[Yesdog]

In preparing for an 800 mile cruise in May, I am concerned about electric power needs. I will be using a Mac laptop (G4 Power Book with 50-watt-hour lithium-ion battery and an advertised up to 5 hours of battery life; however I've heard it's less. The computer is supplied with a 45W power adapter for plugging into 120V outlet.) I'm using the laptop for navigation (with GPSNavX software and BSB charts) and I'll be cruising (and using the computer) between 6 to 10 hours on any given day.

Assuming that I'll need to "plug in" (using one of those little plug into cigarette lighter inverters?) the computer from time to time and taking into account the other 12V power needs, cabin lights, anchor light, waterpump, instruments, VHS, auxillary GPS, refrigerator (which runs on 12V once disconnected from shore power) should I invest in a hard-
wired or other type inverter to power the computer? Also, can this power source be isolated, so that the refridgerator doesn't run off the inverter?

BTW, we PLAN to overnight at marinas that supply shore power every night except one. However, in reality, that could change and we could be on the hook overnight more than
anticipated.

My current (no pun intended) set up is two batteries, a battery switch (OFF, ONE BOTH,TWO), and a portable type battery charger that plugs into a 240V power source at night. No generator (don't intend to buy one) on board and, so far, no inverter.

My question is... do I need another or additional batteries (and if so, a new battery switch)? What type inverter (if any) is suggested and what's involved in the installation? And
anything else to share with this electrically-challenged boater.
javascript:emoticon('hock:')

Many thanks for input.

Joy
m/v PYXIS cruising out of Port St. Joe, FL hock:

[cyberhusky]

Try to get an inverter which will give you 120V AC from 12V DC.
And be sure to charge your batteries in ports if available.

The problem with laptops batt accuracy isn't that good as advertised. Though Apple laptops tend to last longer than PC devices. (This could change with the new intel chips.)
I use an iBook 12" G3 or G4 (that was the reason I bought the 12" Mac because it gave me the longest battery run) which runs about 4 hours (advertised as 5-6 hours) with GPS running on it's own batteries (Garmin 12 or Garmin 72). But using serial or usb data transfer will diminish available power. Using an inverter gives you more power for the laptop, but you must have the possibility to charge the batt. (via motor and generator (Not in your case) or harbor.)
I used to have my Psion 5MX Pro pocket Computer which runs 20 hours on 2 AA batteries (or directly connected to the cirgarette adapter which didn't take so much current) if I sail long distances! The maps were either scanned maps or rendered from a geological internet site. It was enough to see were we are but not for coastal navigation.

Happy sailing
Manou

[JackConnick]

But here are a couple of things you want to look at.

How many amp-hours are your batteries rated for? You don't state their size. Are they deep cycle? Ie a Group 27 deep cycle battery is rated at about 100 amp hours.

If you add up the usage of your on-board devices you can see how many hours you can run them all without recharging. However, deep-cycle batteries don't want to be drawn down more than about half their ratings. It's hard on them.

As far as what you have, I'd be upgrading to a good quality, hard-wired battery charger that's 3 phase. It should be rated to charge your batteries up in a reasonable amount of time, maybe around 40 amps. Too small a charger is also hard on the batteries, most portables are around 12-20 amps.

You can add another battery to one of yours in series and double the capacity, but they should be the same size.

With running a refrigerator, I'd recommend no less than 2 Group 27's in series for your A or house bank, plus another starting battery on the B side. I would add an isolator between the banks and maybe an amp-hour meter for convenience.

I'd just use a plug-in inverter, you don't need a big one for your Mac.

Jack

[Intuition]

You might want to purchase a DC to DC charger so that you don't have the power loss of the inverter to worry about. Here's one example:
http://www.welovemacs.com/aag4.html

For electrical advice I'd recommend Nigel Calder's
Boat Owner's Mechanical & Electrical Manual as an excellent source for understanding charging systems, batteries and calculating system loads.

Best regards,
Mark