Direct Connect Solar Panels to 12v Chargers (for misc electronics)

Question

Can I safely connect 12V solar panels directly to electronics that have a 12V auto charger? The "Car charger" essentially contains the electronics necessary to regulate the voltage from the solar panel down to that required voltage for a given electronic device. And the electronic device controls the charging... my only downside is that the charging will be sporadic and might turn on and off with passing clouds.

I'm thinking of doing this for cell phones, MP3 players, GPS navigation units, etc.. I don't like the solar chargers as most require the use of batteries in the "middle" that you first charge and then discharge to charge your device although they all have switches in them that need to be manually activated.

Thanks!

Answer 1

The car charger is most likely a small switched mode power supply.

This should be fine running from any voltage (assuming a 5v output) of say 7v up to 50v+ (check the chip to be sure of the upper limit).

I am currently working with the MAX5035 family of chips which run from 7.5VDC (or 15VDC for the 12V version) up to 74VDC to give 3.3V, 5V or 12V.

Answer 2

What you plan to do will probably work. If the solar cells have enough light to maintain 12V, then a car charger should work well enough since it's designed to run on that.

The problem is what happens when the solar cells can't keep up with the power demand. In that case the output voltage will collapse. What exactly the car charger and the battery charger in each unit does in this case is not certain, but should not cause harm if the circuits were designed even marginally competently.

The chance of damage are low enough that it's probably worth a try.

Answer 3

What you propose will work in practice in many cases but has some potential problems for some situations.

NimH charging relies in approximate order of use one or more of the following for end of charge detection:

negative delta V
delta temperature
absolute temperature
terminal voltage
timer

All of these may be defeated to a greater or lesser extent in systems that have power turned on or off during the charge cycle. Degree of effect depends on equipment, position in charge cycle, ambient temperature and more. YMMV and probably will.

NimH "like" to be charged at about 1C rate to get a clear negative delta signal. Lower rates tend to mask this signal as it gets smaller or vanishes. Turning charging on an off around EOC is liable to make this unreliable.

Delta temperature MAY be useless if the device is charged intermittently.

Absolute temperature likewise.

Terminal voltage "settles down" given rest periods. It will come back to "correct state" fairly rapidly but few chargers probably use it.

If a timer rests repeatedly it too fails.

So. it seems likely taht a NimH charger may have "a hard time" depending in which combination of EOC techniques are used.

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Lithium Ion will be well behaved in most cases.

NiCd has similar problems to NimH

Lead acid should be OK for main charge aspects but topping cycles and the like would get greatly messed up.

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Targets have to be able to withstand peak voltage - easily established bt fatal in some cases if you get it wrong.

If load Watts of all attached devices approach panel Watts in high sun, multiple clients may oscillate / interact over seconds to munutes in low sun as appliances come on line, start charging, load drops voltage and som devices drop out rater than throttle back.

Many LiIon chargers want the energy source to be able to meet their current demands. If the source sags under the demand they will stop completely rather than backing off. eg a laptop that wants 3.5A will usually not charge at all if only 2A is available.