20W solar...need controller?

[4x4van]

I have acquired a 20W solar panel that shows 20vdc when not connected to anything (in full sun). When hooked up to a 12v battery, I have seen it as high as 17vdc. I'm wondering if I can get away with hooking this up to my RV to help keep the batteries (currently 2 12v, soon to be replaced with 2 6v) topped off while in storage, without needing a charge controller? Although the voltage is high, the amperage is pretty low. Will a 1 amp charge at 17 vdc hurt my batteries?

[BFL13]

LY, that guy made two mistakes IMO:

A. He said to drop to Float when you got to the absorption voltage with the Lifelines. That is even wrong for flooded batteries. You don't drop to float until the batteries are full.

B. He said the Walmart battery is 125AH, but I believe that is not the 20 hour rate, but the 100 hour rate they use for advertising purposes.
Eg, my 27 size AGM 12121-C1s are rated at 121 AH at the 100 hr rate, but are actually spec at 100 AH at the 20 hr rate.

[landyacht318]

A fully charged healthy battery with no overnight loads applied will be overvolted by 20 watts of solar.

Much depends on the battery, how much current does it require to be maintained at a safe float voltage? its different for all batteries and changes as they age

here is a video of a 12watt solar panel overvolting a pair of GC-2 lifelines

https://www.youtube.com/watch?v=z5RtZe9AW2E&list=UUoPqTkOluQsuu3RpGnxVwFw&index=9


https://www.youtube.com/watch?v=IMxlPIdv2D8&list=UUoPqTkOluQsuu3RpGnxVwFw&index=15

And the guy in that video has more experience and knowledge on DC systems that anybody else who posts on the internet

[4x4van]

Hmmmm...interesting. OK, I will revisit the idea of a controller or that voltage regulator linked above. Right now, it is hooked up to a single 12v battery that I have in my enclosed trailer (used for interior lighting) as sort of a test case; I'll keep a watch on it and monitor the battery voltage.

[RoyB]

Glad to see some discussions here...

I am installing a new battery bank which came with a 10WATT SOLAR panel one end of the slide off top lid. I am going to add a second identical 10 WATT Solar panel on the other end.

Was wanting to use the two 10WATT solar panels for trickle charge of the two 6VDC GC2 series battery groups when sitting in the high sun... This battery bank will be sitting on my trailer tongue area...

I too see 16-17 VDC output on the solar panels with nothing hooked up. When I hookup to the batteries this drops back to charge state of the batteries. (12.6-7 VDC...)

Never thought about it may need a regulator on it... Before I started working on my new battery bank I was sitting the one solar panel in the windshield dash area of my truck and had it plugged into an always hot 12V connector. It sit this way for a couple of years and never did any harm to my truck battery. I think the output is around 200 milliamps...


Google image

Roy Ken

[Sam_Spade]

3 tons wrote:

2112 wrote:
You could use something like THIS and a diode between the panel and battery. Adjust it to 13.2V or so at the battery.

This is a very creative idea!!

Might work and might not.

The point is to REDUCE the charging voltage (and current) as the battery charges up. As the internal resistance goes UP, the charging voltage needs to go DOWN.....but a "dumb" power supply does exactly the opposite.

Keeping the voltage constant is only slightly better and would still require some testing and attention.

Get the right tool for the job.
Saving a few pennies up front sometimes costs you DOLLARS on the back end.

[BFL13]

17v on one 12 will be a bit less on two 12s, but still way too high for a float voltage of 13.6v, adjusted for California warm temps to 13.2ish.

So you need a controller that will float in the low 13s.

[3_tons]

2112 wrote:
You could use something like THIS and a diode between the panel and battery. Adjust it to 13.2V or so at the battery.

This is a very creative idea!!

[jplante4]

Moved from General RVing

[2112]

You could use something like THIS and a diode between the panel and battery. Adjust it to 13.2V or so at the battery.

[time2roll]

17 volts every day in storage is too high. Keep an eye on it and start looking for a controller.

[Sam_Spade]

4x4van wrote:
Will a 1 amp charge at 17 vdc hurt my batteries?

YES. Over a long period it might.
Maybe as little as a few weeks.
You NEED a controller.

1 amp will slowly charge the batteries.
As the battery charge (voltage) goes up, so will the charger voltage.....maybe as high as 20 volts and it still will push an amp or more.

Using it for very long without a controller might not be much of a risk but it is a small one......especially if you have wet cell batteries.

[4x4van]

Yup, it was a freebie. And yes, I disconnect my batteries when in storage. I was just concerned with the 17+ volts, but at only 1 amp, I guess it should be fine.

[KD4UPL]

Rule of thumb is that if the charge current is less than 1% of the AH capacity then no charge controller is needed. That means your fine since you've most certainly got more AH capacity than that.
Don't expect that little panel to do much. Sounds like it was free so might as well give it a shot.

[2oldman]

You may as well just disconnect them.

[Ed_Gee]

No, a 1amp charge is barely a trickle.....likely won't maintain your batteries in storage unless battery disconnect is on....