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I'm from Ukraine, so there are very few options to test something and get support.

I need to power my Wi-Fi router and fiber optic modem when utility is down. It is available 4 hours by four down. Wi-Fi and ONU are both DC 12V 1A+0.5A correspondingly. I also have the Li-Pol battery DC 12V 10Ah with some BMS unit. I can buy a 12V relay unit for power transfer as depicted on the picture. I developed it by myself with my poor background, because I didn't find any applicable solution.

Li-Pol battery DC 12V 10Ah with some BMS unit) circuit

Will this circuit work correct? What option would be better (1) or (2). Maybe you could suggest a better solution.

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  • \$\begingroup\$ There is a tool to draw schematics, it's hard to follow your diagram, can you draw a schematic with the tool. \$\endgroup\$
    – Voltage Spike ♦
    Commented Nov 9, 2022 at 19:30
  • \$\begingroup\$ Note that the battery with BMS ... has only 2 poles to be wired, the internal poles don't have to be wired externally. \$\endgroup\$
    – Antonio51
    Commented Nov 9, 2022 at 19:51
  • \$\begingroup\$ thank you for your feedback @VoltageSpike sorry, I tryed but.... sorry \$\endgroup\$
    – Vasyl Makohonchuk
    Commented Nov 9, 2022 at 20:21
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    \$\begingroup\$ It looks like the battery has a voltage of 9.9 to 12.68V, so you would need to connect it to a buck/boost converter to power your 12V devices. Also, a mechanical relay might not switch its contacts over fast enough to keep your devices powered. \$\endgroup\$
    – vir
    Commented Nov 9, 2022 at 20:32
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    \$\begingroup\$ NB: "3s4p" mean ... 4p=4 batteries wired in parallel=1 block ... 3s= 3 blocks wired in series ... \$\endgroup\$
    – Antonio51
    Commented Nov 10, 2022 at 10:12

2 Answers 2

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If you can tolerate a moment "not powered", as @vir pointed out,
something as this can help, unless "error".
If voltage is too low, see also comment @vir ...

enter image description here

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  • \$\begingroup\$ many thanks for your clarification. It was really necessary to justify if the circuit is viable. Any suggestions how to charge the ACC when utility power supply is available? Should I apply separate 12V DC power supply? Or, maybe, or another relay board? \$\endgroup\$
    – Vasyl Makohonchuk
    Commented Nov 9, 2022 at 21:21
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You don't need a transfer switch.

Wire it to power the router off the battery at all times.

Then, have a power supply or battery charger refilling the battery while also powering the equipment.

schematic

simulate this circuit – Schematic created using CircuitLab

When power is lost, the DC battery charger / power supply disappears. The equipment is already connected to the battery, and just picks up the load instantly with no interruption.

This is how cars are wired.

At first glance, it appears your battery kit is already set up to do that. I could be wrong.

The only issue is that the equipment might not like voltages slightly higher than 12 volts. Most equipment will be OK with it. Anything designed to run off a car's "12 volts" is certainly rated to accept 14.4 volts, because car voltages go that high.

A 12 volt lead-acid battery might be a better choice. Why? The batteries and chargers are much easier to find - they are in any auto parts store or truck service center. The chargers are readily available at 5 to 15 amps or more, so the battery can be quickly recharged. Every charger works with every battery. And the equipment is cheap because it is common.

Also, lead-acid 12V batteries "live" between 11.5 and 13 volts, and you probably won't need a buck-boost.

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  • \$\begingroup\$ Of course if the ACC is already configured to be online UPS, it would be the best solution. Just because sale-manager told it's not a case, I've started with AVR. Maybe he's wrong. Li-Pol ACCs are selected for 2 reasons: 1. Utility is 4h up, the next 4h is down. So I need a battery to be charged in 4h. It would be hard with UPS battery 2. A car battery (CB) can be charged in 4h. But using CB inhouse is harmful because of evaporations during a fast charge. Gel and AGM batteries are very expensive (~150-250 USD). Have problems with a deep discharge. And they bulky. \$\endgroup\$
    – Vasyl Makohonchuk
    Commented Nov 10, 2022 at 10:40
  • \$\begingroup\$ Li-Pol for my applications are way more attractive for my application. The voltage deviation can be solved with DC-DC converter connected to OUT terminals, I believe. The only 2 questions left are: 1. Do I need AVR at all for battery health? 2. Will the included DC charger of 2A be capable to supply WiFi+ONU and charge the battery simultaneously? \$\endgroup\$
    – Vasyl Makohonchuk
    Commented Nov 10, 2022 at 10:43
  • \$\begingroup\$ @Vasyl Lithiums need battery protection or they catch fire. I don't know your power requirements, but you need to be able to charge the battery fast enough to be ready for the next outage. I would not plan for 4 on/4 off, I have heard places having 4 on/12 off, and if Russia gets any more missiles that may happen to you too. If power is available 1/4 the time, then the charger needs to supply 4 times the equipment load, but the battery will not charge at 100% efficiency... so really 6 times would be better. \$\endgroup\$
    – Harper - Reinstate Monica
    Commented Nov 10, 2022 at 18:41
  • \$\begingroup\$ the power demand is 16-20W at most, so charging 2A should overcome expected discharge and will not cause ACC overheat. We have now 4/4 power supply configuration. If it gets worse I don't expect any internet services then. However the solution is scalable. \$\endgroup\$
    – Vasyl Makohonchuk
    Commented Nov 11, 2022 at 18:48

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