Do they work? effectively?

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RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

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FACEBOOK: PANZER450

Good question. I assume that they help keep the battery up and can't be used for charging, but do not know if they properly induce the float charge needed.

I could use one, but I'd want to ensure that it wasn't actually reducing the useful life of the battery.

I can tell you what little I know:

There are many different kinds. It is one of the "you get what you pay for" situations. I have the absolute lowest possible end of the market, the Harbor Freight 7.99 model. It has an alleged "charge indicator light" which is a blinking led that blinks whether it is actually charging or not, so that feature is useless. In good sunlight it works fine. In Seattle... My boss has a better one that he says works well. West Marine has a good selection I think. But you know marine is spelled with $$$. PS, you have to make sure the cig lighter is connected while the car is off or it definitely doesn't work! My Toyota had that problem so I had to put an aux cig lighter socket that was connected all the time.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

I know VW offered one in their new cars, it was a 2a trickle charger that could in time revive a dead battery, but they won't sell me one because I don't own a VW (bastards) and even if I did have to replace one I had, it would be like $275.

__________________
RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

____________________________
FACEBOOK: PANZER450

You can buy them on ebay.

I am not sure if our cars are wired to allow reverse current through the 12V charger.

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1974 240D 3.0L 4(M) - 250K -OM617.910 + 300D FW + Rajay turbo power!
1977 6.9 3(A) - 84k