So apparently the wiring in my home is... questionable. Very rarely, plugging or unplugging things will cause a power dip. This is almost always the result of turning on a fan or something, is only for a moment before coming back, but its long enough to cause my printer to reset. I want to add a backup battery supply using either the 12v cell pulled out of an old apc ups (before the suggestion of just plugging it into that... I did, somethings fried in it and it will ALSO power cycle everything connected to it every few hours) or an old car battery. Is there any reason I shouldn't use this circuit, which was originally intended for amateur radio equipment? battery backup circuit

Also is there anything else i should be considering?

  • 2
    $\begingroup$ What does this have to do with 3D Printing? You need to ask this on SE.Electronics I think. $\endgroup$
    – Greenonline
    Aug 23, 2018 at 6:25
  • $\begingroup$ Agreed, although it is a valid question, you are more likely to get valuable feedback from EE.SE. There is nothing specific to this being about 3d printing (except you will need to specify the load of around 200W). $\endgroup$ Aug 23, 2018 at 10:19
  • $\begingroup$ Unless, you reverse the order and ask if battery backup like this is the most appropriate for a 3D printer. $\endgroup$ Aug 23, 2018 at 10:29
  • $\begingroup$ @SeanHoulihane is correct: the correct question is: "what's the best way to ensure power backup if line blackouts are a concern?" $\endgroup$ Aug 23, 2018 at 15:33
  • $\begingroup$ Hello @entropicCreator, and welcome to the 3D Printing Stack Exchange. Your question is only incidentally related to 3D printing and probably can be better answered in an EE focused group. One thing to note is that 12v batteries actually charge at about 14.8v and discharge at 12v. This makes it difficult to simply parallel a battery with a 12v supply. A car alternator puts out 14.8v. Amateur radio equipment is designed to work with this range of input voltages, but 3D printer motor drivers and heaters are not. You may have success, but you might consider a different backup method. $\endgroup$
    – cmm
    Aug 29, 2018 at 16:07

2 Answers 2


There are a couple of points to consider with this question. First, you will need to make sure the PSU is correctly adjusted to the float charge voltage of the 12V battery. SLA should be safe to float charge, but over voltage will damage it over time.

One alternative solution (since powering the heaters will limit your run time) is to detect power failure, cut the heaters immediately, and save state to EEPROM in the firmware. This is the approach taken in the latest Prusa printers, and is a bit more complex, but might turn out to be more reliable.

If the problem you are solving really is brown-outs, then a simpler solution might be to isolate the high current and control sides of the circuit. The MCU will be run from a regulated 5V or 3V3, so a large capacitor (with diode isolation) on that regulator's input would do the trick.


This circuit is correct as it does the job (provides a charge and switch-over when needed).

The only drawback could be that you need to ensure that the battery is sealed as during the charging process the electlorit will slowly evaporate and decrease battery capacity.

I am personally using a UPC as that gives me more options to power-up more stuff including 240V heater and it has a self-test built-in (it is APC1500VA).

  • $\begingroup$ Absolutely: use a UPC and you won't have to worry about spikes, voltage "gaps" upon blackout, and as a side benefit you can keep a desktop controller computer (or arduino or whatever) running as well. $\endgroup$ Aug 23, 2018 at 15:32

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