1
\$\begingroup\$

I'm a pharmacist, not someone in an electronics trade. I'm creating a prototype mobile workstation to be mounted to a medical cart. I have a 12V 2A micro-PC and 12V 4-5A monitor (both have DC inputs as the converter/brick is part of the supplied power cord) that I want to run off of a 12V 40Ah LiFePO4 battery pack. Should the power supply wires have fuses and/or any sort of smoothing capacitors? Any other recommendations?

\$\endgroup\$
2
  • \$\begingroup\$ Why are you reinventing the wheel? Medical cart computers already exist. \$\endgroup\$ Commented Nov 6, 2021 at 4:24
  • \$\begingroup\$ I went that route and received a quotes four times what I expected for a battery pack and twice what I expected for an all-in-one computer. Seems everything medical grade comes at a premium. All-in-ones are nice but if something goes or you want to upgrade then you're likely chucking the whole $2000-3000 unit, but a micro PC at $200-300 is pretty much disposable. If this goes nowhere then at least I have a better understanding of what prices should be. \$\endgroup\$
    – Jay_
    Commented Nov 8, 2021 at 2:51

2 Answers 2

1
\$\begingroup\$

If your battery pack is just bare batteries, you'll need some sort of DC-DC converter in there. The 12V rating is the nominal voltage; when fully charged the open circuit voltage will be higher than 12V. As you discharge the pack, the voltage will quickly fall to something near (but not exactly) 12V, slowly drop until you've almost discharged it, and then nosedive. Since there will only be a short window where the voltage is exactly 12V and since your devices are probably expecting exactly 12V or something close to it, you need something to take in the varying pack voltage and provide a stable output.

9-36V input, 12V output DC-DC converters are very common and the 7 or so amps you're drawing is fairly reasonable so you won't be paying through the nose for one. A 100 or 150W converter would certainly be big enough. The input fusing will be specified by the manufacturer, as will any filtering capacitors needed. Output fusing is a good idea. Without knowing the specifics of your devices, 20% over the nameplate current draw is a good place to start.

Edit: you'll also want to put in some sort of undervoltage trip so that the DC-DC converter doesn't try to discharge your batteries past the minimum cell voltage.

\$\endgroup\$
2
  • \$\begingroup\$ Hi vir, thank you for your input. I will look into a DC-DC converter. If you happen to have a link to an example that would be helpful too. \$\endgroup\$
    – Jay_
    Commented Nov 5, 2021 at 21:02
  • \$\begingroup\$ And, yes, the battery is a bare battery similar in appearance to a car battery. \$\endgroup\$
    – Jay_
    Commented Nov 5, 2021 at 21:03
1
\$\begingroup\$

There are many fanless 24W micro PC's that operate from 9-36 VDC as well as 4k monitors.

Grounding each well to the chassis or by central connection with shielded cables are about all you might inquire about.

High resolution 4k monitors may be fussy about HDMI cable quality and length with the GHz BW's involved.

The charger ought to operate on float most of the time at 13.x V except if disconnected. depending on the model , a Cap may or not be necessary to reduce RF noise.

\$\endgroup\$
2
  • \$\begingroup\$ Thanks Tony. I believe you are right about the micro PC accepting a wide voltage range, but the monitor was more strict (12V +/- 5% maybe). \$\endgroup\$
    – Jay_
    Commented Nov 5, 2021 at 21:34
  • \$\begingroup\$ The monitor is a medical grade touchscreen (for disinfection needs). \$\endgroup\$
    – Jay_
    Commented Nov 5, 2021 at 21:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.