# Replacing a PC's AC-DC converter with a DC-DC converter for mobile power supply

I'm going to be purchasing a mini PC to act as a mobile work station in a set of field experiments. The field locations do not provide power, so I'm trying to come up with a mobile power supply.

For starters, the mini PC utilizes an AC-DC adapter with a rating of 19V and 65W. I've spoken with a few technicians about the mini PC and I was told that it could theoretically demand more than the 65W supply with everything maxed out (apparently never happens), but the system typically operates around 30W. For the sake of being conservative (insert safety factor here), lets assume I'll be operating the system at 60W under full load.

Based on extensive research, in an otherwise non-existent application of portable mini PCs, there are two options I'm looking at

1. Portable battery bank for laptop
2. Auto/marine deep cycle battery w/ DC-DC converter

Let me first identify the power capacity requirements I'm looking for. The system needs to run remotely for 12hrs, with the idea that I could return home in the evening and plug in the battery to recharge it for operation the next day. During the 12hr span of the experiment, the system will be running at full load for 30mins, then "go to sleep" for 30mins. This cycle will be repeated for the full 12hrs.

Based on my power requirements and operating times, I've made the following estimates:

6hrs @ 60W = 360Wh
6hrs @ 20W = 120Wh
------------------
Total = 480Wh


Again, those are pure estimates, which will suffice for this question. Unfortunately, option 1 listed above doesn't present much of a market with solutions in the 500Wh range. The options that did present themselves were relatively expensive in comparison to solutions presented in option 2. I suspect this is due to the Lithium VS Lead Acid batteries. The weight isn't really an issue, so I'm leaning towards option 2, using a marine deep cycle battery with a DC-DC converter. I have found solutions that provide 12V and +70Ah to give +840Wh which is more than enough.

If I go this route, I have to find a DC-DC converter to replace the AC-DC converter to connect the mini PC and battery. Sure, the other option is to buy an inverter, but from what I've read, the inverter is a complete waste of energy, in that it makes a redundant conversion with unnecessary losses. My question is on how to choose a proper DC-DC converter.

I'm not an electrical person, so go easy on me. A lot of this stuff is brand new, but if I cook the system out of ignorance, my boss will have little sympathy. Anyway, if the original AC-DC converter was rated at 19V and 65W, that would give up to ~3.4A. Now I'm assuming the converter is purely a "passive supply", meaning that the mini PC takes what it wants at 19V, but can take no more than 3.4A at a time. That is non-electrical logic for you.

If that is correct, then purchasing a DC-DC converter that is rated at say 100W should be fine? It simply "supplies" more power, but in a "passive" sense, where the mini PC can take it if it wants, or leave it?

I've found some DC-DC converter options that take 12-14V input, and output at a select voltage ranging from 12,14,16,18,20,22V with a maximum output of say 100W. If the logic above is correct, then I should have no problem hooking this up with 12-14V input from the battery and selecting 18V output to the mini PC? The mini PC simply pulls the amps it needs, but can not exceed 100W/18V = 5.5A ?

My confusion comes from how the converters work. Do the "actively" push the wattage through to the mini PC or do they "passively" provide the wattage to the mini PC? If they push it, then it seems that I need a DC-DC converter that matches the rating of the AC-DC converter or less. If they simply provide it, then the DC-DC converter can exceed the specs of the AC-DC converter without any issues.

Any help on clearing this stuff up would be greatly appreciated!

• "Now I'm assuming the converter is purely a "passive supply", meaning that the mini PC takes what it wants at 19V, but can take no more than 3.4A at a time. That is non-electrical logic for you." Yes, that is correct. And with that correct, the rest of your text is also correct. Extra info: DC-DC converters simply make sure that the voltage at the output is at X+-N%, so say 19V +-1%. If it reaches 18.5V then the DC-DC converter will push out more current so it stays at 19V. U=R*I, pushing out more current will take you back to 19V from 18.5V. – Harry Svensson Jul 14 '17 at 2:43
• Since you're going to choose deep cycle lead acid batteries, remember that even "deep cycle" batteries last longer if you don't fully discharge them - as you need about 40AH, batteries around the 80AH range would be recommended (unless you're only planning to use this for a hundred cycles or so) – Brian Drummond Jul 14 '17 at 10:16
• @BrianDrummond Good to know...point taken! – ThatsRightJack Jul 14 '17 at 19:46