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Instead of connecting a solar panel to my 12v 30a PWM solar charge controller, I want to connect a laptop charger to make a DIY UPS.
Will the solar charge controller constantly draw the maximum output of my laptop charger @ 19v 4.74 (90W) and waste the excess power when the battery is full and the connected devices use less power?
Your assumptions re both chargers & solar controllers seems incorrect. It seems likely that a properly designed mains input battery charger will meet your need.
I'll assume you are using a lead acid battery - although similar statements apply to eg LiIon batteries.
Essentially all equipment intended to charge lead acid batteries will only take such input energy as required to charge the battery at any stage during charging and once fully charged. During charging low cost PWM controllers will usually supply whatever current the source will supply when directly connected to the battery and MPPT controllers will attempt to maximise power transfer from the source. In other than very low controllers, cost current is usually able to be limited to a preset maximum (which is chosen based on battery specifications). Lower cost mains chargers tend to supply whatever the battery will take within their capabilty.
Chargers or controllers of more than basic spec will provide a "boost charge" at the end of the charging cycle.
However, once charged either device will float the battery and use only such energy as is required by the battery in 'float' mode plus whatever standby current the charger requires. (LiIon batteries are not usually "floated" - charging is terminated when complete. (Floating at lower than full voltage is permissible but this is not usually done.))
No "proper" battery chargers or solar controllers consume high levels of energy once the battery is charged.
Controllers designed for wind turbine use will divert the input to a "dump load" once the battery is charged due to the need to not allow a wind turbine to operate unloaded.
Just at a quick glance, 30A * 12V = 360W, so it seems like your charge controller will probably try to draw more power from your "charger" than it is capable of supplying. The "charger" will probably go into some kind of over-current fault mode. On the whole, not recommended. There may be some type of charge controller out there that would work in this application. Another option is to use an AC powered battery charger.
The problem with boost converters is not so much over-current, but rather under-voltage from the load impedance being much lower than source on startup.
Always compare your MPT impedance of source vs load.
You can do this in one of two ways;
You best bet is to choose a battery charger with MPPT control and run your 12V device off battery.
Check the router design inside for DC-DC converters and specs.
Does it use 12.0Vdc for anything and depend on this being well regulated?
PWM solar charge controllers use a crude method of regulating charge voltage and current - they simply connect the solar panel directly to the battery, relying on the panel and wiring to limit peak current. If/when voltage or current exceeds the controller's limits it disconnects the panel for a period, with the PWM on/off ratio determining average voltage or current. The battery acts like a large capacitor to smooth the voltage and supply load current during the PWM 'off' periods.
When your laptop power supply is connected to a 12V battery through the PWM controller it will go into over-current protection and may shut down completely. To prevent this you could add some resistance in series. The voltage the resistor needs to drop is 19V-12V = 7V, and the maximum permitted current is 4.74A, so the resistance required is 7V/4.74A = ~1.5Ω or higher. It could dissipate up to 7V x 4.74A = 33.2W, so it should be rated significantly higher eg. 50W.
A fully charged 12V lead-acid battery floats at ~13.2V. At this voltage the power supply will only be able to deliver ~(19V-13.2V)/1.5Ω = ~3.9A. If you try to draw more than this the battery will start to discharge. If the load draws less than this the charge controller will reduce its PWM ratio until the average power supply current equals the load current. At lower load current the resistor drops the same voltage, but wastes less power.
With this scheme the resistor wastes ~30% of the power so the efficiency is ~70%. That's not as good as a proper UPS, but not horrible either.
