[frankthetank]

I'm building a small PV based system more for using on my boat/or in case of power outage.

I'm really only interested right now in powering my notebook computer, because the battery in it sucks!

I have a 400watt(800watt)peak inverter (modified sine wave---seems to work just fine) and a 130AH 12volt deep cycle. Still looking @ panels (probably get a 20ish watt?).

If my notebook says input is 18.5V @ 6.5Amps and my 12volt will put out 6.5amps for 20hours (but only @ 12V)..how do i figure how long my notebook will have juice for using the inverter (there is an efficiency loss when doing this DC to AC to DC but i really don't care!)????

Thanks for the help..i'm no electrical engineer!

[frankthetank]

I'm guessing between 10-20hours...if that helps!

[Devil]

Consumption = 18.5 x 6.5 = 120 W.
Assuming 75% conversion efficiency = power requirement of 160 W
160 W at 12 V = 13.3 A
Battery gives 130 Ah at 6.5 A, probably 120 Ah at 13 A = 9 h QED

[BrownDog]

12VDC to 120VAC back to 18.5VDC seems like taking the long way around the block, since you'll get losses at each conversion. I'm no EE, either, I'm just thinking of the conversion losses. Perhaps somebody who knows better than I do can comment?

[frankthetank]

Thanks devil...man this thing sucks the watts! might want to use the other notebook in times of need!

Wish i had the Pentium M (i've got regular celeron in this notebook) along with a massive 15.4widescreen.

For my other notebook i have a car adapter (has the cigeratte plug in) and i'm guessing that is DC-DC...however, it doesn't work on this (its not suppose to,but i haven't tried it yet) and don't feel like pumping out the dollars for another and i've got the inverter.

10hrs is good enough for me...along with a small panel, i should be fine.

thanks again

[The_Toecutter]

If you have the reserve capacity rating of the battery, you can get more accurate by calculating that battery's Puekert's numbers, and determining the actual number of AH it will deliver when supplyingthe amount of current to meet your needs.

[Zentric]

$this->bbcode_second_pass_quote('BrownDog', '1')2VDC to 120VAC back to 18.5VDC seems like taking the long way around the block, since you'll get losses at each conversion. I'm no EE, either, I'm just thinking of the conversion losses. Perhaps somebody who knows better than I do can comment?

[Caoimhan]

That brings up a point of having a hybrid electrical system in a home.

Some people have taken to wiring their home with both AC and DC systems. You can run both to the same wall panel. This can ease the transition from AC appliances to DC as DC capacity is slowly added by additional PV panels and battery storage. Some heavy power users, such as refrigerators and air-conditioning can be kept on AC for quite some time, with lighting and other appliances being switched over earlier.

A lot of DC appliances can be found by looking in stores that cater to RV users. TVs, VCRs, DVD, Stereo... lots of appliances out there for DC, without resorting to AC to DC conversion.

[MD]

base your DC systems on 24 volt PV panels and four 6v deep cycle batteries connected in series. You can then directly tap 6vdc, 12vdc, 18vdc, and 24vdc supplies directly from the batteries without losing wattage through volatage dividers. (make sure your 24 v panels have actual voltages sufficient to charge your batteries!!)

Most DC devices are fairly forgiving with actual voltage levels.

In other words, your 6vdc supply will likely run devices requiring 3-7.5 volts(with maybe a small dividing resistor on the 3v stuff)

Your 12 volt supply will handle from 7.5 up to 15vdc. (again the small dividing resistor at the low end...watch your wattage!)

18 volts will handle from 15-20 volt devices

24 volt supply carries you up to 28 volt devices.

for the size of your panels, you want to come up with an average load in watt hours, which means you multiply the rated watts of each device by the number of expected hours used in a day, then add all your devices. This gives your total daily watt hour requirement.

Then multiply your PV panel wattage by four or five to get your total watt hours available daily.

Your watt hours available should exceed your watt hours requirement by a safety margin. You pick you own percentage depending on how willing you are to turn some stuff off during long periods of cloud cover

One more thing: If you use this type of system, make sure you balance the load. don't run a bunch of six volt stuff off the first battery, then run 12 volt stuff off the first and second battery. You have to distribute the load or you will get bad battery live and major charging inefficiencies.

[Brandon]

For your notebook computer, just get a car power converter made for your laptop. It'll use a DC/DC switching power supply with likely around 80 to 90% efficiency. One less power conversion than 12v to 120v to 18.5v.

[Zentric]

Good stuff, and thank you, MD.

As far as the solar panels go, are they to be oriented in series or parallel? In other words, if I had two solar panels, would their voltage outputs be >12V each or more like >24V each?

Also, is there any efficiency to be gained in selecting panels that (cumulatively) output, say, 27 volts as opposed to ones outputing, say, 25 volts when charging my four 6 volt in-series batteries? Does some rule of thumb apply?