My car sits for long periods of time without being used, so I would like to get a solar trickle charger. Any advice on what unit would be best for keeping a good charge on a car batt?

 

Are you thinking of a home brewed device, or a commercial unit?

 

I bought a solar trickle charger for only $9.99 on sale. Its output is 2.25W at 12V-13V and has a cigarette lighter plug.

 

Thinking of an off the shelf one...Have a fav?

 

Well, Harbor Freight carries a couple of types:
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=44768
$12, 1.5W. That's not much power. Around 110mA charge rate.

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=41144
$40, 5W. A bit better, around 390mW.

They run sales on things all the time. Haven't seen these particular items come up, but I don't always read the fliers they mail out, either.

 

The first one...would it really keep a car bat up to par?

 

The first one...would it really keep a car bat up to par?

I'm dubious about it being able to keep it up.

It would depend upon how much current is being used by your vehicle when it's not in use. There is always SOME drain, even just to maintain the radio memory and the clock.

And a solar cell only generates power during daylight hours, perhaps 1/2 day. Obviously, more in summer, less in winter.

So, if it's putting out 110mA in the light, it's putting out 0mA in the dark - or averaging around 55mA through the day.

You would need to measure your vehicle's static current drain.
First, charge your battery completely, perhaps for a day on a trickle charger. Then let it sit for several hours for the "surface charge" to drain off.

Then, disconnect the negative cable, and connect in an ammeter. See what the drain is.