find for wrecked leafs.

Is there a way to change the 350v packs down to a 144v pack and effectively double the capacity? or would that be a bad idea?
 

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Hey Duncan!

I will definitely try to look around and see what I can find for wrecked leafs.

Is there a way to change the 350v packs down to a 144v pack and effectively double the capacity? or would that be a bad idea?
Hi if you get the controller and charger and everything out of the Leaf it will all work together

You can re-arrange the 48? modules that make up a Leaf battery in any way you want but I don't see the advantages - you would be better off sticking to the Leaf voltages

If you arrange the Leaf Battery in two packs you get twice the Amp hours but half the voltage
So the same amount of energy and the same range
 

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Discussion Starter #19
I see what you are saying though..

3 leafs for 3 axles with 3 batteries and motors.. Could possibly work. It does look like you can buy these wrecked for around the 3-5k mark.

I wonder if the leaf can get the 100mile range if I could get 100 miles with 3 of them.
 

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The three-Leaf approach makes sense to me. :) The feature which is less than ideal might just be that the Leaf motors are larger and heavier than necessary (if you're mounting motors on axles), given that they are designed for 80 kW (107 hp) each; that's reasonable for the mass of the vehicle, but more than double the power of an original C304.

Other recent production EVs are usable, too, but the Leaf is the most common... other than Teslas. I suspect that multiple Tesla motors and corresponding battery modules would be expensive.

If specifically using the Leaf battery, I think it would make sense to rearrange modules physically to package them into the vehicle, and maybe to connect sets of three modules in parallel then those sets in series (to make battery monitoring easier) if combining them into one large battery... but as Duncan explained there is no advantage to configuring for a lower voltage. Keeping the full design voltage is important to maintaining motor performance at high (motor) speed.

With 3.44:1 ring-and-pinion sets at the differentials, and 2.06:1 reduction in the portal boxes, a motor connected directly to an axle's input will be reduced by 7.09:1, which is almost as much as the stock Leaf reduction gearing. The tires are taller than a Leaf tire, so overall the motors will be geared a little taller than ideal, given the modest top speed expectation for the C304 compared to a stock Leaf.
 
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