I am trying to build a DC UPS to power a modem and a router which have power requirements of 12 V, 2 A and a laptop which has power requirement of 20 V, 3.5 A.

I am planning to use a AC to DC SMPS which is capable of 24 V, 20 A which is connected to a solar charge controller via step down (buck) converter (21 V, ~20 A).

A 12 V, 46 Ah SLA battery is connected to the output marked for battery in the charge controller which means it takes care of trickle current and float current to charge the battery.

A modem and router are connected directly to the output marked for connecting DC load in the charge controller. Output is 12 V and max. 30 A current.

In parallel to the modem and router a step up/boost converter is connected to the charge controller. It steps up the voltage to 20 V to meet the requirements of a laptop. Max current for the laptop is 3.5 A.

I am more of a software developer than a electrical engineer. I have come up with this design using off-the-shelf components with the knowledge available to me by means of other DIY DC UPS on YouTube.

I would like to know if this would work, and if there are any flaws in the design or any heating problems that I should anticipate.

Updated diagram: enter image description here

Old diagram: DC UPS design with off the shelf components

Off-the-shelf parts listed below:

  • \$\begingroup\$ Probably work, but a bit overkill and low efficiency to do it in that many steps. Can’t you find a battery charger instead? Then you just need a low voltage cutout somehow like the one your solar controller would provide. \$\endgroup\$
    – winny
    Aug 29, 2020 at 7:45
  • \$\begingroup\$ @winny Yes, I have now found a battery charger. High current ones are hard to find. amazon.in/gp/product/B07SSS5CKZ/…. Should I update the original question?, since based on it there is an answer given. \$\endgroup\$
    – rej
    Sep 1, 2020 at 8:50
  • \$\begingroup\$ Something like that, but regulated output voltage. Here is a better one: mouser.in/ProductDetail/MEAN-WELL/… As long as your buck and boost converters have undervoltage lockout (UVLO) available and set to about 11 V to prevent damange to your battery, you should be good to go. If not, you need to provide it externally. \$\endgroup\$
    – winny
    Sep 1, 2020 at 14:13

1 Answer 1


A solar charge controller isn't ideal for managing the battery.

A basic controller like that works by alternately connecting the panels directly to the battery, and disconnecting them. This is only OK because the solar panels are a current source. The solar controller isn't capable of floating the battery, so when the bulk charge is finished it just squirts in a bit of high current every now and then.

The buck converter you show is actually capable of current limiting, so it won't blow up.

But the battery will take a beating, being alternately charged and discharged every second or two, as it supplies the loads, then the voltage drops, and the charge controller connects the 20 A current source, only to find the voltage shooting up again.

The way you usually make such a UPS is like this:

15 V DC source -> diode -> load  
15 V DC source -> linear charger -> battery  
       battery -> diode -> load

This way when the mains power is on, there is no other current flowing past or through the battery, and the charger can do the right thing. The loads are powered directly from the DC supply, via the first diode. When the power turns off, then the second diode will supply the load from the battery. Diodes should be Schottkey to reduce the voltage drop. You wouldn't use a 24 V supply, that would waste too much energy in the (linear) battery charger, and it would require your laptop supply to be a buck-boost to cope with +24 V when the power is on.

The rest looks sort-of OK. Be sure to put at least one fuse between the battery + terminal and everything else, so you don't start a fire if something shorts out. And watch out with these cheap and cheerful SMPS boards, I have found them to overheat at well below their claimed rating. I recommend getting them rated at twice as much current as you think you'll need. One clue is simply physical size - if it's a lot smaller and lighter than your current laptop power supply, then it probably won't cope.

  • \$\begingroup\$ could you give an example sample product of a linear charger? I could find for some items that are used with li ion battery charging like this amazon.in/SunRobotics-Battery-Charging-Inbuilt-Protection/dp/… I have edited the question to remove the solar charge controller and the smps. Instead I have used a 12v20a Battery charger. \$\endgroup\$
    – rej
    Sep 1, 2020 at 9:17
  • \$\begingroup\$ @rej don't worry about the charger being linear, IMO \$\endgroup\$
    – user253751
    May 3, 2022 at 13:27

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