Without knowing specifics about your laptop, I'll hazard a
guess. The lower-power adapter probably doesn't provide
enough current to run the laptop at full power, so the
laptop power profile is left on the "battery" profile which
uses less current so that it can conserve power to keep the
batteries from becoming drained. So, yes, this should also
draw less power from the mains side of the power adapter
which would help your 12-volt-battery-with-inverter last
longer.

> Hey Roger,
>
> One question. On getting the adapter for the Dell with
> the higher current (since that is what my laptop is) I
> was wondering if it would be better to not get it and get
> the one with the lower current. The other day I was in a
> meeting and I plugged into one of the adapters we had in
> the conference room. It was one for the lower current.
> The laptop worked great. The only thing I got was a
> message when I plugged it in saying that it knew it was
> the lower current and that it would continue to run on
> the battery profile.
>
> From the point of view of expending less power wouldn't
> it be the better way to go? I'm sure there is some EE
> answer as to why it wouldn't, but just thought I'd ask.
>
> C.
>
> -----Original Message-----
> Roger Hamlett
> Sent: Monday, March 01, 2004 2:34 AM
> Subject: Re: [M]: Help With Deep Cycle Battery - Some
> real numbers
>
>
> > Hey Guys,
> >
> > Thanks so much for helping me understand how things
> work.
> > I can't believe I've forgotten this much from my EE
> classes
> > when I got my CS degree. Haven't done pretty much any
> EE
> > since, except for installing car stereos in my cars and
> stuff
> > like that.
> >
> > Anyways, can you guys help me with some real numbers
> then.
> > I am clear on the fact that batteries should not be run
> down
> > to the ground, so I want to distribute my loads well.
> I am
> > going to get another battery. Hopefully a 115 AH
> battery.
> > Here is what I am thinking of doing. Can someone help
> me
> > figure out how long I can run these things before I get
> > either of the batteries below 30%:
> >
> > What I have to run is:
> >
> > - Laptop: Dell Inspiron. The power supply says it
> consumes 4.5A.
> It says that the supply can _deliver_ a maximum of 4.5A
> (probably at about
> 18v). This is different from the consumption. The supply
> only has to deliver
> this when the laptop batteries are flat, and it is having
> to charge them at
> the same time as the laptop is being used. You can keep
> the actual
> consumption of the laptop lower, using the options to
> switch the processor
> speed down, keep the display dimmer, etc. etc..
> Ideally, get a 12v supply for the laptop. These are
> available from Dell
> themselves, and (cheaper) from a lot of third party
> companies (given that
> your unit has the higher current supply - there are two
> versions of Dell
> supply, according to the Inspiron model involved, make
> sure that the supply
> you get is for the higher current unit). This gets rid of
> the double
> inefficiency of converting to 120v, and then back down to
> 18v, and means
> that the whole system in this area is running at the
> lower (safer if
> conditions are damp) voltage.
>
> - SBIG ST-7: From the SBIG website for the ST-7 it says:
> 5 VDC at
> > 1.5 amps, ±12 VDC at 0.5 amp desktop power supply
> included. So
> > which one do I use?
> Again ideally, get the 12v supply. The same comment
> applies as for the
> laptop. The figures given are the _outputs_ from the
> supply, which is having
> to deliver a maximum of 19.5W (5*1.5 + 12*.5 + 12*.5).
>
> > - Thousand oaks digital dew controller: Unfortunately,
> their site
> > has very little info on current consumption and the
> unit has
> > nothing printed on its back. I'd be running a 10"
> strap, a telrad
> > heater, and a 2" strap for EPs.
> The unit itself, draws practically nothing. This is why
> it does not have a
> figure on it. It is the _straps_ that draw the power, and
> the amount varies
> according to how high the heaters are set. Typically, a
> 10" strap, will draw
> perhaps 3A on full, while the eyepiece strap (and the
> Telrad heater), will
> only draw about 0.2A at the same setting.
>
> > Here's the site:
> http://www.thousandoaksoptical.com/newproducts.html
> >
> > - LX200: As advised, I'll turn slewing speed down to
> maybe 4?
> Perhaps 2... :-)
> I suspect the reason for the laboured sound, is that the
> _peak_ current
> delivery of the 1812 module, is less than the mains
> supply. High speed is of
> no advantage really (if you are going to another part of
> the sky, the scope
> taking a few seconds longer is not a great loss).
>
> > That's it. I have a COleman 800 W inverter that does
> up to 7
> > amps, and I'll probably get another COleman 400 W that
> does up to 3.5
> amps.
> You do not need more inverter power here. If you are
> buying extra stuff, go
> for low voltage supplies instead, and get away from
> running high voltages in
> the conditions round the scope, and the inefficiencies
> involved.
>
> > I was thinking of running the laptop and sbig on one
> battery (to keep
> > them separate from the scope) and since they are the
> ones that consume
> > the most, run them on the big battery (115 ah) and the
> scope and
> > digital dew system on the other battery (the 75 ah).
>
> > Do you guys agree with this? SHould I get the smaller
> coleman
> > inverter or another big one? They are both on sale
> right now. $29.99
> > for the small one and $50 for the big one.
> >
> > Will this be the best use of these batteries? How do I
> calculate how
> > long I should run them before getting to 30% charge?
> >
> > Thanks so much for the help!
> Work like this:
> Take the quoted battery power, and _halve_ it. (this
> allows for the battery
> ageing, and the fact that you will be using it at low
> temperatures), while
> also avoiding fully discharging the battery.
> So for the 115Ah battery, treat it as if it can deliver
> 57.5Ah.
> Now work out the total power needed.
> Laptop say perhaps (working at 3/4 the 'peak') 3A, at 18v
> = 54W.
> ST7 = 20W
> Heaters = (take 3/4 power) 32W
> Scope (look at an average between the 'peak', and
> 'continuous' power, at
> perhaps 0.7A at 18v) = 13W
> Assuming you have switched to using all '12v' inverters,
> instead of 'double'
> converting, then add these together, and add an
> 'inefficiency' for the
> inverters at 20%.
> Total = (54+20+32+13)*1.2 = 143W
> Current (at 12v) = 143/12 = 11.9A.
>
> The battery can then give about 5 hours.
> In fact the figure for the laptop, heaters, and scope,
> are all higher than
> 'real'. My own Inspiron, only draws about 2A, provided
> the internal battery
> is charged, while the heaters typically operate nearer to
> perhaps 30% power.
> I see more like 10hours operation, with a similar setup,
> on this sort of
> battery. The figure represents a 'worst case', for a
> really cold night.
> However if you work through the 'double inverters' (using
> the Coleman to go
> up to 120v, then down again to the voltages required by
> the individual
> units), you would need to think in terms of:
> Total = 143*1.2 (to allow for the inefficiency of the
> second inverter) =
> 170W
>
=== message truncated ===


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