Electronic – Large load connected to small solar panel and battery

battery-chargingsolar cell

I have a smaller solar panel (say 12v, 0.5 amp and therefore 6 watts) connected to a 12v car battery (assume lead-acid) directly though with the included blocking diode, there is no charge IC attached. In full sun the panel will deliver up to 15v. What happens when a load (It must be assumed that the load isn't particularly voltage sensitive, within reason) in excess of the solar panels output is connected? For the example, lets say the load is 20 watts.

I would imagine that for a small load (below that of the solar panels output) the load would simply use the provided electricity from the solar panel (15v and whatever amperage) and the battery would charge at a lesser rate, however, once this load is exceeded, what occurs? Does the solar panel burnout? Does its voltage output drop? Or Does the battery simply provide a small quantity of current whilst the panel provides the bulk? If the latter is the case, how does this work?

Thanks for any assistance,

Best Answer

When the load is off, the panel will fill the battery with the voltage above permitted for the chemistry of the battery, the electrolite in the cell will boil (gassify) and destroy the battery.
When the load is on, both battery and panel will provide current. Of course the panel will work less efficiently if the output voltage of the panel isn't held on maximum power point voltage.