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:
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.
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.
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 :)