DIY Panel + questions

Re: DIY Panel + questions

I believe one issue with most all voltage converters connected to a solar panel without any energy buffering in the system is this...

Solar panels output a constant current rather than a constant voltage.

Virtually all of the power supplies that you can purchase (with the exception of MPPT solar charge controllers and linear current boosters for water pumps, etc.) all assume that the amount of energy on the input is >> than the amount of energy the voltage converters/regulators will draw.

With solar panels and constant current output--that is not a good assumption...

As long as the converter is drawing less than Imp (or whatever current is available with the current sunlight conditions), the Voltage will be above Vmp and the current will be below Imp.

As soon as the voltage converter/power supply draws 0.0000001 amps more current than is available from the solar panels, Vpanel-out will start to collapse. If you are using a switch mode power converter of some sort, its input power is P=V*I--

So what happens when V starts to collapse, the converter wants to draw more I--(to keep Power=needed energy for conversion)--So the Vpanel will continue to drop as the converter tries to make up for lost voltage by drawing more current... This is a "positive feed back loop"--Basically, the converter will draw down Vpanel to its minimum input voltage--and either stick there, or "turn off"... So, either the converter will not get enough power (and may continue to supply some output current--an OK thing) or it will turn off, Vpanel will rise, converter will restart, and the whole process start all over again.

For "smart" charge controllers, the constant resetting caused by Vpanel collapse (and local microprocessor resets) can cause the battery charge controller to forget the current charging state--and try to recharge the battery from some sort of initial condition state... And probably will result in less than ideal charging performance.

A PWM type charge controller just lets whatever current is available / appropriate for charging the battery, through to the battery for charging. And the battery really sets the Vpanel output voltage (charge controller only reduces current as the battery approaches full charge--by monitoring the battery voltage, temperature and possibly other conditions). There is no collapse involved and no fancy electronics tracking Pmp=Vmp*Imp going on...

A MPPT type charge controller, on the other hand, has sort of two sections... One is the input section that monitors voltage and current from the panel and controls the current draw based on the MPPT calculations and current panel voltage--And and output stage that monitors the directly connected battery (or utility power in the case of a GT Inverter) for proper output conditions.

With a Solar panel to DC Voltage Converter to LapTop input Charge Controller, you have a very difficult task to design a circuit that can meet the input requirements of the laptop battery charging/power management circuit (that probably assumes one of two state--Either unlimited power, or no power/unusable power input).

If you had a DC Converter that did the MPPT calculations to only take Pmax (based on Vmp*Imp)... And did its best to keep the Voutput voltage at 19 volts, but let it drift down as input solar power drops--You may get something to work (really dependent on laptop input power design--can it take 100% at 19 volts input and 50% at 18 volts and 0% at 17 volts--and still be happy?).

By you sticking a solar panel with Vmp > ~17-19 volts, and placing a ~20 volt clamp / shunt regulator on the output--You may not have the most elegant MPPT type charge controller--but you may have at least something the computer's DC input could work with--Again, if it gracefully drops charging current as available voltage/current slowly drops.

You could do the same experiment with a bench top lab power supply (something like this one will do 30 volts / 3 amps, or 20 volts * 3 amps = 60 watts for your testing--~$200; or a smaller one that will do 30vat 1.2 amps, or 20 volts * 1.2 amps = ~24 watts--will not work at full power, but will allow you to understand your computer's low energy input ability--only $90 or so)...

Set the DC output voltage to ~19 volts--And then start "playing" with the output current regulation settings. See if the computer can take significant power when the DC current regulation is set to less than the input current it is demanding at that time (battery charging and battery charged, and just supplying energy to the computer operations, screen off, screen on, idle, sleeping, etc.).

A little experimentation will tell you a lot before you try and build a large linear shunt controller (or regulated digital shunt controller with load resistance) for your solar panel (which would have to dissipate your full panel wattage of XX watts--not a small controller).

Not saying it cannot be done--just that we don't know enough to understand if it can be done with Brand XYZ laptop computer.

-Bill

Or that it may be simpler / easier and more convenient to purchase a MPPT (or PWM) charge controller and a "smallish" AGM battery to build a solar PV power system and use the LVD (low voltage disconnect) available on some charge controllers to cut the power to the laptop when the battery is nearing full discharge.