Instructables

Will 22 volt solar panel hurt my marine 12volt battery?

 I bought a small solar panel to charge a marine 12volt battery, but when I checked the voltage of the panel in the sun  it read 21 - 24 volts - will that hurt my battery?

seandogue4 years ago
If you charge directly, yes, since you're applying far too much voltage across each cell for safe charging, due in part to hydrogen generation at elevated cell voltages.. Ideally, you will route that solar panel thru a charger circuit, and at the very least a voltage regulator that brings the voltage down to a more reasonable level for charging a 12V battery. Goggle/yahoo, etc are your friend for details. "Lead acid battery charging techniques"
Actually, like Andgadget has said, once connected to the battery, the output voltage will be more-or-less exactly the voltage of the battery. It will hopefully be just a smidge higher, forcing some small amount of current through the battery. The rest of the solar cells excess voltage doesn't magically disappear, however. That couple of watts will be dissipated as heat, by the solar cell, itself.
klee27x klee27x4 years ago
Here's an analogy. The battery (relatively low impedance power souce) is the surface of the earth, which, on a theoretical grid, is positioned at 12 inches high. The solar cell (a relatively high impedance power souce) is an ant, suspended at 22 inches high, 10 inches above the ground. Now press play (connect the wires). By and large, it's the ant that falls the 10 inches to the ground, not the other way around. Ironically, everyone is concerned about the earth being hurt, and no one has considered the ant.

Essentially, you're partially shorting the solar cell when you connect it directly, on a sunny day. But that scant 2W of power is easily dissipated within the solar cell, itself, without harm. So the earth is fine, the ant is fine, and no one gets hurt.

If you were dealing with a situation where the excess voltage/heat could actually damage "the ant," , a low value power resistor placed between the solar cell and battery would effectively shoulder part of the burden of heat dissipation. E.g., if the solar cells were arranged to output 112V, you could put a 0.1 ohm resistor in series. Using a voltmeter, you could measure the voltage drop across the resistor. It would be somewhere between 0-100V, depending on the internal resistance of the solar cells. You could then adjust the value of the resistor until it is dropping most of the 100V, without overly restricting current at times when the output of the cell is lower than max. Of course, with a voltage differential this large, efficiency would be pretty bad.

Now if the solar cell were a large meteor (output in the dozens or hundreds of watts), then you'd have to take measures to protect the earth.
klee27x4 years ago
+1 AndyGadget, +1 jeff-0, +1 Lemonie. Collectively, they answer your question.

Once fully charged, it's actually desirable to continue giving a lead acid battery a small trickle charge to keep it topped off. Your 5W solar panel puts out that trickle charge. On a good day. In other words, the panel might have enough output to keep an already charged marine battery at full, but not a whole lot more than that. There's very little risk of damage from overcharging, unless you're talking about a toy boat with a <1Ah battery.

Adding extra circuitry would likely end up in worse performance, if anything. All I'd do it put a diode between the solar cell and the battery and call it good.
lemonie4 years ago
I doubt it. Lead-acid cells are tough, it's current you need to consider and they'll happily take amps (plural). How much power does the panel produce?

L
mtlt (author)  lemonie4 years ago
I think the panel is rated at 5 watts - I will be looking into a charging regulator.
Thanks for your answer(s).
lemonie mtlt4 years ago
At 5 watts, if it will output more than 12V (and 13-14V would be good) you're OK. Charging regulator not at all necessary for a lead-acid battery from this source.

L
jeff-o4 years ago
That's the open circuit voltage, which is typically several volts higher than the loaded voltage output.  It'll probably be fine...
AndyGadget4 years ago
if it's a small solar panel, no.  The voltage will be pulled down to the 12V terminal voltage of the battery.  If there isn't one in the circuit already you will need a diode in series with the charging lead so the battery doesn't discharge through the panel during the night.