Measured power draw of various items aboard.

[Sunbeam]

Having some time to spend at anchor recently, I finished setting up the battery monitor and also measured the power draw of various items I have aboard. I thought the figures might be of interest to others here.

(I had been meaning to do this for awhile, but the immediate reason was that I could see a .19 draw on my Victron battery monitor and I wanted to figure out where it was coming from. After disconnecting everything - luckily the boat is fairly simple - it was still there, and it finally dawned on me that I had not "zero calibrated" the unit after installing it. That was a simple matter once I remembered to do it. After that I reconnected things one at a time and measured the draw. Of course the ones I really pay attention to are things that I may have on for hours (fan or anchor light, for example), not things that are only momentary. Anyway, here is my list. These figures are in amps.)

1) Battery monitor, Victron BMV-600S ---------------------------------------0.0


2) Bilge pumps, basic Johnson pumps with Water Witch 101 switches
2a) Idle -------------------------------------------------------------------0.0
(I wasn't sure if the Water Witch switches might draw when idle)
2b) Bilge pump running, 1,000 gph pump--------------------------------------1.5
2c) Bilge pump running, 500 gph pump----------------------------------------0.7

3) Battery combiner, Blue Sea ACR 7610 --------------------------------------0.0 
(Note: the Blue Sea folks told me this does draw a small load unless
 re-wired to be "off" with battery switch "off."  I did re-wire mine 
 this way because I didn't want a draw when boat idle for longer periods,
 so I'm probably not going to investigate it further myself).

4) Trim Tabs, Bennett M-120 Sport
4a) Relay, when not active---------------------------------------------------0.0
4b) Trim Tab indicators------------------------------------------------------0.0
(LED lights are always on when wired according to Bennett) (Note that
 before I "zeroed" the battery monitor these showed a .04 draw, which
 is probably correct but just that coming from zero it's too negligible
 to show when it's the only load.  I may put in a switch to shut these
 off anyway just to not always have the lights on at night when I may 
 want it dark.)
4c) Trim tab(s) cycling (one or both is same total draw)--------------------12.0

6) Chartplotter, Garmin 2006c
6a) Getting satellites, backlight fully on------------------------------------.87
6b) Backlight on lowest setting-----------------------------------------------.35

8) Depth/fish finder, Garmin 160 Blue/Airmar P66 Triducer---------------------.38
8a) Backlight on (forget which setting, sorry)--------------------------------.52

9) VHF, Standard Horizon Eclipse+
9a) Stand-by-------------------------------------------------------------------.25
9b) Transmit, Hi--------------------------------------------------------------4.90

10) Fan, Hella Turbo
10a) Low-----------------------------------------------------------------------.19
10b) High----------------------------------------------------------------------.26

11) Horn (stock one for my boat)----------------------------------------------5.0

12) Cabin light---------------------------------------------------------------1.5 
(stock ones for my boat, horizontal lens cover slides sideways as switch,
 incandescent bulb) 

13) Cockpit light (squarish, on aft side of cabin door bulkhead)--------------3.9

14) Nav. lights
14a) All-around light----------------------------------------------------------.12
(stock Perko removable pole fixture with Marine Beam LED bulb)
14b) All-around plus sidelights------------------------------------------------.37
(stock Perko 120 sidelight fixtures with Marine Beam LED bulbs)

15) Computer
15a) Inverter, Powerline 90388-------------------------------------------------.22
(small type that plugs directly into cigar lighter fixture)
15b) Above, with Apple/Mac 60W power brick plugged in (total figure)-----------.41
15c) Above, with older Macbook Pro 13" plugged in and charging---------------~4.0
15d) Above, w/computer on, charging, not "doing" much, full display backlight-5.0
15e) Above, display backlight on lowest setting (visible, albeit dim, screen)-4.4
15f) Above, no display backlight on (screen dark)-----------------------------4.3

16) Battery charger, dry cell 
(Maha MH-C401 FS, used for charging AA and AAA Eneloop batteries, plugs 
 directly in to 12v cigar lighter socket)
16a) Just plugged in, doing nothing--------------------------------------------0.0
16b) Charging 4 AA batteries----------------------------------------------------.30
(Of course this may vary at different stages of charging, and AA's take
 longer to charge than AAA's, so this is incomplete info.)

17) Music player 
17a) Cigar lighter to USB adapter, just plugged in and doing nothing-----------0.0
17b) Charging fully discharged Apple Nano Generation 1--------------------------.15
(I didn't time this, but it took perhaps 3-4 hours to fully charge, as a 
 rough estimate.)

Sunbeam

 

[hardee]

Sunbeam,

Nice research. Nice device to have on board. Lots more detail than I ever thought about though. I can see it as useful information for long term cruising.

Harvey
SleepyC:moon

 

[Sunbeam]

Glad you found it interesting. Even for shorter term though, a higher draw device could take the batteries down more than you might think (or, conversely, you might hold off on using said device unnecessarily). For example, sitting here at anchor for a couple of days with Internet access and a laptop, I could draw the house bank down pretty easily (i.e. this could be a weekend, not "long term").

Still though, I hear you, and if motoring a reasonable portion of each day and on a shorter trip, it may not be a "thing."

For me, it is so worth the relatively reasonable price and easy installation. But then I like my data One handy feature of the display is "SOC" or State of Charge. I can glance over and see that the house bank (really just one dedicated battery) is at, say, 87% charge. Then I know how much power I have to spend or save as desired to keep above whatever percentage of charge I want to (and considering how soon I may be motoring again - since for me right now the engine is my source of charging). It also shows the voltage and/or amperage going into the engine while I'm motoring - I found it interesting that the main engine was charging at a fairly high rate even when just idling along in narrow canyons. The alternator on smaller "kicker" sized outboards that I know of don't really put anything out until at a high percentage of throttle). At least this is how I'm understanding it.

Sunbeam

 

[thataway]

Nice study. We have also mapped out our specific lights (and one of the reasons to go over to LED). For the low draw appliances, we find it more accurate to use a digital volt meter, which does better than the shunt for low draw--in the milliamp category.

Thanks.

 

[Sunbeam]

For the low draw appliances, we find it more accurate to use a digital volt meter, which does better than the shunt for low draw--in the milliamp category.

Ah, good to know! Thanks. I have a digital voltmeter aboard, in case I get curious.

 

[Foggy]

So, what is your average daily use of the listed equipment (amp/hrs) at anchor, not plugged in to shore power and not running you engine(s)?
What do you have for your 'house bank' batteries in amps?
Are you satisfied with what you have or are you running low on amps?

Aye.

 

[Sunbeam]

So, what is your average daily use of the listed equipment (amp/hrs) at anchor, not plugged in to shore power and not running you engine(s)?

Hi Foggy,

Since this is my first real trip with the boat, I only have preliminary thoughts - others here have years of experience on their C-Dorys to draw on. Also of course everyone is different in how they like to use their boat. I did live/cruise aboard a larger boat for some years, so the concepts/calculations are not new to me; but this boat/usage is so different (and so far, quite fun).

First off, let me say that the boat is, so far, pretty simply set up in terms of things that use power. Really the only "big draw" at anchor has been my laptop, and then only if surfing the web (like now) when I have a connection (otherwise just downloading camera, which is a minor use). This could easily be a huge, battery-gobbling draw! At anchorages where we didn't have Internet, the state of charge (SOC) of my house bank after a few days was still upwards of 95%. Contrast that to here, where we have been holed up for two days and DO have Internet (and I am out of paper books!), and we are now at about 70%. I might not have allowed myself to draw it down that far (charging the laptop, which is basically a luxury), but I'm reasonably sure we'll be motoring again in the morning.

Other than the laptop at anchor, other draws we've used (that may or may not occur when at anchor) that come to mind:

1) Occasional charging of buddy's phone or tablet for backup charts (Navionics). We tended to do this when underway, to use "extra" power (battery already fully charged).

2) I once charged a set of AAA batteries for my headlamp, although I have plenty of charged batteries along, but we were motoring, and the batteries were full, so I figured why not keep the stock topped up. Same thing for a set of AA's that power the LED lantern.

3) I charged up my Nano music player, mostly to see what the draw was for my chart above, as I have not used it yet.

4) Chartplotter and/or depthsounder on overnight for anchor/depth alarm (sometimes).

5) "Non-draws," which could be draws, if that makes sense (which I'm sure you are familiar with since you are a long time boater, but for the sake of the thread):
* Nav/Anchor light -- these are are LED, so draw little.
* Cabin lighting -- we've used our headlamps, a candle lantern or an LED lantern we have that runs off of AA batteries. We've also pretty much gone to bed after dark (except for some rounds of cards; also buddy's e-reader has a backlight, and I use headlamp for paper book reading at night).
* Fans -- temps are such that we have no need of cabin fan(s).
* VHF on stand-by -- but since there basically is no VHF here (too rocky/bluffy/canyony) we haven't always had it on.
* RADAR on standby -- but the boat does not have it (yet).
* Refrigerator/powered cooler -- we have basic cooler with ice.
* Water heater or pump -- we have a teakettle and foot pump.
* Heat and cooking -- neither of them as currently set up on this boat use electrical power.

What do you have for your 'house bank' batteries in amps?

For the house "bank," I have one Group 31 AGM battery that has a 115-amp-hour capacity. I chose the 31 because it's basically the same physical size as the 27, fit efficiently into the lazarette, and has more amp hours than the 27.

Are you satisfied with what you have or are you running low on amps?

I'm pretty happy with the way things have been going, but I can also see there may be times I would want to use more power (that darned hungry laptop a heater/fan, or other added items). I think I would next consider a solar panel to augment motor charging (because I don't tend to spend time in marinas and like to hang out at anchor). It's also quite possible I'll want to "complicate" the boat further, but I'm not sure about that yet.

I don't have shorepower on the boat, but do have the "ingredients" for a simple system as outlined in other posts here. Not sure if I will hook it up or not, because I'm not typically a marina person (but then again, use will likely evolve).

I guess if I had to summarize in one sentence so far, it would be that I'm happy with what I have, and am enjoying the relative simplicity; but laptop use is the wild card (note to self: bring more books!). A solar panel would be nice for many locations. I can also see why people are attracted to things like the Engel/Waeco coolers (powered), and wouldn't rule one out. But the corollary to that is that after years of dealing with more complicated boats (other people's), I'm not wanting to rush into complications either. Obviously more time on the boat to figure things out is what I need

 

[Foggy]

SNIP

What do you have for your 'house bank' batteries in amps?

For the house "bank," I have one Group 31 AGM battery that has a 115-amp-hour capacity. I chose the 31 because it's basically the same physical size as the 27, fit efficiently into the lazarette, and has more amp hours than the 27. SNIP

My understanding of Group size is that all Group 27 size batteries are all the same length, width and height. Same for Group 31 - all the same 'size'. Amps may vary with Mfg. Plus, for recharging your batteries, it is suggested you have all your batteries the same: voltage (12V, obviously), type (gel, AGM, etc) and maybe capacity in amps. You may want to check on that. I'm going to. Modern smart chargers and/or isolators may take this into account so it's not that important now days.

If your average daily amp usage equals 50% of your battery capacity in amps, you should be good on your house bank alone w/o external charging.

Aye.

 

[hardee]

Hi Sunbeam et al,

Since I am looking at replacing my batteries (2 group 27's) this year it was nice to learn that the G-31 is the same physical size. My batteries are going on 6 years now, and I'm thinking next spring will be the time to make the change.

I have used them pretty much all the time on the "Both" on for all use with no problem. I check the voltage frequently (via a cheap cig plug monitor), and it gives me the trend info I need to keep the batteries up. It is really running on the KISS side of things.

I have thought about solar charging and rarely plug in at the marina, finding that running the boat at least every other day is adequate for my electrical needs, mostly a CPAP at night, and vhf or music during the day if I am not running. I do charge my phone each night.

This has been an enlightening thread because I have not really gotten into how much juice some things use. My last trip, 10 days, was even without a cooler, just to see if I could do it. It wasn't too bad. Had some fresh veggie's, some nuts, dried fruit, some bread stuff and some canned proteins.

Thanks again for sharing,

Harvey
SleepyC:moon

 

[Sunbeam]

My understanding of Group size is that all Group 27 size batteries are all the same length, width and height. Same for Group 31 - all the same 'size'.

That's my understanding as well. However, the Group 31 physical size is close to the Group 27 physical size (not the same, but close). I couldn't easily fit a very much larger battery, but I could fit a Group 27 or a Group 31. The 31 gave me more "bang" in amp hour capacity for the physical size I could conveniently fit, so I chose it. I've found these specs vary slightly by mfgr. but these are the two for the company I was buying from:

Group 27:
12.09" x
6.65" x
8.31"
105 amp hour capacity
66.5#

Group 31:
12.91" x
6.77" x
8.43 "
115 amp hour capacity
72#

Amps may vary with Mfg. Plus, for recharging your batteries, it is suggested you have all your batteries the same: voltage (12V, obviously), type (gel, AGM, etc) and maybe capacity in amps. You may want to check on that.

My house battery and start battery are both 12volt, both AGM, and both the same age. They are not the same size, by design (the start battery is smaller), but I believe this is okay. As far as charging, I have the batteries (house and start) connected with a Blue Sea ACR and switched with an On-Off-Combine (for emergencies) switch. My boat came with a different "system," which is also popular, which was two equal sized batteries (happened to be two Group 24 liquid electrolyte batteries), and a 1-2-Both-Off switch. With that system, "house" and "start" would simply be alternated from day-to-day, or something similar.

I don't have a smart charger installed, but so far am relying on the outboard (of which the alternators are not "smart"). I do have a small "smart" charger that I can hook up with alligator clips and plug in if I'm going to be near an outlet for awhile when on the trailer.

If your average daily amp usage equals 50% of your battery capacity in amps, you should be good on your house bank alone w/o external charging.

That would be my lowest acceptable SOC if not in an emergency situation. Although added to that in my mind is "how much am I going to be motoring and how soon," because currently that's the way I'm getting charge into my batteries. If I want a full battery tomorrow night, but am only going to motor a short time tomorrow, I'll put that into my mental plan,and probably not draw down to 50% (due to the limited ability to put back, with a short motoring period).

I agree there is a lot to consider, many acceptable options (and some not so acceptable) -- and a lot has changed over recent years. Also electricity is not my strong suit, but I try.

Sunbeam

 

[thataway]

The group 31 can be from 95-135 Amp hours, The group 27 from 85 to 105 amp hours. The battery with the higher capacity will weight more, because of heavier lead plates. We try and get the highest amperage battery we can find.

We found that in AK with 40 amps of the 130 Honda we would run at least 3 to 4 days before visiting a marina--and we usually did that for water and often good hot showers (even though the 25 has a shower). We had a 1000 EU Honda generator and an extra 30 amp battery charger, plus the built in 10 amp charger, but never used it. The usage included a built in refrigerator. This changed at Lake Powell, where ambient temperature was much higher, and we had to run the generator every few days to keep the battery well charged. However we would stay at anchorages often for a number of days.

We like the chest type of refrigerator/freezer such as Dometic/ Waeco. To run this we have a 125 amp, group 31 AGM battery, and run the generator with the 30 amp charger an average of 2 hours a day at Lake Powell--daytime ambient temperature of 80 degrees. The house battery--currently group 27 flooded (will be replaced with group 31 AGM), lasts at least 2 to 3 days without charging. We just only use a few hours of LED lights for reading when at anchor. We will put the boat's 20 amp charger on the house battery every few days to keep it topped off, if we were not running the outboard.

 

[Foggy]

Running your engine will only partially charge your batteries. To get a full charge, which they need periodically to prevent sulfonation, the batteries should have the proper charge system, via shore power or solar panels, to take them past the 'bulk' charge phase which is the limit of your engine alternator.

So, I'd rethink your idea of "If I want a full battery tomorrow night, but am only going to motor a short time tomorrow,".

Aye. I'm learning too.

 

[Sunbeam]

Running your engine will only partially charge your batteries. To get a full charge, which they need periodically to prevent sulfonation, the batteries should have the proper charge system, via shore power or solar panels, to take them past the 'bulk' charge phase which is the limit of your engine alternator.

So, I'd rethink your idea of "If I want a full battery tomorrow night, but am only going to motor a short time tomorrow,".

Aye. I'm learning too.

I do get that the outboard engine alternator is not "smart." And too, that it is not as sophisticated (or as good for the batteries) as a smart charger. Yet my experience on this trip has been that if I am motoring for a fair bit during a day, the SOC of my battery does get to 100% (according to the battery monitor, which I think is accurate) before I anchor. Of course if the battery starts at 95% this takes less motoring than if it starts at 70%, which is why I said I will take my next day's planned motoring into account when deciding how low I "can" draw down the house battery. Meaning I don't want to "have" to motor more just to charge the battery, if I can avoid it.

Since I'm not planning to be in places where I couldn't purchase a battery (with some notice of course), I decided to take my chances of reduced battery life due to "dumb" charging -- for now -- rather than put in a more sophisticated system that I might not even be able to use (if not at a shorepower location and since I do not have a generator).

I learned while cruising that AGM batteries don't seem to be fond of spending life at 70% charge, but in that case even more time was spent at anchor, and we had even more limited charging ability (basically one solar panel, and no modern charge controller on it), and too, replacing a bank prematurely meant a much larger cash outlay than my one Group 31 and in places where the batteries might be hard to find. In actual practice on our type of boats now, what I've read (and talked over with people who have more C-Dory type experience) indicates that the one AGM with the outboard's charger doesn't die as prematurely as you'd think it would (presuming some attention to keeping SOC up).

Sunbeam

 

[NewMoon]

Another bit of analysis to throw into the discussion:

When figuring our boat's electrical needs, and thus the desirable battery banks and alternator, I looked at the documentation (or the case) of each significant current user on board, and plugged their amp draw figures into a spreadsheet, along with conservative (high) estimates of the amount of time the devices would be in use during an overnight at anchor.

This made it easy to calculate the estimated AH usage for each, and the total. And to fiddle with the numbers a bit until I thought I'd got the estimates to a fairly realistic (but a bit high) figure. I also moved the users up and down in the list so as to sort them in order by total projected usage per overnight. The fridge was by far the greatest user, and the anchor light (before I changed it to LED) surprised me by coming in second.

The spreadsheet said we might use as much as 60-70 AH per 18-hour spell at anchor. I haven't yet verified the rates of current draw figures for each device with our Link 2000 AH meter as Sunbeam has, but overall usage is as predicted, ranging from 30 to 70AH per night.

Cruising electrical wisdom suggests that the house bank capacity should be three (or even four) times that AH requirement, since you don't want to draw down below 50%, and pumping back in the last (top) 15% of charge is much slower than going from 50% to 85%. So we chose a house bank of 210 AH, two G31 AGM's from Deka.

Not being limited to the charging system of an outboard, we also chose a nominally 75-amp external alternator and smart regulator to replace the stock one on our diesel engine. The alternator's amp output should be at least 25% (maybe 40% or more with AGM's) of the house bank size in AH. The spreadsheet also calculated how long it would take to recharge in various scenarios.

If anyone is interested in a copy of the AH usage spreadsheet, or of another one in which I collected the stats on a variety of batteries, and compared various possible setups, I'd be happy to email.

 

[Sunbeam]

Good points, and I have worked it that way before. On this boat (my C-Dory 22), I've kind of worked "backwards" for now. That is, I fit in the largest practical house battery (without going to heroics or moving the battery location entirely), in both physical size, weight, and the amount I wanted to be "responsible" for re-charging with my current available methods (outboard's alternator and small smart charger for occasional trailer use). So I'm fitting my electrical draw desires into that (LED's make this so much easier!) instead of figuring out my draws and then designing a system around them. Since things are simple, this works - at least for now.

Once I've used the boat more, I'll decide if I want or need anything more, and then change things as necessary (within the constraints of size and weight presented by the boat).

Wind is down today, so I'm off to do some exploring, which will put some amp hours back into the battery

Sunbeam

 

[thataway]

Both Foggy and Richard have brought up the charging of the last 15% to 20%, and this is important. This is the reason we try and hit a marina with shore power or run the little generator every few days--to be sure that battery is topped off regularly. There are other ways to help prevent sulfation--and that involves the pulse chargers--some of which actually run on 12 volts--but again you have to be able to keep those batteries topped off with a shore charger.

On our larger boats--where we had very large electrical demands--such as an auto pilot, 10 hours of navigation lights, etc we had not only 200 amp alternators on the engine--and in one boat on the gen set--plus 130 amp chargers on the inverters--which battery banks which were over 1200 amp hours. But this type of technology is not either practical or necessary in a C Dory.

For a 25 or Tom Cat, about the max is 2 engine start (dual engines) one or two in a house bank--and then a separate inverter or refer bank. It is fairly easy to do the 2 group 31's, in the larger banks, some have gone with an 8D battery or a couple of golf carts in series. However, doing this makes any of the boats considerably heavier, and will affect performance.

 

[Sunbeam]

Just as a random data point, since it has come up in the thread: Yesterday morning when we left our anchorage, the battery SOC was at 64.7% (after two days and two nights at anchor, mostly aboard, weathering a front/wind - and nearly all of that draw down was my laptop/Internet, so nothing "essential).

We cruised for 3.5 clock hours (3 engine hours), all between idle-forward and hull speed (say around 800rpm to 1400 rpm). We covered 14 miles (and incidentally used around 2 gallons of fuel, give or take (hard to measure that little). When we put our anchor down again at the end of that time, the SOC was at 92.6%

Yesterday afternoon, last night, and this morning were more typical of what we have seen (no Internet). While at anchor (a bit under 24 hours) we didn't use any power to speak of (charged a camera battery and a phone slightly). Then this morning we puttered around 30 minutes (1200 rpm or so). We wanted to get back to "Internet land" to see about the park closure and look up weather so we could make plans. Now the SOC is at 93.8% (all as measured on the battery monitor).

I will have to keep an eye out to see what it takes to put that last 5-6% in. We've only been below 90% a couple of times, so I don't have a solid feel for it yet since this is the first cruise with the boat. I could probably kill a battery in short order with easy Internet access and no additional charging methods (but I will exercise some self control!).

Sunbeam

PS: I mentioned this before, but my biggest (happy) surprise is that the engine puts a fair number of amps in to the battery at idle/hull speeds (10-12 amps). With smaller outboards like you'd have on a smaller sailboat (at least in my experience, which granted is not with all motors), the engines don't really put out much until you are closer to WOT (or rather "full normal running speed). I have never run a "large" outboard before so did not know what to expect. My Yamaha is listed as having a 20 amp alternator, but (unlike some other brands, and I have not researched further) not listed is whether this is "available" total or just the total (some of which would go to running the engine). I suppose since it is carbureted, less goes into the engine than would with a newer engine). Also, I haven't yet noted how many amps are going in at higher speeds (maybe it's the same). Also will have to see if that number drops as I get over 95% SOC.

 

[srbaum]

Sunbeam,
Why do you use an inverter to run/charge your computer, vice powering it from the 12 VDC (cigarette lighter socket)?

 

[Sunbeam]

Sunbeam,
Why do you use an inverter to run/charge your computer, vice powering it from the 12 VDC (cigarette lighter socket)?

Good question, and a slight peeve of mine :amgry With all my past computers (I've tended to live on 12 volts, so they've all been laptops) I have purchased a direct 12 volt charger, and I always considered using an inverter sort of a waste (going "up" then back down again not being as efficient).

However. The newer Macs have a connector called a MagSafe. It's a nice feature, basically - the computer end of the charger line is magnetic, so if you trip over the cord it it just pulls out instead of bending or breaking it -- or dragging your laptop off the table (BTDT). But, it's proprietary. So there isn't an aftermarket charger you can just pick up and buy. And Apple for some reason has chosen not to make a 12 volt charger themselves (they have a connector you can use to run it on an airplane, but it does not charge it).

There is a way around it. You can buy another Apple charging "brick" (to get the MagSafe end) and then mate it to a "whatever brand" 12 volt charger made for laptops (or make the one end usable for both with a joint). I just haven't done it yet (I guess I was lazy and just started using a small inverter in the road vehicle, where power is not as precious). Now I probably will re-visit getting one of the "home made" direct 12 volt chargers (there are places you can buy them too, but it's a little bit of a runaround).

Now you're probably sorry you asked! But it has been an annoyance and I would have wondered the same thing as you.

Sunbeam

BTW, A bit more running today (at cruise speed) and the SOC is up to 99.99%. At the moment I don't have the battery monitor where I can see it from the helm (I think I will move it so I can), so not sure exactly how much things slowed down for the last bit of "juice" going in, but it didn't seem to be too awfully slow. Perhaps it's just as much or more that it's not as good for the battery(s) as it would be with a smart charger (that has float stages and etc.). I assume using the non-smart engine to charge will shorten the battery's life some, but decided to go with this system for now and see how it goes. Worst case, I buy a new battery sooner than I would have, and re-evaluate. I will at least keep the charge percentage up above 50-60% to help the battery out as much as possible.

 

[willronco]

Sunbeam (and maybe others who have faced this Mac annoyance),

There is a company called HyperShop that makes a variety of external batteries, connecting cables, etc. to help charge Apple laptops more conveniently:

http://www.hypershop.com/

I haven't used that particular one, but I have used a similar kind of product with a magsafe plug on one end and a 12v adapter on the other which worked OK. If I get a chance to try out the hypershop device, I will report my findings.

A thousand thanks for all the current draw information, it is incredibly helpful!