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I'm using a DC/DC converter (has an incoming voltage of 8-40 V, and an output voltage of 12 V) to connect it to a UPS instead of a lead-acid battery and power up some devices from a large capacity 4S lithium battery pack.

The battery pack provides 11-16.8 V, then the DC/DC convertor provides a stabilized 12 V output, and the UPS in offline mode (no AC connected) considers it to be a lead-acid battery and converts the incoming 12 V voltage to 220 VAC.

What would happen if I try to connect the UPS to the AC power source? Will it try to charge the "battery" and provide some voltage (like 13.6 VDC) to the output port of the DC/DC converter, and damage it? Or are DC/DC converters usually protected against some external voltage on their output?

If the DC/DC converter has some kind of protection against a situation like this, my intention would be to leave the UPS always connected to AC source, and it would automatically switch to using my lithium pack during blackouts.

Note: of course, I'll charge the 4S lithium battery pack manually using a specialized charger. The UPS is only used to get 220 VAC from the battery pack, not for charging.

EDIT: maybe I can put a diode to ensure this kind of protection (prevent charge current from UPS)?

Details about DC/DC converter from the Aliexpress where I've bought it:

  • All epoxy sealed containers with waterproof die-cast aluminum housing;
  • Light compact, convenient to use and transport;
  • Non-isolated; High efficiency: > 95%;
  • Operating temperature: -40 ° ~ +85° (max: +85°);
  • With overload / over-current / over / low voltage protection, stable performance; Auto recovery.
  • Input range: 8V to 40V
  • Output Voltage: DC 12V ±1%
  • Max Output Current: 15A/20A/25A
  • Ripple:<100mV
  • No-load Current<0.02A
  • Line Regulation:±0.02%
  • LoadRegulation:±0.02%
  • Efficiency: >95%

the DC/DC converter is epoxy sealed

The UPS model is Eaton 5E650IUSB, rated for 650 VA / 360 watt My load is Starlink (35-75 watt consumption), each lithium battery pack is a 4S pack of 128 batteries (18650 Li-ION) with BMS, the total capacity of each battery pack is around 1 kWh

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    \$\begingroup\$ This can be found out by reading the data sheet of the device. Look up the absolute maximum ratings and see what it says. There is no general answer to your question; some will survive some will burn. \$\endgroup\$
    – Andy aka
    Commented Jan 22, 2023 at 12:39
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    \$\begingroup\$ Without knowing which DC/DC converter it is and how it works, this can't be answered. \$\endgroup\$
    – Justme
    Commented Jan 22, 2023 at 13:10
  • \$\begingroup\$ I'm kind out of like here. It is just a noname DC/DC converter from AliExpress, without datasheet. aliexpress.com/item/4001256491867.html 300 watt model \$\endgroup\$
    – Dmitriusan
    Commented Jan 22, 2023 at 14:21
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    \$\begingroup\$ No datasheet = no sale. I’d give it a 90 % chance of no damage. \$\endgroup\$
    – winny
    Commented Jan 22, 2023 at 20:21
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    \$\begingroup\$ @Dmitriusan I did ask all the details above to find a solution for what you have. My idea is to still use a lead-acid battery to tame the UPS internal logic but to assist it from the lithium pack+converter. Нехай сила буде з вами. Нехай Україна процвітає! \$\endgroup\$
    – datenheim
    Commented Jan 25, 2023 at 21:48

3 Answers 3

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The short answer first:

It can work and the DC/DC converter will not be damaged - if you use a "diode" for decoupling, leave the UPS's lead-acid battery in place and regard the given "but"s.

Possible solutions in order if "goodness":

  • power the Starlink directly from your lithium pack with a dc/dc step-up converter (11-17 V => 48 or 56 V)
  • use a small inverter directly from the lithium pack that is just powerful enough to be able to deliver Starlinks peak consumption continously (200-300 VAC)
  • re-use the stuff that you already have

The first option has by far the best efficiency, injecting the power via non-standard POE wiring is the hard part. Voltage depends on dish's form.

The latter options have a lower efficiency - somehow comparable.

Now let's look at the third option: re-using an UPS and giving it an external battery extension.

Some aggregated info:

  • The UPS uses a single lead-acid battery as far as I can tell
  • This battery seems to be rated 360W/9.6V meaning it is a normal battery that get's charged with around 13.6 V max, has a standby voltage of ~12.4 V, and a end-of-discharge voltage of 9.6 V
  • the lead acid battery should still be in place and in working condition, but it must not have full capacity any more
  • your main load on the UPS is just the Starlink
  • You mentioned Starlinks consumption as 35-75 W, so I hope 35 W is the mean, and 75 W does only happen some times
  • Starlink max. draw seems to be 180 W, but it can be reduced/limited by the user (read about operation modes)
  • 35 W AC output may translate to 60W power draw from the battery, which in the worst case (9.6 V) gives ~ 6.5 A taken from the battery and @ 75 W AC => ~ 120 W DC => 12.5 A
  • your DC/DC converter seems up to the task (power wise). Maybe 12.5 V or 13.0 V output would have been better, but it still may work with 12.0 V

Here a quick and dirty sketch: Updated sketch

From left to right:

  • lithium charger on the left is shown for completeness
  • lithium battery (B1) as is, followed by an optional fuse and switch S1 (relay)
  • converter (U1) taking whatever B1 delivers and giving 12.0V / 15-25 A
  • optional fuse F2 and optional resistor R1
  • followed by the diode D1 - the important part
  • the switch S2 is optional (relay)
  • from the diode the current goes straight to the lead-acid battery that is part of the UPS

Danger zone, the big BUT:

If the UPS is on mains power there might be a dangerous voltage on point P1 (the negative pole of the lead acid battery B2). If so, try to reverse polarity of the mains plug. If it is still so - we are in trouble. (It would mean the whole other circuit must be isolated etc. I would dare to go further in this case.)

But if you find that the negative pole of B2 is somewhere near neutral (mains N) then we can continue.

Details

Let's start with main power available, the UPS has a fully charged B2 (is held on 13.6 V) and a full Li-battery B1 on 16.8 V. Starlinks gets main power from the protected port of the UPS (ignore the optional switches, fuses and resistor for now).

B1 is connected to U1 which is running and providing 12.0V to D1. Behind D1 we have 13.6 V from the UPS battery and thus no current is flowing towards the converter U1. From B1 only the standby consumption of U1 is drawn.

In fact very little current may flow in D1 due to reverse leakage of the diode depending on the diode type. (To give this current a return path in case the converter is off the resistor R1 can come into play. Also a reverse protection diode going from converter input + to output + may be an idea).

Now the mains goes off. The UPS supplies power from it's own battery to the Starlink. As soon as the voltage of B2 goes under (12 V - forward voltage of D1) current from the converter increases the more B2 goes down. At around 11.4-11.2 V on B2 the most current will comes from the converter and the lithium battery, and B2 will stay more or less at 11 V. If power peaks happen, both batteries will supply power, sometimes even B2 might get a bit of charge back from B1.

This means, as long as B1 powers the inverter, the UPS-logic will see 11V on it's battery - which may translate roughly to a charge state of maybe 10-20%. Because of this it would be a bit better if the converter would give 12.5 or 13.0 volt. Anyway. Because the discharge-end voltage of B2 is 9.6 V by specification I think the UPS will not switch off.

The Diode

You will need a diode (or a bunch of parallel diodes) with some cooling. It depends of what you have or can get.

Schottky would be good, but silicon might work as well. The important point is the voltage drop at typical as well as maximum power draw must be as small as possible, because it a major contributor to power loss. The absolute maximum power draw of the UPS gives a diode current of 35 maybe 40 A - so a specified diode drop near 0.5 V would be good, but still give 20 W of waste heat. A drop above 0.8 V at 40 A should be avoided, because your converter only gives 12 V and the switchover must stay safely above 9.6 V (UPS shutdown voltage).

I suggest THT or screw mount types, some examples:

  • MBR1040 (10 A) drop 0.55 V at 10 A, 4-5 parallel
  • FERD30H100S (35 A) drop 0.75 @ 30 A, 1-2 parallel

With anticipated mean current of 7 A and 0.5 V drop you waste 3.5 W - acceptable.

Of course an ideal diode (FET-transistor) would be of great advantage and increase total efficiency a lot (50 mΩ Rds @ 30 A = 1.5 W - not much for a TO220-FET. But you must have it at hand...

Whatever rectifier device you use, itself and it's cooling should be able to survive the maximum current that your converter can deliver.

R1 actually is optional, but my idea was to give a potential reverse leakage current of D1 (in case the UPS is charging it's battery an U1 is off) a load, maybe 5-50 mA depending on the chosen diode. Alternativly a 12 V fan can do this and provide cooling for the diode just in case.

For wiring: use 6 mm² or at least 4 mm².

Switches S1 and S2: these could be two normal-closed contacts of a relay whose coil is operated from mains supply. If main is present, the whole circuit is fully disconnected. The outage is catched by the UPS at first, shortly after the lithium is connected.

I'm in a hurry right now - will add a bit more later - I don't trust the draft saving :)

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  • \$\begingroup\$ your answer is exactly what I've been looking for. Also, your explanation is very useful in describing the caveats of this approach. I've some old PCBs from broken devices for my DIY attempts (like broken UPS, computer power supply, chargers, and so on), so getting components should not be a problem. \$\endgroup\$
    – Dmitriusan
    Commented Jan 28, 2023 at 18:16
  • \$\begingroup\$ Especially about the caveats of this approach you mentioned and what I did not think of (like bigger amperage when converting from DC to AC, I expected something like 20% of wasted power). I'm lucky that I've bought components with a large spare in specifications. \$\endgroup\$
    – Dmitriusan
    Commented Jan 28, 2023 at 18:27
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What would happen if I try to connect the UPS to the AC power source? Will it try to charge the "battery" and provide some voltage (like 13.6 VDC) to the output port of the DC/DC converter, and damage it?

The charge controller of the UPS will become confused as you will have the DC DC converter and the battery. Bad things could happen including overcharging. It would not be a good idea to do this. The best thing to do would be to put the DC DC on the output of the UPS, if you need to convert from AC to DC then that would need to be done. You could potentially connect it to the inverter or DC output in the UPS if you could find it. Another bad thing that could result is the DC DC converter could drain the battery beyond it's undervoltage point, rendering the batteries unusable.

Or are DC/DC converters usually protected against some external voltage on their output?

They aren't, you need to follow the input specification of 8-40V and ensure the input voltage is in that range.

On thing you might be able to do is detect when the UPS is off, and switch a relay from the battery. The UPS probably has a portion of the circuit that does this already and you could tap into it. You would need to make sure the battery has undervoltage protection and the DC DC converter shut's down beyond the 4S battery packs undervoltage point.

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As I understand this, you wish to replace the UPS lead-acid battery with a different battery chemistry, is that right?

I sense an XY problem here. I think the better approach is to obtain a UPS that supports the battery chemistry you want (e.g., LiFePO4). Otherwise you’re throwing good money after bad.

That said, there’s two problems at work here with your proposed solution:

  • reverse current / voltage in your power supply
  • UPS charger behavior when on mains voltage

Of the two, I think the latter issue is more of a problem. The UPS has a battery management controller that expects to interact with a specific battery type (lead-acid). It may decide that the faked-out ‘battery’ (that is, your pack plus 12V stabilizer) is no good and go offline.

You could block the UPS charge path with an ‘ideal diode’ isolator. Blue Sea and others make these for solar setups. That would at least protect your power supply. But I still think the UPS won’t like it.

Get a different UPS.

MORE: Now that I have a clearer idea of your goal, here's an idea: convert your Starlink system to run on DC power. Here's someone who did exactly that: https://tuckstruck.net/truck-and-kit/geekery/modifying-the-starlink-power-supply-to-run-on-ac-and-dc/

Another thought. Solar power controllers for RV's can integrate multiple sources (line, solar, car alternator) to support a battery of a given chemistry. Couple that with a high-efficiency inverter and you have a workable system. Example: zampsolar.com/collections/all

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    \$\begingroup\$ Ok, the problem I'm solving in 2 words: I live in Ukraine, and Russia is attacking our critical infrastructure to cause blackouts throughout the winter. I'm a software developer, I need electricity and the internet to work. I've built up a few 4S lithium batteries with total capacity of 4 kWh. That is enough to power up the laptop (with automobile charger via 100W USB Type-C), my devices and Starlink for a very long time. But Starlink Residential needs 220 VAC to work. That's why I use a DC/DC converter and an old UPS to use my battery packs as 220 VAC source. Thanks for suggestions and tips \$\endgroup\$
    – Dmitriusan
    Commented Jan 25, 2023 at 20:09
  • \$\begingroup\$ maybe you are right, and my intention to use the UPS additionally to switch between power sources is an overkill for my task. Also, it's a good point about the battery management controller of UPS, I did not think about it. In the latter case, probably manually switching the socket during blackouts is "good enough", so your answer may be accepted unless somebody provides suggests "cheap&dirty" hack \$\endgroup\$
    – Dmitriusan
    Commented Jan 25, 2023 at 20:12
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    \$\begingroup\$ Maybe in addition to M1A1’s we need to send some UPS’s. Slava Ukraini! \$\endgroup\$ Commented Jan 25, 2023 at 20:48
  • \$\begingroup\$ Glory to the heroes! @hacktastical, the help of the USA and European Union already drastically changes the situation and saves Ukraine. We get not only armor and ammunition, but also sanctions for Russia, power equipment to repair our energy network, also Europe exports energy to Ukraine and sends generators. We are very grateful for all the help, and UPSs are lesser of a problem. Diesel generators, Ecoflows and Bluetti, and usual powerbanks are available now at the market after some period of unavailability and high prices. My power setup is just an inhouse DIY solution for the same purpose \$\endgroup\$
    – Dmitriusan
    Commented Jan 26, 2023 at 18:42
  • \$\begingroup\$ thank you for the ideas! I have a spare 48v dc/dc converter, and will consider the Starlink modification you have pointed out \$\endgroup\$
    – Dmitriusan
    Commented Jan 28, 2023 at 18:02

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