This is my latest attempt to try building a ups ish system for my 3D printer the printer is custom-designed with 5 meters (no, it's not a typo) on each axis.

we use MKS Gen v1.1 with custom marlin core.

this board can handle 12-24v.

with our calculation, our current draw is 20 to 60 amps when bed and nozzle are heating up.

so a commercial UPS is not affordable.

I came up with the following circuit as our power source.

I could use some tips to optimize it.


simulate this circuit – Schematic created using CircuitLab

I know it's not pretty

My charge controller is Arduino Mega with the following algorithm:

if the battery voltage drops below 11.5 the relay is connected to charge the battery up to 13 volts then the relay is disconnected.

V1 is a power source with 80 amps capacity.

V2 is 3 car batteries connected in parallel. (to handle the sudden current shock)

There is a backup generator behind V1 which starts if V2 drops below 11.2.

There are other loads on the printer side that are mission-critical and can not be turned off even for 1 second.

P.S: I was thinking about adding a capacitor to smooth out the voltage but I could use a bit of help to calculate its capacity.

P.S2: we are planning to add solar panel as V3 later this year...

P.S3: I know, I know its idiotic, but we are tight on budget and cannot afford a big UPS we are pretty much out of money and this printer is our income source ...

( it took me around 35 days to build this printer, speed is 3mm/min, it's huge )

  • 1
    \$\begingroup\$ Could you state the question more clearly? \$\endgroup\$ Jul 25, 2019 at 18:19
  • \$\begingroup\$ @DmitryGrigoryev I need some optimization on my circuit to lower any risk, if possible \$\endgroup\$
    – far2005
    Jul 25, 2019 at 18:21
  • \$\begingroup\$ Optimization requires to define criteria and constraints. E.g. optimizing for max current with limited cost will give a different result than optimizing for min cost with a minimum required current. \$\endgroup\$ Jul 25, 2019 at 18:28
  • \$\begingroup\$ @DmitryGrigoryev I'm looking for optimization in terms of clean voltage and clean current with cost in mind to minimize any possible damage to the printer. I'd sleep better if I could keep the voltage on 12v \$\endgroup\$
    – far2005
    Jul 25, 2019 at 18:30
  • \$\begingroup\$ And why do you need an UPS? There are plenty of off-the-shelf power supplies for 12..24V. 60A is a high current but not unheard of. \$\endgroup\$ Jul 25, 2019 at 18:37

1 Answer 1


A low-noise 80A battery charger should be able to deal with this. It's the same kind of issue that folks confront at trade shows to keep car batteries alive while they're powering all their gizmos on the show floor.

Example: http://soundsgreatusa.com/cascade-aps-90-amp-1200-watts-pro-grade-power-supply-dual-volt-marine-ready/

Another: https://www.powerstream.com/charger-low-noise-high-power.htm

The power supply would connect to an automatic transfer switch, which would be part of the genset or a separate item.

  • \$\begingroup\$ I already have a big power supply. is the circuit safe? \$\endgroup\$
    – far2005
    Jul 25, 2019 at 20:01
  • \$\begingroup\$ The unit I linked is designed specifically to be tied in parallel with batteries to not only charge them but to supply the load as well. I can't say the same thing about your power supply - you seem to have doubts about it given the complex diode-and-relay solution you propose. Looks sketchy to me, plus development time... better to buy the pre-engineered solution I think. \$\endgroup\$ Jul 25, 2019 at 20:14
  • \$\begingroup\$ So this unit has a built-in charge controller? I prefer to be safe than sorry thus the diode solution \$\endgroup\$
    – far2005
    Jul 25, 2019 at 20:17
  • \$\begingroup\$ Yes, it has built-in charge control. It would be safer than what you propose assuming your power supply isn't designed to charge lead-acid batteries. \$\endgroup\$ Jul 25, 2019 at 20:18
  • \$\begingroup\$ Ours is a Meanwell with 80 amps capacity and variable voltage, I cranked up the voltage to charge the battery and made my own charge controller \$\endgroup\$
    – far2005
    Jul 25, 2019 at 20:24

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