Charging my car battery

simagic

So, My car sometimes sits idle for a week or two (or three) and I thought I might put a 5 watt panel on my dash and run it to a Morningstar 4.5 Amp Charger and of course then to the battery. BUT.......I just read on this "great" forum that there would be a big loss thru the glass windshield. Am I just 'spinning my wheels here????????? or will this do some good???. Because of the windshield loss, should I use a 10watt panel instead???........thanks guys

BB.

Re: Charging my car battery

Volkswagen uses this panel for keeping their batteries charged (stuck to windshield with suction cups on the inside of glass):

Max Power: 3.2 W
Voltage at max power: 18.8 V
Current at max power: 170 mA

So--3 to 5 watts should be fine for a standard car battery without charge controller.

A 10+ watt panel, you should probably use a charge controller.

Does your car have "significant" loads when off? My in-laws' Lexus would use ~180 mAmps (computers, gps, old On-Star system, LoJack, etc.)--so would need a significantly larger panel to keep up.

Of course, when the trunk was not fully closed, the trunk light would stay on and kill the battery in a couple days.

-Bill

niel

Re: Charging my car battery

look at it this way, it normally would not receive any external power and is made to sit there on its own. in giving it this small charge it could help to maintain the battery longer even if it has loads such as bb pointed out. small side loads like those would just mean there may be a limit to the benefit of adding a panel of a certain small wattage. adding a pv like you propose can be a big plus as it is adding a small float charge circuit that could bring your car battery to its fullest charge point (100% soc) that the car alternator system fails to do and could lengthen battery life by heading off sulfation from forming as fast.

yes, you are correct that a bit of power will be lost in going through the glass and a bit more for being off angled to the sun's real position. the larger pv wouldn't hurt anything if you wish to employ it, but anything put there is a plus. of course this will not be a good way to charge the battery because one was listing to the radio all night long as it is meant only as a float charger. bigger loads like using the car stereo would demand a larger pv system to keep it at 100% soc.