Finding always-on 48v in the cabin or bed

[Woodrick]

I'm going to run a 48V feed from the roof to my bed for a CPAP. Any tips or pics of how you routed the wire?

120V and turning the outlets on won't cut it?

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[Eyesore]

120V and turning the outlets on won't cut it?

We get conversions losses going from 800V DC to 120V AC, then again from 120V AC to 24V DC.
I'd rather go from 48V DC to 24V DC, and not run the inverter all night.

 

[Crissa]

We get conversions losses going from 800V DC to 120V AC, then again from 120V AC to 24V DC.
I'd rather go from 48V DC to 24V DC, and not run the inverter all night.

You get fewer losses converting from 120 AC to whatever than from DC to DC, but fair point, the overhead from the AC on at all is pretty high.

-Crissa

 

[Woodrick]

We get conversions losses going from 800V DC to 120V AC, then again from 120V AC to 24V DC.
I'd rather go from 48V DC to 24V DC, and not run the inverter all night.

Yes, the 1-2% maybe 5% with a crappy power supply, is disastrous.


How do you think that the truck charges the 48V battery? Because a CPAP is probably going to drain the 48V battery in an hour.

It's all coming from the 400V/800V main batteries. So down-conversions are a given.
And you probably charged from 240V, so that got up converted.

i.e. don't worry about it.

Just looked up CPAP draw, 30-60W/hr. Worse case, 720W/12hr.
If you only got 50% efficiency, then there's enough for a few months.

 

[Eyesore]

I've decided not to try to power my CPAP from the 48V roof tap. I can't get the the roof tap to stay on unless I leave the climate control on (camping mode) or the inverter on (outlets always on). This defeats the purpose of reducing the demand on the truck battery to run the CPAP.

Cybertent mode does not leave the roof tap powered.

I may try an e-bike battery instead.

 

[jschall]

Yes, the 1-2% maybe 5% with a crappy power supply, is disastrous.


How do you think that the truck charges the 48V battery? Because a CPAP is probably going to drain the 48V battery in an hour.

It's all coming from the 400V/800V main batteries. So down-conversions are a given.
And you probably charged from 240V, so that got up converted.

i.e. don't worry about it.

Just looked up CPAP draw, 30-60W/hr. Worse case, 720W/12hr.
If you only got 50% efficiency, then there's enough for a few months.

The main problem I'm looking to solve is running a fridge 24/7/365. It needs to be powered always.

Currently no way to do this without leaving a ton of systems on drawing 500W or more using sentry mode. Less concerned about the inefficiency of DC to AC back to DC conversion, more concerned that my fridge will frigging turn off after 12 hours.

I assume the truck won't let the 48v battery die, so probably I will end up wiring it up directly.

You get fewer losses converting from 120 AC to whatever than from DC to DC, but fair point, the overhead from the AC on at all is pretty high.

-Crissa

Modern DC-DC converters are extremely (95%+) efficient, are simple and cheap, and work by switching at hundreds of khz, allowing extremely small and inexpensive input and output filters. Converting AC efficiently is comparatively far more heavy, expensive, and annoying, because the zero crossing of the AC waveform means your input power disappears for what is effectively an eternity on the timescales that modern switching power supplies operate on. Meaning you have to bridge the gap by storing energy, typically with large electrolytic caps, which have relatively high ESR and therefore dissipate a bunch of energy as heat.

AC-DC conversion is extremely complex and inefficient relative to DC-DC, and DC-AC conversion is inefficient as well.

 

[Crissa]

Modern DC-DC converters are extremely (95%+) efficient, are simple and cheap, and work by switching at hundreds of khz, allowing extremely small and inexpensive input and output filters.

Yeah. ..if the dc-dc conversion is built for a specific load point, it's that efficient. It's not so efficient when the loads are very small or very variable.

And then it also costs money to install or design. Which is probably why not much power is available pre-wired in the truck.

That's the problem when theory hits the road.

-Crissa

 

[jschall]

Yeah. ..if the dc-dc conversion is built for a specific load point, it's that efficient. It's not so efficient when the loads are very small or very variable.

And then it also costs money to install or design. Which is probably why not much power is available pre-wired in the truck.

That's the problem when theory hits the road.

-Crissa

I literally just need it to leave USB C on 100%. It is enough to run my fridge.

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