Installing roof AC and have a few questions

Paddy

Adventurer
So we need AC and we don't have room or desire for a gen set. So, is it practical to use the sprinter 2.1L engine as a gen set if equipped with an oversized or secondary alternator? The sprinter has a very complex electrical system so I hesitate to dive into possible mods on that, however if there is a proven way to do that I'm all ears. I've also heard talk of secondary alternators, with the second one powering just the house.

So, if going that route does the van need to run the inverter system off the batteries, which are then replenished by the engine and use an AC-AC? (Meaning a 120vac compressor) or do we skip the inverting and go with a 12vdc compressor for AC, if that even exists?

https://www.nationsstarteralternator.com/2010-2017-Mercedes-Sprinter-2500-Van-2-1L-280-Amp-p/dcp1-12385-280-2.1.htm

Here's a 280a replacement alt and they claim 200a idle output. That seems like it would power a small roof AC.
 

bdog1

Adventurer
Split AC systems are now available in 18v. Never looked into the details. There set up for big home solar.


Sent from my iPhone using Tapatalk
 

luthj

Engineer In Residence
You have some options. Before I delve into the various approaches I need some details.

How long do you need to use the AC in one go?

How often do you intend to use it?

What conditions will you use it in? Temperature, humidity, direct sun?

If you already have a roof AC unit, what size is it? And what is its rated watt/amp draw?

How well insulated is your van? How many windows? Do you have good/tight fitting window coverings? Is the floor insulated?

How many people will be in the van? Lots of door opening in closing?

Is the AC a comfort item, or is it a absolute necessity? If you have to stop using it an hour earlier than desired, will a person or animal suffer distress?

Are you willing to have a small AC unit designed to cool just the bed area? Is the AC only for sleeping? If so there are options specifically for cooling a bed.
 
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_hein_

Observer
There is a thread on the Sprinter-source worth reading:
https://sprinter-source.com/forum/showthread.php?t=48214

You will need a large battery bank 400-500AH
and large 2500Watt inverter big enough to start
and run the AC on 120V. You will see loads of 100A
or more on the DC side. I'm not sure it's feasible
to run directly off the aux alternator without any
batteries. You need batteries to absorb the transient
loads.

You can recharge the batteries with the existing
alternator or add the aux alternator. The charge
rate is dependent on the batteries state-of-charge
and not so much the capacity of the alternator.
But you can use the extra capacity to power the
AC with the engine running.

Running the engine as a generator creates
some heat and unless you have the high-idle
option may not be that great for it. Really do
need to be able to run AC on batteries for at least
a couple hours.

There are some 12V air conditioners. They are
usually smaller capacity but still take a whopping
amount of current. It takes a considerable amount
of energy to cool a space. Can't change that.
If you plan to use shorepower for the AC then
having a 12V unit will complicate that somewhat
because you will need a 120V to DC converter
with a capacity as high as the current draw from
the AC. That's more than a typical inverter/charger
can deliver. Means you may actually be depleting your
batteries if you run the 12V AC while on shore power.

Insulation and reducing solar gain are essential.
I would be happy to explain further in a phone call.

All the best,
Hein
DIYvan.com
 
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Paddy

Adventurer
You have some options. Before I delve into the various approaches I need some details.

How long do you need to use the AC in one go?

How often do you intend to use it?

What conditions will you use it in? Temperature, humidity, direct sun?

If you already have a roof AC unit, what size is it? And what is its rated watt/amp draw?

How well insulated is your van? How many windows? Do you have good/tight fitting window coverings? Is the floor insulated?

How many people will be in the van? Lots of door opening in closing?

Is the AC a comfort item, or is it a absolute necessity. If you have to stop using it an hour earlier than desired, will a person or animal suffer distress?

Are you willing to have a small AC unit designed to cool just the bed area? Is the AC only for sleeping? If so there are options specifically for cooling a bed.
Great questions thank you. The purpose of the van is exploration of mountain and desert areas, so it's a luxury comfort not totally needed but would be nice during the day for a few hours max.

No animals

Low to mid humidity

One or two people in van, it's a small sprinter after all.

No existing AC or high idle systems are installed.

Van is pretty well insulated. It's a sport mobile if that gives any indication.
 

Paddy

Adventurer
There is a thread on the Sprinter-source worth reading:
https://sprinter-source.com/forum/showthread.php?t=48214

You will need a large battery bank 400-500AH
and large 2500Watt inverter big enough to start
and run the AC on 120V. You will see loads of 100A
or more on the DC side. I'm not sure it's feasible
to run directly off the aux alternator without any
batteries. You need batteries to absorb the transient
loads.

You can recharge the batteries with the existing
alternator or add the aux alternator. The charge
rate is dependent on the batteries state-of-charge
and not so much the capacity of the alternator.
But you can use the extra capacity to power the
AC with the engine running.

Running the engine as a generator creates
some heat and unless you have the high-idle
option may not be that great for it. Really do
need to be able to run AC on batteries for at least
a couple hours.

There are some 12V air conditioners. They are
usually smaller capacity but still take a whopping
amount of current. It takes a considerable amount
of energy to cool a space. Can't change that.
If you plan to use shorepower for the AC then
having a 12V unit will complicate that somewhat
because you will need a 120V to DC converter
with a capacity as high as the current draw from
the AC. That's more than a typical inverter/charger
can deliver. Means you may actually be depleting your
batteries if you run the 12V AC while on shore power.

Insulation and reducing solar gain are essential.
I would be happy to explain further in a phone call.

All the best,
Hein
DIYvan.com
Hmm. The van does have a large inverter capable of 2200 continuous watts. The house battery is small but I'm also planning on building a larger lithium bank and could beef it up even more if that is a necessity for the AC. Was going to go with 180ah cells giving useable 140ah, but could easily fit 400ah in the space currently taken up with 140ah lead.
Thanks for the link I'll read that later.
 

luthj

Engineer In Residence
There is a pretty big range of insulation on SMB vehicles. And good insulation will make a major difference. The same goes for solar gain reduction, and window coverings.
For temps around 90F; a well insulated van, in the shade, you can get by with 8,000btu. In my van, I can cool a partitioned sleeping area quite well with 5,000btu. In full sun you will need 10,000btu to achieve temps in the 70s. The quality of window coverings (the number of windows as well) and insulation will have a big impact.

Running a high efficiency 8,000-10,000 BTU unit will use at minimum 100A from a 12vDC power source. Probably more for the larger unit. DC native units are expensive, but can be 10-30% more efficient. I have seen 6,000BTU DC units quoted as using 45-60A.

Here is some real world numbers from my rig. We travel in widely varying climates, and sometimes need to use AC during the day/night for our own sanity.

I have a (not very efficient) 5,000BTU 120V marine AC unit mounted under my bench seat. It uses around 75A @12VDC in temps of 90F. (I need a better condenser). It will keep the van comfortable if we are parked in the shade, but takes a while to drop the humidity when its high. I can run it for an hour or two during the day if we have lots of solar charging, or if we are driving regularly. If the temps are truly brutal at night (over 85F, high humidity), we partition off the bed, and run the AC unit to just cool that area. With a roughly 50% cycle, I can generally get 4-8 hours from our 510AH lead acid battery bank before I get below 40% SOC.

Keep in mind that any amp hours you use running the AC must be returned at some point. Lead acid batteries will need these AHs returned, and a 100% charge reached every week at a minimum, ideally every few days.

Which brings me to your second alternator suggestion. Idling the engine for most any reason is not very desirable on modern diesel engines. They need lots of EGR flow to keep the NOx levels down, this can accelerate soot buildup in the intake, and EGR cooler. The I4 seems much less prone to problems than the 6 cylinder though. If you are going to do it, I suggest a High idle control be installed.

The second alternator is a great way to get lots of amp-hours back into your batteries when you drive. The factory alternator runs at a fairly low voltage, which will restrict your charging rates somewhat, though lithium batteries may charge quicker than LA at that voltage. A second alternator with external regulator and temp compensation will charge MUCH faster (up till 90% for LA) than the factory alternator. The external regulator is basically a charge controller, and will have the second alternator putting out the optimum charge voltage for your bank (most are adjustable).

It depends on how often you are using your AC from the batteries, and what type/size they are, but you may find yourself in need of additional charging if you use the AC regularly.


If you choose to idle the engine, you can still get a substantial amount of the ACs power needs from the stock alternator. The factory recommendation is ~50A. With reasonably sized wiring, some folks have been doubling that with no immediate problems.


After it's all said and done, you should consider the price of a regular 120VAC rooftop unit, and a Honda 2000w generator (gasoline fueled). There is probably a method to carry the generator that will work for you, and you may save lots of $$$, and get longer runtimes. You will need to match the AC unit to the generator obviously, as many larger units may not readily start from a smaller generator.
 

86scotty

Explorer
I agree with the above, that basically you're going down a rather deep rabbit hole to achieve this. The best compromise I have had was a Honda 2000 mounted on the back rack of my old van with a Polar Cub 9500 btu roof unit. The Honda 2000 won't start a typical 13500 btu RV roof air. It will run a Polar cub or smaller window AC just fine though. The 9500 btu unit was great for my EB Ford pop top, but just. Any more space or really hot outside and it could barely keep up. In average conditions it was more than enough. This setup was nice because you could run it while going down the road with the generator on the back bumper and you never knew it was even running over the noise and vibration of the van but while parked it would rattle you to death. The solution for me was to sit the generator on the ground away from the van. If noise is a concern drag it further away and run an extension cord. I plan to do this again with a new build using a cheap window AC unit stealth mounted somewhere. By that I mean not in a window.

I think idling the van for any type of AC is counterproductive since you're heating the exhaust, doghouse and floor with engine heat and probably reducing your cooling to nothing.

Good luck, I'd love to see what you come up with.
 

DiploStrat

Expedition Leader
Been down this rabbit hole. There is a huge thread on this in the 12v forum.

Listen to luthj; he is giving good, real world advice.

 

Paddy

Adventurer
Nice, they've come out with a 13,5 that can run on a 2000 watt genny! That wasn't available a few years back when I bought my Polar Cub, and it is in fact cheaper. Hard to beat that.
Yes this looks good. I think I'll go that route with upgraded alternator and house batts and see what that gets us. Not ideal but we really don't have room for a genie on this van. I hear the point about heating up the floor and this is a very good point however. I wonder if a fiberglass wrap on the long sections of the pipe wouldn't be a good idea?
 

86scotty

Explorer
Just FYI on the 9500 BTU Polar Cub I mentioned above. Another thing I liked about it was it was not as tall. It worked well with a regular roof rack. Every rig is different but I'm not a fan of the huge AC box on top of vans. Your situation might be different.
 

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Paddy

Adventurer
Just FYI on the 9500 BTU Polar Cub I mentioned above. Another thing I liked about it was it was not as tall. It worked well with a regular roof rack. Every rig is different but I'm not a fan of the huge AC box on top of vans. Your situation might be different.
Yes I liked that also. These vans are tall enough and we get into tight spots with them all the time where we travel.
 

luthj

Engineer In Residence
With a bit of ducting, a window AC unit can be mounted under a bed or cabinet. Pretty cheap, and no height loss. Once you get around 8.5ft, extra height doesn't matter as much, especially in urban areas.
 
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