Running a computer (Mac mini) on direct 12v

Thread Starter

curts

Joined Apr 3, 2020
2
Hi,

I am in the process of converting a Mac mini to run directly on 12v and ditching the AC-DC power supply. I have it working but the computer seems to stutter and lag with the varying voltage of the battery instead of a constant 12v. In my search I have found Buck-Boosters but they seem to be not so clean and generate a lot of heat? I would think this is something fairly common to get battery voltage (10v-16v) and output it to a constant 12v? But in this case the amperage is fairly high (12.5A max but usually not more than 9A in my tests)

The Mac mini when running in my tests idles around 1A but underload around 9A. It would not boot using a 10A power supply, but it will boot on a battery since it can get the 12.5A it asks for to boot. I'm hoping for something I can place inline.

I found this which seems to be a nice solution but only 10A and worried it may not allow the extra needed to pass for boot.
https://www.murata.com/en-us/products/productdetail.aspx?partno=UWE-12/10-Q12PB-C

Someone suggested something like this but from what I can tell its not a voltage regulator, it seems like it just filters noise.
https://www.teradak.com/products/117.html

I have only made a few simple circuits in my time, so I apologize if my terminology or references aren't that good.

Thank you!
 

Papabravo

Joined Feb 24, 2006
13,533
You obviously don't appreciate the basic rules of power conversion, or you would not be trying to do this in the way you have chosen.

Rule #1: In any power conversion scheme you will ever see besides all the ones you won't, the power out will always be less than the power in.
Rule #2: Sometimes the power out will be much less than the power in.
Rule #3: A well designed SMPS (Switched Mode Power Supply) will usually be more efficient than a linear power supply.

Your minimum power output requirement appears to be 150 Watts (= 12 VDC x 12.5 Amperes). To work off of a battery you need the conversion to be a buck-boost. This is like tying one hand behind your back. Let us say you have a design is 80% efficient, that means you need an input power of 187.5 Watts (= 150 Watts / 0.8). If the Battery Voltage is 13.8 Volts, that corresponds to a current demand from the battery of 13.59 amperes (=187.5 Watts / 13.8 Volts). Now, as the battery discharges to say 10.2 Volts, you are now asking for 18.38 Amperes (187.5 Watts / 10.2 Volts). You can see that you are fighting a losing battle.

Drawing that kind of power from a battery when you don't have to is just plain foolish. If you insist on continuing with this nonsense, what you need to find is the battery with the largest energy capacity, measured in Ampere Hours, and the most efficient buck-boost converter you can buy (95% would be good). Don't try to design and build one yourself, your chances of success are slim. Short of accomplishing those two things I think your project is doomed.
 

Thread Starter

curts

Joined Apr 3, 2020
2
I like being doomed. With that said I am a novice, so I appreciate any advice. It seems like there are many things that do what I am looking for or perhaps I am looking at them wrong.

This takes 9v-36v and outputs 12v 150w
https://www.mouser.com/ProductDetail/Cincon/CQB150W-24S12?qs=gt1LBUVyoHkciX5KMyvFeA==

Or even this, even tho it is very large
https://www.cui.com/product/dc-dc-converters/isolated/vhk150w-din-series

These isolated converters seem to be a good solution? And in terms of battery, that's not an issue as the smallest I would be using is 20Ah LiFePo4

And then there is this buck/boost that seems to be another option.
https://www.mini-box.com/DCDC-USB-200?sc=8&category=981

Thank you.
 

Papabravo

Joined Feb 24, 2006
13,533
Specifications for continuous running are fine, but what you need is a device with an appropriate transient response. This is more difficult to judge, but you have already alluded to your need for it, when the load goes from 1 Ampere, to 12.5 Amperes, how does the output voltage behave. Does it remain at 12 VDC or does it sag to 8 or 9 volts for 30 seconds before it recovers. I think this might be important to you.
 

wayneh

Joined Sep 9, 2010
16,384
Part of the solution here could be software. I think it could be quite a challenge but you might be able to alter the boot process in a way to avoid that peak power surge. I’m certain there are Mac mini forums where you might ask such a thing. You’re probably not the first person to attempt such a thing.

The redneck solution of course is a giant inverter, and then just use the regular power brick. That’s what most laptop users do in their cars.
 

MisterBill2

Joined Jan 23, 2018
5,795
One thing not mentioned is the resistance of your connections. No matter what the power source, using connections that drop the voltage will cause problems. I have seen that before and the solution is to not have the skinny connection wires and the poor connections.
 

wayneh

Joined Sep 9, 2010
16,384
One thing not mentioned is the resistance of your connections. No matter what the power source, using connections that drop the voltage will cause problems. I have seen that before and the solution is to not have the skinny connection wires and the poor connections.
Yes, that applies as well to the redneck solution I mentioned of using an inverter. Once you get past 100W or so - 10-15A - you can't use the cigarette lighter style. You need to wire straight to the battery with a substantial cable.
 

MisterBill2

Joined Jan 23, 2018
5,795
I doubt that the 12 volt battery is an automotive battery. And I am wondering about the power connection. Of course it also matters that I have no idea what a MAC MINI is. Evidently not a laptop computer. And that does seem like a very large boot up current draw. Do you have the original supply on hand? Has the original supply failed?
 

wayneh

Joined Sep 9, 2010
16,384
I doubt that the 12 volt battery is an automotive battery.
I haven't done the math but anything less may be a problem for a 150W load for any significant length of time.
Of course it also matters that I have no idea what a MAC MINI is. Evidently not a laptop computer.
It's a small form-factor desktop computer, a medium-featured Mac inside an 8" square, 1-1/2" high shell. They're popular as both an introductory model (because of the lower price) and as a home-theater component.
 
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