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macrumors newbie
Original poster
Apr 27, 2017
Hello, I'm new to the forums.

I have been working for the last month on a large battery bank from four retired laptop batteries. These are all the standard 18650 lithium batteries of varying age. I realized that it would be a problem that they are all going to be varying capacities, so I got around this by wiring them all in parallel and only charging them up to about 4.05V. I conservatively estimate the battery to be able to store about 60 amp hours at standard lithium voltage or about 180 watt hours, but they will be discharged at about 40% to save life. So I have about 72 useable watt hours.

I have a step down unit that charges the cells. It can accept a variable input voltage and is designed to be set to charge basically whatever configuration I want.

I have succeeded in being able to charge the batteries from a PSU from the wall at 12V to th 4.05V I have it set to.

At the output end of the batteries, I have several step up units. There are a few modules that will convert the power to 5V in the form of USB outputs. I also have two modules that can step up to almost any voltage and can reliably have 6A running through them.

I set one of my output modules to send out about 12.5V which is what the AC/DC power supply inside the mac mini puts out. I have modified my mac mini so that it will take direct DC power. I have removed the stock PSU and connected the power input on the motherboard to a standard 2.5mm DC jack and connected that to the exterior of the Mac Mini. I followed the procedure outlined here:

This procedure was successful. I can run my mac mini from an external PSU that supplies 12V. In fact, I am typing this message using the mac mini now.

(sorry for the long introduction)

NOW, to my issue:

The mac mini will turn on just fine when I use an external AC/DC PSU connected to my 2.5mm jack. HOWEVER, when I plug it into the output of my batteries (which I multimeter tested at 12.5V before plugging in), I will get some reaction, but it will not boot.

The reaction I get is a small pulse of the led, then it shuts off and no response. I hear the disk inside just barely try to spin up for a split second. I do not hear any mac chime like normal.

Here are some pictures of my setup to give you a better idea of the setup. On the left of the batteries is the input module. On the right of the batteries are the outputs. I have one diode that prevents any backflow to the batteries, but the outputs are not isolated from each other.
IMG_20170427_172103.jpg IMG_20170427_172114.jpg IMG_20170427_172121.jpg IMG_20170427_172136.jpg

Tell me what you think!
[doublepost=1493329017][/doublepost]I also recently purchased an isolated power converter which I connected to my 12V step up. The mac mini did the exact same thing. I also tried connecting the mini to both step ups thinking that it wasn't getting enough initial amps.

One other clue that may help in my testing of the power outputs of the power supply that works, I tested the current coming out of the standard AC/DC unit and it was all over the place. It seemed to start high when I touched the probes and then go down to 0. I tested the current on the output of the batteries and I got a flat 5.5 amps. I tested it after I had it connected to the isolated power output and it had a flat rating also, but at 3.5 amps.

My mini has an SSD as the main drive, so doesn't need to spin up to boot.

Perhaps does the mini need a spike of current when it initially boots or is it the reverse in that it needs very low current as a safety feature and will block booting if there is too much?
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macrumors 68040
Sep 2, 2006
United States
Sounds to me that your setup isn't producing the required voltage at the necessary amperage for your mini. You need both.

Since you indicate that the amperage is fluctuation, it sounds like you need to stabilize the power you're sending to the Mini.

Yes, any electronic device will require greater amperage as it is first powering up, and bringing current to its component parts. It will then stabilize.

That's often demonstrated by lights in the house dimming momentarily as you power up some high powered equipment.

You need to find out what the maximum amperage is that your mini can draw. Then configure your power to deliver the proper voltage at at least a little above that amperage requirement. And have the voltage and amperage stabilized under load.


macrumors newbie
Original poster
Apr 27, 2017
I had that same thought. The weird thing is that the PSU that I am currently using (as I write this) is supposed to only be able to supply a max of 4A which is underneath what my output on the batteries is doing. This unit is proven to work, but the other does not.

I also combined the output from both step ups to double the amperage and still got the same result.
[doublepost=1493332353][/doublepost]The max draw of the stock PSU on the mac mini is 7A continuous usage. It supplies about 12.5V.


macrumors 603
Aug 5, 2008
San Jose, CA
I don't know how your step-up modules are designed, but very likely your setup has a much higher internal resistance than the stock switching PSU, which means that the voltage will drop too much when you draw higher amperage. Modern CPUs in particular need very precise voltages and computer PSUs have regulators that result in a very low internal resistance.
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