How would you wire small computer to run on both batteries and AC power? [closed]

Question

I'm trying to use a motherboard (Mac mini 2018 model) for a project that will require it to be able to run off batteries. The project is essentially building a custom laptop using the Mac mini mobo and power supply internally with an mxm GPU board for upgrade able graphics in the future.

How would I wire up the batteries with the power supply in order to be able to hit swap between AC and DC power and also keep the batteries charged when plugged into the wall? I could either use the power supply from the external GPU or the power supply from the Mac mini I think.

External GPU I’m considering is the Sonnet Breakaway Puck. Max power from the included power supply is 160W or 220W depending on which model I buy and it marketed at being able to power itself and a laptop connected via USB-C so maybe this is enough for both Mac mini power and GPU power?

Answer 1

This is certainly possible. There are DC/DC converters made exactly for this purpose. You might want to google for "dc/dc atx power supply 12v". These converters are available at different power ratings. Many of them are designed for carputers (computers in cars). In the picture you see a small one providing 80 W power. It is mounted as part of the ATX main board connector. Stronger units usually come as separate boards, connecting to the main board via a cable.

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It is also possible to attach a car battery charger to the battery, making operation and charging completely independent.

Note that 12 V is the nominal voltage of the battery. Actual voltages for a lead type car battery should be anything from 11.8 (empty battery) to 14.7 V (charging, battery close to full).

Answer 2

In theory, you could connect a 375V~400Vdc battery where the PSU converts AC to DC.

Possibly get a used 2016 Nissan Leaf battery pack that operates at 375V and hot wire it into the PSU and operate it for 100h.

In theory yes, but lethal if you make just a little mistake.

UPS solutions with large batteries, seem to be the best choice if you cannot find a laptop to perform as well.

Answer 3

First, lets check the feasibility. I put a power meter on my new desktop computer. It is a large tower, but it isn't loaded up with cards. The OS is running on a SSD, this helps keep the power down. It also has a traditional HD. The power varies between 23 and 110 watts. The average is probably about 40 W when doing a mixture of realistic tasks. My large laptop has a 56 Wh battery (0.7 lb ~0.3 kg). So, assuming no losses, this computer could run about 1.4 hours on this battery.

This is a small battery compared to the size/weight of the desktop computer, you could use a battery 5-10X this big and still keep the weight reasonable.

For an off-the-shelf solution, you could use a small battery designed for solar cell applications, and run the PC off an inverter. This is not very efficient, maybe 50-70%.

For efficiency, you need to scrap the computer power supply and find or design something that can charge and use a battery directly. Sort of like what is in a laptop, only capable of more power.

Is this even possible using a computer power supply to power the motherboard and keep the batteries charged?

Probably not, a useful battery is going to have a higher voltage than the desktop will output at any reasonable current. But, a battery powered desktop is feasible, see my last two paragraphs.

Answer 4

Is it only a motherboard ? could explain why you want to do that to a desktop instead of just using a laptop.

Since you want to get rid of the AC part , the motherboard run off DC voltage (12V max) (-12V min)

ATX PIN Layout

ATX pin layout reference

Instead of taking the route of inverting Low DC to High DC and back just to supply a PSU , you can directly feed DC to the motherboard.

There are a couple of ways to do this , one of them is going to be to design your own PSU, using two batteries to supply 12V and -12V, using those voltages to regulate them down to 5V and 3.3V. Another would be to take a PSU and tinker about to use the functions you require from it(which is different for each PSU but should be somewhat similar) which could prove a hassle and perhaps dangerous.

ATX POWER SUPPLY CIRCUIT EXAMPLE

ATX power supply circuit reference

If you're going the first way , make sure you keep in mind (PS_ON,PWR_OK) pins and their functions. Also make sure you supply the motherboard with enough current. A car battery is of reasonable size, and a suitable 12.6V and a lot of juice. But keep in mind this is highly DIYish. You'll have to take into consideration, power protection and battery empty measures to protect both the battery and the motherboard.(battery shouldn't fall below 40% as voltage will go down as well).