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Solar Trickle Charger For Car Battery

by Chris - June 21st, 2011.
Filed under: Solar Products. Tagged as: , , .

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This article reviews the use a solar trickle charger for car battery charging. A small solar panel mounted on a car or truck can help compensate for small quiescent loads on the battery that may lead to a no-start, dead battery condition if the vehicle is allowed to sit for long durations.

A friend of mine has a car that he doesn’t drive very often. He lives on one of the San Juan Islands and leaves his car at the ferry terminal on the main land. He only comes across once or twice a month. Without fail, his battery is almost always dead when he gets in to start it. Swapping the battery with a new one has not solved the problem. He’s taken to lugging a battery powered jump-starter with him on the ferry, which is highly inconvenient. I decided to write this article to help him out and others like him.

Luckily, this is actually a really easy problem to solve. Sunforce makes a solar trickle charger for car battery charging. All you need is a Sunforce 50022 5-Watt Solar Battery Trickle Charger. Period. No other parts are nessessary.



This solar panels comes with an auxiliary power connector (12v cigarette adapter) as well as mini alligator clips. You can also tie it directly into the power lines, so you have several options for interfacing the panel to your batteries. The integrated diode prevents current from discharging back into the panel.

Normally, I do not recommend that people connect a solar panel directly to a battery. It’s best to use a charge controller to prevent over charging or over discharging of a battery. However, this is one exceptional case. As mentioned in my article on How to Build a Simple Solar System, a panels output should not exceed 1.5% of a batteries amp-hour rating, if you plan to use a diode instead of a charge controller. Most car batteries are between 70 and 100 amp hours (AH). 1.5% of 70 AH is 1.05 amps and the maximum output of this 5 watt panel would be 0.416 amps, which means you’re OK to *not* use a charge controller.


Finally, the solar panel will not compensate for significant quiescent loads! This is not band-aid for poorly designed power systems. Most cars will have a key-off load that averages less than 100 mA (0.1 amps). Assuming a worst case key-off current of 100 mA, that means your car uses 2.4 amps per day of current. Assuming the Sunforce solar panel output it’s rated 350mA for 8 hours per day, it generates only 2.8 amps per day – this barely covers your worst case scenario.

If your car or truck is in a similar situation as my friends, try out this solar panel. If your vehicle is parked in a sunny area and you find that the solar panel is still not cutting it, then it’s safe to assume that your car is averaging more than 0.1 amps of current draw when it’s turned off. Your only option at this point is to physically disconnect your battery when you leave it. Leave the solar panel connected however, as it will keep the battery fully charged.






2 Responses to Solar Trickle Charger For Car Battery

  1. Modern cars have up to .75 amp key-off drain (model year 2008-up).

  2. Really!? That seems like an incredibly high key-off drain current. Do you have a specific make/model in mind?

    At a current draw of 750mA, I would expect the car battery to be dead within 3 days or less. I’m more familiar with heavy duty trucks, and a ‘normal’ key-off (quiescent) load is less than 0.1 amps (100 mA). I’ve seen them as high as 0.5 amps, but like I said, vehicles batteries will be too depleted to start the vehicle after only a week or so of sitting.

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