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If I add an electric compressor to the existing AC system can I run the AC off of an external battery?
 

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If I add an electric compressor to the existing AC system can I run the AC off of an external battery?
Unfortunately, it's not a viable option. Even if the mechanicals were possible, the power requirements of the van's AC system are very, very high. It could never be run off a battery bank from the van.
 

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Just as an example. I found the below example of a small air conditioner:


It says the power output is ~1000 W (930 but rounding up for simple math). I personally have a 6000WH battery. This means that I could run the linked AC for a total of 6 hours of actual run time. I personally also have 200W of solar so you might argue that can bring my AC usage down to ~800W during sunny times when I am wanting to use the AC which would bring me up around 7.5 hours.

Anyway work out the math for your system. If you have a very large battery (> 10k WH) or a large solar system (>600W) it might be possible but it would require careful planning.
 

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Perhaps I am incorrect, but I assumed the OP was asking about the AC compressor on the van's engine.
 
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Very large power requirement for sure, but there is more to it than just adding a second compressor to the factory A/C system. You would also have to power the in-dash HVAC system (fan, blend door, etc). I suspect that would be in direct conflict with the CAN bus, which doesn't play nice with mods. This is one of those ideas that looks attractive on the surface, but if it was possible, someone would have marketed it years ago.
 

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Very large power requirement for sure, but there is more to it than just adding a second compressor to the factory A/C system. You would also have to power the in-dash HVAC system (fan, blend door, etc). I suspect that would be in direct conflict with the CAN bus, which doesn't play nice with mods. This is one of those ideas that looks attractive on the surface, but if it was possible, someone would have marketed it years ago.
Isn’t that basically what plug-in hybrids do, except they don’t use a second compressor but replace the engine belt-driven compressor with an electric one. I can’t wait until OEM air conditioners can be used to cool vans for camping without having to add anything on roof or off the back, or wasting space to fit window A/C inside van.

I thought of replacing my van’s compressor with electric, or adding second compressor like Sportsmobile use to do, or splitting my rear A/C from engine A/C to use OEM evaporator with new condensing unit, but the practical engineer in me decided it was not worth the cost or added risks. For a fraction of cost I added a standalone window A/C and kept factory unit at full capacity and OEM reliability.
 

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... I can’t wait until OEM air conditioners can be used to cool vans for camping without having to add anything on roof or off the back ...
Yes, EV van conversions will be cool (pun intended). But in the meantime, like you, I'd rather not to mess with complex OEM systems that can fight back. That path is strewn with unintended consequences.
 

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You would need so many check valves it would be a plumbing nightmare.

It would also be up to you to make the dash fans, blend doors, and everything else work correctly, without making the body control module throw error codes.

I wouldn't attempt it. If you are mechanically inclined you can install a second unit with its own condenser, evaporator, fans, and compressor. If I convert another van in the future, I think that is what I will do. My internal window unit works just fine, but I might like to try something different next time.

The best way is to find a kit on Alibaba. They are the same types of systems undermount AC and Cruise-n-comfort offer but for $300-400 dollars. like this (but there are lots of choices)

 

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Isn’t that basically what plug-in hybrids do, except they don’t use a second compressor but replace the engine belt-driven compressor with an electric one. I can’t wait until OEM air conditioners can be used to cool vans for camping without having to add anything on roof or off the back, or wasting space to fit window A/C inside van.

I thought of replacing my van’s compressor with electric, or adding second compressor like Sportsmobile use to do, or splitting my rear A/C from engine A/C to use OEM evaporator with new condensing unit, but the practical engineer in me decided it was not worth the cost or added risks. For a fraction of cost I added a standalone window A/C and kept factory unit at full capacity and OEM reliability.
Our 500e Fiat has a compact on-board heat pump unit for heating and cooling. It runs from the traction battery (400 VDC) and draws about what you'd expect for a 4-7k BTU heat pump. Running it is almost perfectly silent and smooth, but noticeably cuts into the driving range of course. These units are much more powerful than the little 12-24V heat pumps I've seen, but of course require 400 VDC to operate. Let's see what they put into the electric PM for Amazon. :)
 

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Our 500e Fiat has a compact on-board heat pump unit for heating and cooling. It runs from the traction battery (400 VDC) and draws about what you'd expect from a 4-7k BTU heat pump. Running it is almost perfectly silent and smooth, but noticeably cuts into the driving range of course. These units are much more powerful than the little 12-24V heat pumps I've seen, but of course require 400 VDC to operate. Let's see what they put into the electric PM for Amazon. :)
A 48V compressor may be available from mild (low voltage) hybrids. I believe Mercedes has some new engines with no belt drive at all. Everything is powered by electricity including engine water pump and air conditioner. If I recall correctly, the article said it was based on 48V electrical, except for some 12V loads which receive charge from the 48V battery by using DC-DC step down converter to charge the conventional 12V battery.
 

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A 48V compressor may be available from mild (low voltage) hybrids. I believe Mercedes has some new engines with no belt drive at all. Everything is powered by electricity including engine water pump and air conditioner. If I recall correctly, the article said it was based on 48V electrical, except for some 12V loads which receive charge from the 48V battery by using DC-DC step down converter to charge the conventional 12V battery.
Before the pandemic, there were low miles lease return Fiat 500e models on our local lots for $7-8k. At that price the Bosch motor and drive/controller, traction battery pack, and climate control module were bargain priced!
 
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