Low power draw laptops that can run on direct 12v

[Etzu]

What are you guys using.

I've seen a few guys running a laptop hooked directly to a 12v socket with the original cable\power brick sitting on the floor disconnected. The power brick was 12v. He made his own cable, and said it's been working fine for years.

I'm going to need a low power machine with a lot of spare battery capacity shortly and was doing some shopping, but thought I'd see what you guys are doing.

PS I'm no longer a power user, I don't game or do any video editing. 99% it's just emails, recipes, to do lists, browsing and maybe the occasional movie in bed.

[booster]

We just went through a similar routine, but we had been running on 12v for a long time. I don't know if there are any native 12v computers around any more, as what we found were all higher.


What we did find was that there was really now reason to use the 12v power to run or recharge the computer. With native 12v you should still use a voltage stabilizer and at high DC voltages, you need to use a 12 to whatever DC voltage adapter. When I tested the new 12v to 19.5v adapter that we got for the new computer, it actually used as much power as running an inverter and the normal AC brick that came with the computer. Using a small higher efficiency inverter should make it use less than the 12v converter setup.



Details of what we found here


http://www.classbforum.com/forums/f2...ters-7727.html

[BBQ]

.

Understand the inner workings of a voltage step-up "black box".

To step up from 12v DC to 18v DC,
the most common method is to
feed the 12v DC through an inverter circuit to turn it into AC,
then convert the AC to 18v DC.

[avanti]

You can get laptop-specific 12VDC power adaptors on eBay very cheaply. They are available for most popular laptops. But, I agree with Booster: in this and many other areas, it is starting to look like going native 12V isn't worth the effort vis just using small inverters.

If I were doing a new van today, I would consider doing the opposite of what I did last time: I would push as much as possible to 120VAC and put my effort into a set of efficient inverters. LED lighting is a likely exception.

[Bud]

Originally Posted by avanti

You can get laptop-specific 12VDC power adaptors on eBay very cheaply. They are available for most popular laptops. But, I agree with Booster: in this and many other areas, it is starting to look like going native 12V isn't worth the effort vis just using small inverters.

If I were doing a new van today, I would consider doing the opposite of what I did last time: I would push as much as possible to 120VAC and put my effort into a set of efficient inverters. LED lighting is a likely exception.

And at this point Avanti, what about the compressor refer?

Thanks.

Bud

[avanti]

Originally Posted by Bud

And at this point Avanti, what about the compressor refer?

Actually, the recently-reported successes of folks who have been trying out modern high-efficiency 120VAC fridges is the thing that got my attention in this area. I have long assumed that native 12VDC compressors would be vastly more efficient (both in fridges and A/Cs). But, apparently, it ain't necessarily so.

I would still look carefully before deciding, but 120VAC with a well-matched inverter now has my attention.

[booster]

Originally Posted by avanti

Actually, the recently-reported successes of folks who have been trying out modern high-efficiency 120VAC fridges is the thing that got my attention in this area. I have long assumed that native 12VDC compressors would be vastly more efficient (both in fridges and A/Cs). But, apparently, it ain't necessarily so.

I would still look carefully before deciding, but 120VAC with a well-matched inverter now has my attention.

Definitely something to look closely at if the need for a new frig arises. The Danfoss has multiple speeds available as well as computer controlled speeds for efficiency. It is likely they are using an AC frequency converter for that purpose, so they probably already are AC based motors in the refrigeration unit.


Durability would probably be the biggest issue.

[avanti]

Originally Posted by booster

It is likely they are using an AC frequency converter for that purpose, so they probably already are AC based motors in the refrigeration unit.

Yes. IIRC all fully-sealed compressors use synchronous multi-phase AC motors in order to avoid having to have brushes sealed inside. They therefore have fairly fancy controllers.

[booster]

I actually just completed our "AC charging station" a couple of days ago.




We have another of the same inverter in the video cabinet.





I really like the little Samlex 120 watt PSW inverters as they a very small, high quality, and seem to be very reliable. They run at about .3 amps idle, but we shut ours off with separate power switches anyway. The downside of them is the single two prong plug on them and no built in on/off switch.


Both of ours have had the two prong outlet removed and a power strip direct wired into the unit. A small piece of fiberglass sheet epoxied in place fills in the missing plug are and holds the cord grommet. It works nice this way because you can bond neutral and ground together when you put in the cord, and that is how standalone inverters should be wired.

[cruising7388]

Originally Posted by avanti

Actually, the recently-reported successes of folks who have been trying out modern high-efficiency 120VAC fridges is the thing that got my attention in this area. I have long assumed that native 12VDC compressors would be vastly more efficient (both in fridges and A/Cs). But, apparently, it ain't necessarily so.

I would still look carefully before deciding, but 120VAC with a well-matched inverter now has my attention.

I think the jury is still out, but I harbor some doubts about this because I think there are some subtle influences that can be difficult to quantify.

Finding a well matched inverter isn't a slam dunk. Capacity is certainly no longer an issue but operating efficiency under load and comparative efficiency in stand-by has to be part of determining whether or not you come out ahead or behind going to 120VAC.

For 120VAC reefer operation to be more efficient than 12VDC operation, it's efficiency would have to be high enough to offset the operating inverter conversion loss. This specification is typically provided for the most efficient operating point of the inverter, but actually, depending on the load on the inverter, it's efficiency may vary from optimum (seems typically around 90%) to something less than that under different inverter load demands.

The variable duty cycle of the reefer makes efficiency even more difficult to quantify with an inverter involved. With a 12V system, when the reefer is off, there is no battery drain. But with an inverter involved, when the reefer shuts off, (assuming no other inverter loads), the inverter goes into standby but still consumes battery power and the manufacturers seem reluctant to publish stand-by drain. The Powerstar units provided by Roadtrek apparently consume 5 amps in stand-by which is similar to the compressor demand when the reefer is operating. The consequence is that while the reefer may only be in a 50% duty cycle, with respect to battery drain, the effective duty cycle is 100%. I notice that the compressor reefers provided by Roadtrek were initially 12V/120V dual voltage units but currently they operate at 12V only.

Perhaps an advantage of 12V native operation is that while on a trip, even if your inverter crashes, you would still have refrigeration.

[booster]

As has been mentioned by a couple of posts, as soon as you are off the pure 12v use in your device, the electronics of a 12v adapter are making AC then transforming and then going back to DC, so can take lots of power, even compared to an inverter.


The compressor frigs do the same thing, but don't go back to DC because of the AC frequency drive motors, so no more steps there either.


5 amps of parasitic is ridiculous, but we all have gotten pretty used to seeing Roadtrek picking less than stellar components. Most good 2-3K inverters will use about 1.5 amps. A compressor frig would likely run OK on the tiny little Samlex 120 watt inverter shown. Getting under .5 amps of parasitic is pretty easy these days.


We just need some folks to test what they have and post up the results. A Kill-a-watt meter on the AC units will give accumulated watts over time, as will a Watts Up for the DC version. Very simple to do. From the little bit of information we got on the new AC frigs from some posts a while ago we may find them a lot closer to the same than any of us expected.

[Boxster1971]

Nice setup Booster!

- - Mike

[avanti]

Lots of open questions, but at least we are starting to ask them.

Another one:
We have to understand the tradeoffs between one big Inverter and lots of little ones. I have no good intuitions as to which will turn out to be better. The obvious advantage of lots of little ones is that they can be best tuned to their particular load. And, of course, a DC-DC converter may make the most sense for low-voltage devices such as laptops--that is basically what 12V-powered laptop chargers are. OTOH, part of the motivation is to stop screwing around with many permutations and to standardize on 120VAC, so maybe these small loads aren''t worth worrying about.

As for fridges in particular, high end inverters have "sleep" modes, where the standby current is near zero, with periodic "pings" of current to determine whether a load is present. These often do annoying things in an RV (such as flashing the display on the microwave). But it would be ideal for a unit dedicated to a fridge (except that many fridges probably need constant power if they use electronic thermostats.) An exotic solution might be to have a fridge with DC-powered electronics and the ability to turn on a dedicated inverter when the compressor kicks on.

Lots of space for experimentation here.

[booster]

Yep, totally agree with Avanti's statements. Lots of questions not answered, but at least now we are looking at them, where before, many of us, me included, just assumed the 12v adapters were more efficient.


Something I did just think about would be the design parameters for the proverbial dorm frig. IIRC, home frig testing and specing has a range of room temps that they perform the tests in, which would be typical house temps. I would certainly think that in an RV, the range of temps would need to be considerably wider than in a house, so the dorm frigs may not be capable at the limits.


One thing I will say that may be an advantage for using several small inverters would be the ease of having isolated outlets for small load inverting. A small inverter and an outlet or power strip and your done. No need for transfer switches and such that you need to change outlets from shore power to inverter.

[cruising7388]

Originally Posted by avanti

As for fridges in particular, high end inverters have "sleep" modes, where the standby current is near zero, with periodic "pings" of current to determine whether a load is present. These often do annoying things in an RV (such as flashing the display on the microwave). But it would be ideal for a unit dedicated to a fridge (except that many fridges probably need constant power if they use electronic thermostats.) An exotic solution might be to have a fridge with DC-powered electronics and the ability to turn on a dedicated inverter when the compressor kicks on.

This is precisely what happened when RT installed the Powerstar inverter. It had the power saving feature you describe which would pretty much negate the issue of standby power dissipation. But what would happen is that each inverter interrogation would make the microwave oven lights flash and more irritating than that, it would make it beep. If you were away from the coach, you probably couldn't care less but when in the coach it would be pretty irritating.

Unfortunately, rather than fixing the problem at the microwave end, RT subsequently had Powerstar deliver the inverter with the power saving mode deleted. Subsequently, owners who left their coaches for significant periods without remembering to shut the inverter down before leaving, would return to find their batteries deader than a door nail.

[booster]

I think a lot of the power saver/sleep setups have a minimum power setting that is user settable, so you can eliminate small loads like the microwave clock getting messed up and trying to come on. Our Magnum said something about it IIRC, but I never have looked into because it is nothing we would ever use as the inverter is nearly always off.

[cruising7388]

Originally Posted by booster

I think a lot of the power saver/sleep setups have a minimum power setting that is user settable, so you can eliminate small loads like the microwave clock getting messed up and trying to come on. Our Magnum said something about it IIRC, but I never have looked into because it is nothing we would ever use as the inverter is nearly always off.

I've never programmed it on the Magnum but I remember that on my Prosine 2.0 I could turn power save on or off, and with power save on, I could define the threshold in watts that would trigger the inverter on and IIRC, the duration between inquiries. The RT Powerstar inverter probably also has a startup threshold but there doesn't seem to be any way to program it. The other shortfall of the Powerstar is that it doesn't do any load analysis and in order to ensure that the breaker doesn't trip, the maximum charging permitted is a little anemic. On our 800ah Eco, the factory set the maximum charging on the Powerstar to 36 amps and you can grow old waiting for batteries to fully recharge using shore power.

[avanti]

Originally Posted by cruising7388

I've never programmed it on the Magnum but I remember that on my Prosine 2.0 I could turn power save on or off, and with power save on, I could define the threshold in watts that would trigger the inverter on and IIRC, the duration between inquiries.

Yes, our Outback is the same. You can set the "turn on" threshold and the probe interval, but not the probe voltage. This was probably ok 15 years ago turning on incandescent lights in an off-grid cabin, but is pretty much useless with modern "always on" electronics. The inverter guys really should work on this. There must be some way to do this unobtrusively using an RF signal, or very low currents or some such.

I actually once took my $80 microwave apart, hoping to mod the control board so that I could run the electronics with 12VDC and only use AC for the magnetron. It turns out the dirty tricks the Chinese engineers use to build a microwave that can be sold for $80 were pretty horrendous. No real boundary between the AC and the DC parts of the board. I gave up.

[Boxster1971]

Having a more efficient 48VDC-120VAC inverter like the Volta System would help make going to a standardized 120VAC system a real winner. Then a small 48VDC-12VDC converter to handle lighting loads would work nicely.

[WJones]

Originally Posted by Etzu

What are you guys using.
PS I'm no longer a power user, I don't game or do any video editing. 99% it's just emails, recipes, to do lists, browsing and maybe the occasional movie in bed.

Seems to me that the needs that you describe would be very well taken care of by a tablet instead of a laptop. It might avoid all these power issues.
Just a thought!