Solar Panel Car Battery chargers - any good?

SteveJW

I would check something isn't drawing power when the car is parked, look for a boot light or something like that

My Mazda stays outside and is usually used once a week for the shopping, it start ok

Last winter November time it stayed outside at Manchester airport for three weeks, came back it had over a foot of snow on it, turned over no more than half a dozen times and fired up

Someone posted The car charges itself at 4 Amps when it's running

Don't know where this came from, most modern cars have alternators rated at over 45 amps, partly discharge the battery by leaving lights on, or sat idling with lights, heater, heated windscreen at traffic
lights will drag a battery down. The state of battery charge is sensed (voltage etc) and the alternator / electrics auto senses this and ups the output accordingly.

Yes when battery is fully charged alternator output is reduced to an amount that takes into consideration the current drawn, this could well be around 4 amps dependant on what is in use

datostar

I had one on a motorhome a few years ago. I never thought it made much difference and still had to use a mains charger if it hadn't been run for a couple of weeks or so.

Strider590

The voltage regulator of the charge system does not measure the relative currents charging the battery and for powering the car's loads. The charge system essentially provides a fixed voltage of typically 13.8 to 14.4 V (Volt), adjusted to ambient temperature, unless the alternator is at its current limit. A discharged battery draws a high current of typically 20 to 40 A (Ampere). As the battery gets charged the charge current typically decreases 2 A to 5 A. A high load results when multiple high-power systems such as ignition, radiator fan, heater blowers, lights and entertainment system are running. In this case, the battery voltage will decrease and the charge current as well.

http://en.wikipedia.org/wiki/Automotive_battery

Essentially a battery shortly after starting the car, will not be discharged enough to draw any significant current from the alternator. There's a formula somewhere that'll work out the charging current versus voltage difference, but with alternator doing around 14v and the battery fully charged voltage being 12.9v there's always going to be a few amps of current going into the battery.

On a discharged battery, it'll draw a lot more current from the alternator (great way to earn yourself a large repair bill though).