While i have not read the whole thread, 1 questiin that i have is why is a buck boost converter needed if it is a DC-DC converter? The output should be regulated right?
You can check with a multimeter attached to the outputs. There will be a trim pot to adjust output voltage or may be you can ask meanwell to send you their schematic.
The output droops when the inverter has no power input, the load runs directly off the battery.
I think I made a mistake when reading the specs. ENC 180 is a charger and not a UPS like I thought it is. 
Sounds like we need a 3 stage charger that goes into the third stage after a set time, instead of a set current threshold.
All three of my inverters appear to be time-based or voltage-based, they don’t even have a shunt inside to measure current if they wanted to.
Ok. Got it. When there is input what is the voltage at the terminal with load
When there is no input what is the voltage at the battery terminal with load
With no input can the output voltage be adjusted and set to 12v ?
With input load and battery connected what is the voltage at no load and with a load like a car bulb connected?
I’d assume the voltage is whatever the battery terminal voltage is.
Normally, the inverter will be either boost or float charging the battery, so the buck boost converter takes that 13.5V/14.4V and bucks it down 12V
With a power outage, the battery drops to 12.8V and keeps discharging to 10V or less during extended outages, so buck boost converter then has to boost it up to 12V because it’s powering networking equipment which may be voltage sensitive.
Load is a few amps, I would guess.
Most inverters are designed to output a constant voltage. If the output voltage is dependent on input voltage without regulation then the inverter is probably wrong.
Can you share the link of the inverter if you do not mind?
We are getting confused with battery charging voltage vs output voltage. The inverter should accept a wide range of input voltages , irrespective of the battery terminal voltage.
At the output a few millivolt variation is acceptable, but not to the level of a few volts.
In this scenario, we’re using the inverter only as a battery charger.
The load (routers, switches, wifi access points) is connected directly to the inverter battery.
This is for efficiency, bypassing slow transfer times, protection against the inverter shutting off due to low voltage (at the risk of draining the battery flat)
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Yes sorry, the whole idea is how to make a robust DC ups out of an inverter battery and get rid of a bunch of adapters.
Ok.. Now i understand the scenario.
In that case why not just buy a battery charger ciruit or diy a voltage, current regulated charger
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Why not try using a battery charger, battery and a few 12v 5 amp voltage regulators?
@Heisen’s concern is that if the load is directly drawing power from the battery, then the inverter will never switch to float charging and would end up prematurely killing the battery.
LDO’s will work when the inverter is powered on, but they won’t be able to boost a low voltage during a power outage when the battery voltage drops.
12AH lifepo4 would be around 4-5K. Maybe cheaper if you built one yourself. Definitely costlier than lead acid. If you know a good cheaper 24v 12-18AH battery in compact size then do share.
But if switch to float charging is time based then I guess it won’t matter.
I read up on how to do proper lead acid battery charging and my insecurities stem from that.
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Why do you need an inverter? Just a battery charger.
Knowledge refresh.
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Oh just because everyone already has one and it’s convenient.
But @Heisen is pursuing a standalone solution.
If thr inverters can output 15+ volts then it is easier. Inverter to battery charger will do the trick.
To me, it feels like all the challenges can be solved if you’re using 5x LFP cells? They don’t require floating, use standard cc/cv curve for charging, and charger voltage is always going to be > 14v, which can then be directly fed to a buck converter for stable 12v output. The battery voltage is also never going to drop below 12v without hitting the bms low voltage disconnect. They can remain at 3.5*5 = 17.5v happily all day, and if needed can be discharged to 100% dod without causing any damage. Are they more expensive than lead acid, sure, but not by much these days.
Edit: For a more readymade solution, use a 24v system with a 24v battery, ideally LFP, and use that to power your buck converter.