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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?

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  • \$\begingroup\$ Your assumptions re both chargers & solar controllers seems incorrect. Essentially all equipment intended to charge lead acid batteries (assumed LA) will only take such input energy as required to charge the battery at any stage during charging and once fully charged. During charging current is usually able to be limited by charge controllers to a preset maximum (which is chosen based on battery specifications). Lower cost mains chargers tend to supply whatever the battery will take withing their capabilty. Once charged either device will float the battery. \$\endgroup\$
    – Russell McMahon
    Commented Nov 3, 2019 at 22:26

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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.

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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.

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  • \$\begingroup\$ thank you so much for pointing that out. indeed, it may trigger some kind of over current protection. my main purpose is to make some kind of cheap UPS for 12v devices such as routers. so, I may have to buy a lower amp solar charge controller, say 12v 5a = 60w (assuming that there is a 12v 5a ones). still, I have concern -- will the solar charge controller constantly draw its maximum input power of 60w even if the battery is full and the devices just use below 60w? \$\endgroup\$
    – selnet
    Commented Nov 3, 2019 at 0:42
  • \$\begingroup\$ Very difficult to predict what will happen. The behavior of the PWM controller is not fully known. Maybe an MPPT controller would be a better choice. A very small one. If you are able to test safely and monitor temperatures and disconnect safely if anything is over-heating, you can try it. I can't be responsible for any damage that may occur as a result. If you have any doubts don't try it. But, again, if you have AC power, I think an AC powered battery charger is a better option. \$\endgroup\$
    – user57037
    Commented Nov 3, 2019 at 0:49
  • \$\begingroup\$ I have an AC powered battery charger but it may always output to its manually set ampere at 10A/20A. I am concern about the efficiency, thus asking about whether the PWM solar charge controller will draw constantly from my laptop charger at 60w (12v 5a) even if it has less load when the battery is full and the connected devices use below 60w. \$\endgroup\$
    – selnet
    Commented Nov 3, 2019 at 4:56
  • \$\begingroup\$ There are chargers out there that will only put out the power needed. They maintain constant voltage. Look at chargers for boats and RV's \$\endgroup\$
    – user57037
    Commented Nov 3, 2019 at 5:23
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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;

  • The Vmp/Imp=Zmp
  • the slope of the V vs I , PV curve =Zmp, which for PV current source happens to be the same at any MPT point for different solar inputs.

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?

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  • \$\begingroup\$ Consider the ESR of caps being charged as initial load or dV/dI =load \$\endgroup\$ Commented Nov 3, 2019 at 4:15
  • \$\begingroup\$ So, mine is really much lower. source: 19 / 4.74 = 4.008 load: 12 / 30 = 0.4 An MPPT solar charge controller does the impedance equalizing? I have read that an MPPT solar charge controller tries to read the max voltage and max current output of solar panel which in this case, will force my AC-DC adapter to output to its max power. it will not damage the AC-DC adapter? and also I have read that some AC-DC adapters have no output limiter and will get damaged when it cannot handle the load. Thanks for sharing your knowledge on this, I appreciate it. \$\endgroup\$
    – selnet
    Commented Nov 3, 2019 at 4:40
  • \$\begingroup\$ This Z ratio does not apply to a dynamic charger which starts with ESR in xxx milliOhms \$\endgroup\$ Commented Nov 3, 2019 at 4:42
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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.

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