What size cable.
 

Could someone tell me what size cable (mm2) I need to use for a run of 25 meters to run a 12volt DC to DC converter, 90w/19volt output, from a 12volt, 1.5Kw battery bank?

This is to run an Asus notebook using about 50w.

Thank you for your help.

It depends on many factors, mainly on current, type of cable (Cu,Al or other) and surrounding. You did not mentioned if this 25 meters are from battery to converter, or from converter to notebook. You can get small voltage drop at the end of cable (due its conductivity). In output you have 90W/19V=4,8A, in input you get litle more than 90W/12V=7,5A. Get 15A for safety, and from properties of cables you can read it dimension. For copper cable maximum current 15A is for about 1,5mm2. For AWG cable you can get properties from wiki page.

IMHO you would want the longest run between the battery and the converter. Another important piece of the puzzle would be the input operating range of the DC-DC converter. Is the 19 volt output at full load? The approx voltage drop using 1.5 wire would be about 2 volts which might be ok but you would need to know the battery discharge curve (@full load) also to see if the run time to the converter min operating voltage plus the wire voltage drop meets your needs.

see http://www.powerstream.com/Wire_Size.htm and the calculator at the bottom of the page to see the losses you'd encounter to find an acceptable wire size. You are looking at about 85 feet of wire at maybe 5 to 10 amps. An acceptable loss is usually something under 5% meaning the 12v side could be wired with a wire loss of a half volt or so while the 19v side could maybe accept almost a volt of wire loss.

The 1.5 kW battery bank doesn't mean much as that is a power rating. Generally, with lead acid batteries, you have an available energy capacity of about 15 usable watt hours per pound when run at a watt per pound power draw. A number of factors can influence this by +/- 10% or more so planning should allow for a significant reserve.