Should I leave the solar battery charger plugged in?


I have an '87 Toyota pickup. I drive it a few times a year. To keep the battery charged I plug a solar charger (1.5 watt) into the cigarette lighter socket. It puts out about 22 volts, about 17 of which make it to the battery. A friend tells me this will hurt the battery. I hope it will be little enough current to matter. Who’s right? Is there a sensible way to attach the solar charger’s output to the voltage regulator?


First, please verify the amps, volts and watts of the solar charger? Is it, in fact, a device designed to maintain a charge on a 12 volt battery?


I would be a bit concerned, solar power systems have electronic regulators to make sure the batteries don’t get overcharged. Also, the ‘battery tender’ company now makes a solar charger with the regulator circuitry built in. If yours put out little power, then maybe it won’t hurt, but it won’t charge much, either.

Since the solar charger likely puts out only a small amount of energy, it is very unlikely to cause any problems.   While it may show 22V that is before the resistances down the line are accounted for and in real life situations it likely to be far less than 22V.  If you dig out an old analog volt meter, chances are you would be more like 12.02V


Solar cells have internal resistance. Likely much greater than that of the wiring.
While the cells will deliver 22V with no load their voltage sags when current is drawn.
The bigger the area of the array the lower the resistance.

My WAG is if the array is no bigger than about 8x10" the voltage will sag enough under load.

Check the voltage after the charger has been connected and in the sun a few hours.
About 13.2-13.8V is best.

Edit: I missed the 1.5W part. Not enough to even do much good.


The open circuit VOLTAGE is meaningless… The small panels OUTPUT is only 1.5 watts! That’s about 100ma charge rate which is perfectly safe. As a matter of fact, it’s probably not enough to maintain the battery except under ideal conditions…

A “12 volt” solar panel must have an open circuit voltage of at least 17 volts to effectively charge a 12 volt battery…


You’re right, Caddyman, 1.5 watts is close to 0. On the ‘battery tender’ web site they list solar chargers starting at 5 watts up to 15 watts. They also have a stand-alone regulator, but for 1.5 watts, no need.

This highlights the problem with suggestions to ‘just put solar cells on the roof of the EVs (or hybrids)’. Solar panels don’t put out much power, you need a lot of them to charge an EV.


At ~10W per square foot you’d need 100’s of sq ft for an EV charger.


How hard is it to unplug something from the cigarette lighter?

Just unplug it in the remote chance your friend is right. You’ve got nothing to lose.


I used to leave the battery disconnected, which worked for years. In the last few I’ve had to push it anytime I didn’t drive for a month, which annoyed me, and could be a problem. I got this one on sale, $10, willing to lose the money. But it works. I have let it sit for 6 months and had it start within a minute without pushing. Now the clock keeps the time.

The voltage I measure on the battery when the charger is connected is no different than when it isn’t: the battery overwhelms the little the charger provides. But over months it makes a difference. This answers Mr Whitey’s question: I don’t know months beforehand when I will drive so I have to leave it plugged in to be ready because it probably needs a month of charging to make a difference.


Thanks to Mr. texases for looking it up on the battery tender web site: this is evidence consistent with leaving a 1.5 watt charger connected without regulation being innocuous.