Hello so I have a pico psu lying around that can supply 120w from a 12v source so I'm thinking about using a 12v car battery and a solar panel array to run a old server. But my question is can I just connect the pico psu directly to the battery or do I need to make a circuit to limit the current ?
It would run - and it would probably burn (EDIT: the PSU if it has a malfunction as the battery will deliver its max Amps when it can);
I would highly recommend a safeguard. May it be a circuit or a fuse as the short-circuit current of a car battery is nothing to underestimate.
Yes, you should put an inline fuse in for sure though. 120W/12V=10A*125% means you should probably have a 12-15A in line fuse for short circuit protection.
Thats not how batteries work man. Wat. I mean yeah put a fuse on it but its not going to spontaneously combust just because you put a battery capable of high current on it.
I may should have been more specific - I did not reference to the lead acid battery with "burn" I was more like the PSU will/would/could burn in case of a malfunction as the battery will just deliver what it can no mater what.
A lead acid (car) battery will not combust by itself anyhow - its only advice to have a well aired room as the fumes are combustible.
Never said that - or did not intent to imply it ^^
You could use a DC-DC Buck-Boost converter to supply consistent voltage and current to a picoPSU. Just make sure you get one rated for the voltage and current you need.
I have used a few of the cheap chinese ones to drive some LED's in current limiting mode, and they have held up for a good while now. There are many on ebay and amazon for cheap.
There are also large capacitors used in car audio applications that might protect against low voltage drops. However I am not sure how well these would behave on a battery that is being consistently drained. This would all depend on how fast your panel could charge back up the system.
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According to the data on Amazon, the Pico PSU has an input voltage rating of 11.5-13.0V. If you can make sure that your car battery will deliver that, I don't see a reason why your idea shouldn't work. The problem however is that a battery's voltage is not the same at all times. In fact, a standard car battery (led-acid) will usually operate between 10.5 and 13.5V (give or take) depending on its charge. This means that you need to make sure that there is some form of under-voltage protection, or otherwise the Pico PSU might start dropping the output voltage to your server and then bad things might happen (data corruption, etc.). In addition to that you probably want a UPS-like shutdown trigger if the voltage drops too low.
I assume you also planned a charge controller (for charging the car battery) to go along with that solar panel array? Usually car batteries charge at 13-14V. This is probably fine for the Pico PSU, I assume, but it might run hotter, depending on the load.
And as previously mentioned, an appropriate fuse is probably not a bad idea either, but I think managing under-voltage and charging is probably a higher priority at this point.
EDIT: I haven't seen @Kur0's post before submitting my post.
That is probably the easiest way to make sure the Pico PSU gets a clean input voltage. But don't forget to plan in cloudy days. 
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Since solar is the main idea here, putting another car battery in series to give 24V would be a good idea. Some of the DC-DC converters will run on a very wide range of power, 24V would give these a more efficient range when stepping the power down to 12V, thus a longer run time. 24V panels are widely available.
This is a similar unit to the one I have used: Ebay Listing DC-DC Buck Boost
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Provided the pic PSUs input voltage is within the battery voltage range it will work just fine (Keep in mind voltage drop from the load put on the battery)
You do not need anything to limit the current, the PSU will only pull as much current as it needs regardless of how much current the battery can provide.
A fuse is still recommended, but it's only there to keep anything from getting lit on fire if your pico PSU dies and shows up as a short for the battery.
Assuming your batteries have enough capacity and that your solar panel can charge the batteries within the amount of time that they're exposed for light you're going to be fine.
If this is a practical thing to do on the other hand is another question entirely, you're going to need a very big battery bank to keep a system that needs 60-120w continuously running along with a decently large solar panel (300w or more)
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I use an M3-ATX Pico PSU from Mini-Box hooked up to a pair of 50AH batteries in parallel and a 145 watt solar panel with charge controller. That particular model is capable of dealing with the range of voltages seen on a car battery or solar power system. It also has extras for connecting the computer power button through the Pico PSU and controlling when it turns on and off, which is user adjustable. I bought it a long time ago before they had some of the newer and cheaper WI (wide input) models without all of the extra features.
It works flawlessly for me, but my rig draws ~20 watts running full tilt. The machine you are trying to run, specifically how much power it uses, will dictate which PSU you need and how much solar power and energy storage you need. Mono and polycrystaline panels are fairly cheap and last a very long time, so it is worthwhile to get more wattage than you absolutely need.
My panel still manages to top off my batteries even on cloudy days with a PC and lights running. Siting the panel properly and understanding where the sun is and how much of it you get on the winter solstice (including possible cloud cover) is the important part. After that there shouldn't be any issues for the rest of the year, assuming vegetation doesn't grow in the way.
I also use other Pico PSU's for my machines connected to grid power. It doesn't make much sense to buy them and try to rig up buck/boost converters in my opinion. Get one designed to handle your intended use and it should work fine. I have a 10 amp fuse inline to protect things in case of a disaster. After recently having an imbecile destroy some of my wiring recently, I'm glad I added extra fuses to my system. Never underestimate the destructive forces of pure stupidity.
There needs to be much more details given about what you currently have and your intended uses before any intelligent recommendations could be made.
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