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?

[Harvey51]

I would be afraid to connect 17 volts to a battery connected to a vehicle - who knows how much voltage the computer can handle? A Toyota mechanic told me our Highlander can handle 16 volts. It’s easy to drop the voltage by 1 volt, just run the current through two diodes in series - there is a drop of about half a volt in each diode. The Highlander parasytic current draw is less than half an amp so a 1 amp battery maintainer is adequate for its current addiction. I have heard that disconnecting the battery on a late model vehicle can cause problems, such as having to replace sensors because the computer learns to live with sensor aging and when it loses memory it has to learn again from a new sensor.

[MEXICOWANDERER]

Con't the trickle issue...

For several years the industry touted "self regulating solar panels":

Naturally they did not regulate and naturally they destroyed batteries. I had brought down several extra Ramcar RV batteries (Neh) and fixed her up with a Trace C-30 controller against the wishes of a half dozen self appointed solar experts on the beach. Because the vendor appointed 17.2 volts OCV as a stat.

Rubbish.

She ended up with a functioning 20 watt system. She was a real shrew with electrical power.

[MEXICOWANDERER]

Con't the trickle issue...

For several years the industry touted "self regulating solar panels":

Naturally they did not regulate and naturally they destroyed batteries. I had brought down several extra Ramcar RV batteries (Neh) and fixed her up with a Trace C-30 controller against the wishes of a half dozen self appointed solar experts on the beach. Because the vendor appointed 17.2 volts OCV as a stat.

Rubbish.

She ended up with a functioning 20 watt system. She was a real shrew with electrical power.

[2112]

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!!

I use them with a 19.5VDC laptop power adapter to float and equalize my batteries while in storage. I power three modules with a single adapter so I can float three batteries at once. The 19.5V input is always on so no isolation diode is required. A diode would be required if using solar to prevent a 20mA battery drain at night.

[4x4van]

Well...that escalated quickly! :B

[2oldman]

MEXICOWANDERER wrote:
Perhaps in 115 degree weather or if the AGM battery is really old. One to 2% is common self discharge.

If that. My Lifelines' self-discharge was hardly noticeable.

[MEXICOWANDERER]

That's why I recommended float but total disconnect protocol needed to be stated once again. I saw the number 5% per month self discharge used for AGM recently on this site. Perhaps in 115 degree weather or if the AGM battery is really old. One to 2% is common self discharge.

[BFL13]

Trojan on AGM Float:

"applications where batteries are infrequently used, it is desirable to compensate for
self-discharge by keeping a low charging voltage on the batteries. This is called the float voltage.
Avoid using continuous float charging on VRLA batteries that are charged more than once/month."

Seems they prefer you disconnect and let them sit instead of getting the wrong float voltage? AGMs do sit without significant self discharge for longer than a month.

RV "disconnect switch" does not usually disconnect everything (LP alarm, radio settings, etc, stay on) so watch for that.

[MEXICOWANDERER]

Two indicators are omitted in he discussion IMHO

The first is with AGM the determining charge rate once the charge acceptance rate reaches 1/2 percent @ 14.40 volts should be reduced to the appropriate float potential. The greater the wattage potential Delta T @ (OCV) at 14.40 volts the greater the positive plate shedding. This is also called overcharge. Yes voltage will rise above 14,40 but at 14.5 it is destroying the battery. Curiously enough a flooded calcium battery demands higher voltage but does not exhibit the "brick wall" voltage/amperage effect as does an AGM battery.

At 50c I've seen the .5%@14,4 volts effect as low as 13.1 volts. The higher the voltage differential the more self destruction takes place.

Refer to the Lifeline manual for temperature compensation.


When sustained floating a flooded battery electrolysis indicates a battery is being overcharged. Hydrogen and O2 bubbling must be avoided.

A sum total of the number of hours the battery is being mistreated to (i.e. solar versus shore power) is very useful as the total number of hours the battery is accumulating damage.

An amp hour meter is useful here. Any totalization exceeding 120% is cause to for additional surveillance.

I use decay characteristics as a handy indicator of finding a proper float point. Hear me out...
Let's say your controller says "All is well"
Using a watch, impress 14.4 volts while using a digital ammeter. If amperage starts out at greater than .5% and takes longer than 5 minutes to recover to .5% the battery is being floated at too low of voltage. Don't forget to allow for a possible error in temperature compensation.

Trends and tendencies. This is beyond the scope of 'usual' battery charge float control. But for the ultimate (added 20% lifespan) in float parameters it's worth tinkering with.

[landyacht318]

The screwy31 was that same battery. UsBattery g31. 125 amp hours.

Still is. Lower case looks exactly the same. But blue. It has not yet shorted a cell despite dire predictions of that inevitable impedending occurrence, 4 years ago

Mainesail is a well respected marine engineer., with current experience with all battery types

. And I know a lot of half witted fools with college degrees.

[BFL13]

red31 wrote:
A. 1:49 "until it gets to about .5% acceptance rate of its 220 ah rate (capacity) then it should terminate and go to float"

B. usbattery G31 20hr rate 130, 100hr 144
https://www.usbattery.com/products/12-volt-batteries/us-31dc-xc2/
https://www.usbattery.com/wp-content/uploads/2019/04/usb_31DC_data_sheet_2019.2.pdf

neither has anything to do with CC need.

Right. I played it again and heard "point" this time. 😞 Sorry.

Not sure that 31 is the same battery though.

[Sam_Spade]

RoyB wrote:
I think the output is around 200 milliamps...

It would take a LONG time for that to cause any problem.
BUT....it probably isn't enough to really help anything either.

The charger needs to be disconnected for a few minutes every week or so and a battery voltage measurement taken......until you can determine that it is keeping the battery in the right range.

[red31]

A. 1:49 "until it gets to about .5% acceptance rate of its 220 ah rate (capacity) then it should terminate and go to float"

B. usbattery G31 20hr rate 130, 100hr 144
https://www.usbattery.com/products/12-volt-batteries/us-31dc-xc2/
https://www.usbattery.com/wp-content/uploads/2019/04/usb_31DC_data_sheet_2019.2.pdf

neither has anything to do with CC need.

[MEXICOWANDERER]

Pretty bold statement. Shouldn't it read using a prefix "IMHO"?.............

BFL caught it and so did I....

Too many ambiguous statements to believe the good fellow is even college educated.


battery university has a deep well of knowledge that is difficult to challenge.

But it was a thoughtful presentation and I liked the bridge showing what happens rather than using only numbers.