5 watt solar panel question

[Taranwanderer]

So I just purchased a Sunforce 5 watt "12 volt" battery trickle charger for my home generator--this is my first foray into the solar panel era. The Generac has a 12V tractor-type battery to start it up, and I want to keep that battery full so it's ready for use when the power goes out (I know, this isn't a camping/RV specific question, but there's so much pooled knowledge on these forums, I knew this was the place to ask.) So I'm reading the directions, and it says you can measure the output of the panel with a voltmeter (which I was going to do even before I read it,) and it says that the voltage "should be between 16 and 25 volts in full sun." 16-25 volts?!?! What happend to it being a 12 volt charger? Now I know it's only a 5W panel, which should come out to about 350mA of current, and while it does have a blocking diode to prevent battery discharge at night, it doesn't have a built-in charge controller. So is that much voltage going to hurt the battery? And do I need a charge controller for a 5W panel? I'm a little concerned...

Thanks in advance.

[Kuba]

You need a charge controller.

[Taranwanderer]

Sunforce's website states that the product has "built in overcharge/discharge protection," but states it doesn't have a charge controller, just a blocking diode. Their site also states that any panel under 10W shouldn't be able to overcharge a 12V car battery.

I'm not opposed to buying and installing a charge controller, I'm just wondering if I need it if the manufacturer is saying I don't. I'd think they'd be saying I do need it so I'd buy more of their products...

[cadman99]

I have had a 5 watt panel charging a 4 amp hr battery for 2 years.

[RPAspey]

Those voltages are normal. You're measuring open voltage. Thats just one spec of a solar panel. I doubt you'd ever over charge your starting battery.

[Taranwanderer]

Originally Posted by cadman99

I have had a 5 watt panel charging a 4 amp hr battery for 2 years.

Is this with or without a charge controller? And has it ever overcharged your battery?

Originally Posted by RPAspey

Those voltages are normal. You're measuring open voltage. Thats just one spec of a solar panel. I doubt you'd ever over charge your starting battery.

See, that's what I'm thinking too--at 5W, its not producing enough amps to cook my battery--is it? It probably getting 350mA (at best) for 3 or 4 hours a day.

[cadman99]

No charge controller - a waste of money on a 5 watt panel.

[RPAspey]

The only time that panel will charge is when its voltage is higher than the battery's. So once the panel starts charging, say 12 v for the discussion, your panel could theoretical make .41 amps. Watts/volts=amps. 5watts/12volts=.41amps. Thats on a perfectly clear day and at 90 degrees to the sun. Check the specs on your battery, I'd guess you are losing that much due internally. Again I doubt you will over charge the battery. A charge controller is a waste at that wattage.