My 5 watt solar panel car battery charger manual states that charge controller is not necessary for panels under 15 watts. I'm trying to understand the physics of why that's the case. Even if my panel is only putting out say 15 volts at 350mA couldn't it eventually overcharge a battery? What if the 5 watt solar panel were connected to a smaller battery, like a 7amp 12v, could it overcharge that battery? BTW, it has a diode to keep the battery voltage from discharging into the solar panel.
A typical lead-acid battery can safely be "trickle charged" at C/100 rate, where C is the capacity (if you are talking car battery, that is about 50Amp-hours typical). C/100 for a 50Ah battery is 500mA, so 350mA won't hurt it. The trickle-charge just overcomes losses due to battery self-discharge, which is fairly high in lead acid batteries. Your current is also limited by the internal impedance of the solar cell and diode.
On a small, 7Ah, 12V battery then, yes, you do run the risk of overcharging if you leave your solar panel connected because you will be charging at about C/20.
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