is it okay to leave panel disconnected while in sun?

[rug_burn]

Gentlemen:
I have an off-grid small solar system to keep my 12 volt airplane battery charged. The panel is supposedly a 20 watt, but the best I've ever seen it do is about .5 Amp x 14 volts or about 7 watts, which is good enough. Typical 36 solar cell panel, about 14" x 21". I am going to add a couple other batteries and a microcontroller switcher to keep all topped off, but it's not done yet.

My only concern is something mentioned by a friend who plays around with electronics who said that it's best to leave the cells connected to some non-zero load while in the sun, even to the point of shorting them out or running them thru a 5 ohm resistor, etc. This is because the cells supposedly degrade unless they have some current running thru them.

At this point, when my plane battery reaches 14 volts or so, I have just been disconnecting it from the battery, and putting a 5 ohm resistor across the output of the panels. I have a small Coleman solar charge controller which will do this automatically, but without connecting the resistor to it, and the controller's not connected yet anyway.

Is this story about keeping some load on the cells true? Anybody else heard this claim, or should I just let the panel voltage float when all batteries in my system are in full charge range?
Thanks in advance...

[Sunking]

No problem leaving the panel disconnected or shorted out in the sun. Only time you will see full current out of a panel is when shorted out or connected to a battery that is discharged. With a decent controller you will see very little if any power when the battery is fully charged up. Your Coleman cannot do that.

I do suggest you get a different controller, a PWM type and just leave it connected to the battery. Toss that Coleman unit in the trash.

[Mike90250]

No panels that I know of, have any sort of connection/sun warning. (except for some unique + gnd panels)

[mapmaker]

No panels that I know of, have any sort of connection/sun warning. (except for some unique + gnd panels)

Some cdte (cadmium telluride) panels should not sit at Voc in the sun. --mapmaker

[rug_burn]

I do suggest you get a different controller, a PWM type and just leave it connected to the battery. Toss that Coleman unit in the trash.

Man, that's rough- I haven't even used it yet...!

[PNjunction]

Shorted or open, it won't harm the panel. What WILL harm the panel long term is a persistent shadow across one or more cells. Shaded cells are now reverse-biased, and there is a good possibility that those panels at that power level don't have any protection against that.

Are your panels mono/polycrystalline, or are they thin-film? Just beware of low-end "boutique" panels, usually tossed to consumers as "ideal for camping", when in reality they are not.

What are we talking here- RC model plane, or genuine aircraft? If a genuine aircraft, what exactly is the battery that you are using?

And yes, those Coleman / Sunforce 7A charge controllers are a joke. Old school tech from the 70's that just ping pong between two voltages, relying on the hysteresis of the battery. Super inefficient. Not to mention those CC's are not temperature compensated, and can have wide variance in voltage. A pwm charge controller of decent quality is a must. At the very least for this application, a Morningstar Sunguard 4.5A would be a good start until we find out more about your battery and panel.

[inetdog]

Could not resist jumping in on this one.

1. Shadows of any kind are not a problem while the panel is open circuited. Just when it is in use.
2. As PNjunction noted, commercial panels bigger than tens of watts and most DIY panels will incorporate bypass diodes. Those will limit the reverse bias on any shaded cells within the panel and for silicon based panel should minimize the risk of any damage. Reverse bias by itself is not so bad as long as the current is limited to keep from creating localized heating. Just where the threshold is for having bypass diodes in a panel is hard to guess. Best to look at the panel or apply a small forward voltage to a dark panel and see how much current flows.
3. For a small battery maintainer panel with an output voltage on the order of 14 volts, all it takes is a couple of shaded cells to drop the voltage low enough that you not be forcing much if any current through the shaded cells. Just do not short circuit the panel instead of open circuiting it if you are going to leave it partially shaded.

[rug_burn]

Okay:

The plane it's connected to is a full size plane, (1955 F35 Bonanza) sitting in a hangar without power. Also occupying that same hangar are several RC model airplanes, which I have played around with for many years.

The battery is a lead-acid Gill rated at 35 Ah... I'm going to also connect it to two other batteries, as mentioned in the first post above. One is a fairly new car battery, probably about 45 Ah or so, and another made of 6 of the old dry charge Willard cells which are WWII vintage, believe it or not. I've had these things for about 35 years of their life, and they're in perfect condition, never had acid in them. The dates on the boxes are 1943! I also have the battery acid, standing by. This should give me probably a bit more, maybe 50 Ah.

The panel is (I think) amorphous, it's kind of hard to tell because it has this bead-like diffusion plastic sheet panel in front of the actual solar cells. The color is bluish, but I looked real good, and I don't see the polycrystalline pattern at all.

It came from an emergency/survival power pack I got at an electronics swap meet. I got the panel for $45 and had to disassemble the thing, take out these unusual flat gel cell batteries (2 x 6 cells, series parallel) and reassemble, keeping the respectable mounting frame. It has a reverse polarity prevention diode. It was actually a pretty nice little unit until the gel cells gave up the ghost. Came with a small light that you can plug into any handy 12v cigarette lighter outlet, which it also came with.

I have it mounted atop the hangar with an oversized piece of 1/8" plexiglass about 3/4" above it with air space between to keep off rain and bird turds, 30 degree angle.

I suppose eventually after I get the multiple battery switcher going, I could spring for a better controller. How do the PWM controllers work the PWM part? Do they taper off the duty cycle as the battery approaches full charge?

[Sunking]

I suppose eventually after I get the multiple battery switcher going, I could spring for a better controller. How do the PWM controllers work the PWM part? Do they taper off the duty cycle as the battery approaches full charge?

Precisely. They are effectively a on/off switch operating anywhere from 17 to 60 Khz. At 100% duty cycle, the switch is full on, and at 0% duty cycle full off. They are no tall that efficient but cheap and will not harm the battery like your Coleman unit can do.

[Mike90250]

.... 6 of the old dry charge Willard cells which are WWII vintage, believe it or not. I've had these things for about 35 years of their life, and they're in perfect condition, never had acid in them. The dates on the boxes are 1943! I also have the battery acid, standing by.....

Have you tried to sell the Willard Cells to a collector before you put acid in them ? Their lead plates, are not likely to have survived without being severely damaged (atmospheric oxygen & 35 years, they are most certainly toast, unless sealed in a steel can) and the cells are not likely to store any power. So try selling them to a collector, and use the $ to buy a reasonable battery or a charge controller for your existing batteries.

[PNjunction]

What gauge of wire is the run from the top of the hanger down to the batteries? In addition to the dinky panels, you may have lots of voltage drop.

What you have now is really not doing much for that Gill.

How about going aviation-specific and taking it to the next level. I just wouldn't mess about with a cool plane like that. See why aviation-specific (usually tied to Gill / Concord specifically) is the way to go. Yes, like anything aircraft related, open wallet and empty it.

http://www.batteryminders.com/categories/applications/aircraft.html

Aside from AC chargers for your Gill, (maybe a healthy recharge with one of these may be enough NOT to go solar), there are solar kits or just the aviation-specific charge controllers. Funny how they use the same body as the Coleman, but use VASTLY different programming inside.

While I'm not going to tout the desulphation routine, it does NOT use high-voltage pulses. Consider it a benign added feature if controversial desulfation makes you barf. I know I did and avoided all products that hyped this. However, I got one on a lark, and the REST of the charger is amazingly high quality and attention to very specific voltages, charge stages, temperature compensation (both ambient and remote). I use two of them on my Optimas, Odysseys and Deka agm's, but they are conventional and not aviation specific. If I had a Gill, I'd go for an aviation-specific model.

Had to toss that out there as something to consider, not to sell - as enthused as I am about them.

[rug_burn]

Thanks for the info. The thing is that sitting for a week or two, then flying for only an hour or two, the battery just slowly discharges, and only gets a surface charge from the hour flight. a year or two of this, it dies entirely, and needs to be replaced. The guys at Gill, who I called once or twice, recommended a deep charge every so often if the airplane sits a lot, like most airplanes.

The nice thing about solar, even with the cheapie controller is that it will give it a slow, deep charge and keep it topped off all the time. My experience has been good so far , because when I connected it the first time, the battery was down to about 11.3 volts or so, it wouldn't start the engine. After 1 week, it bumped it up to about 12.5 volts, and after a few more days it was at about 13.8, when I checked early in the morning, after a night of sitting. Probably hit 14.2 or something close the day before , that's a little high, I see from reading the Gill battery FAQ page... I'll have to watch that. One time shouldn't hurt it, just probably boiled off a little water out of the acid, which I'll also check.

Maybe in the end, I need to go with the do-it-yourself microcontroller charger where I can set the cut-out voltage at about 13.4 volts or so instead of 14.2, like the cheapie Coleman does. Do any of you guys ever read Nuts and Volts magazine? There have been several articles about solar charge controllers, one with a controller which charges several different batteries in turn. If you really wanted to get into it, you could even do a PWM charge routine.

As for "aviation grade" I don't always go along with all that high priced stuff. The technology on that plane is strictly 1950's. It's got a big torquey engine, putting out about .5 hp per cubic inch, leaks oil at about .4 qt per hour, and is reliable as heck. It uses 87 octane fuel. I do all the work on it myself, after a few unimpressive displays of mechanical ability on the part of paid mechanics. Runs like a champ, and about the best you can do on those engines (Continental E-225-8) is a leak rate of about a quart every three hours, almost where I am now.
Yeah, the wires could be thicker, but 1/2 amp ain't much current, and a slow charge is okay in my case.

[Sunking]

Rug Burn one thing that sticks out seems to be your understanding of what the best charge rates are for a battery, and how to charge a True Deep Cycle Battery. I am a pilot so I know a little about planes. The starter battery is not a Deep Cycle battery. It is a Starting battery which means it can provide very high currents for short period of time. Deep Cycle batteries cannot deliver the very high currents needed to crank an engine.

Moving on for your application all you need is a Float Charger to act as a Trickle Charger to keep the battery topped off while sitting. The aircraft magneto/alternator will do the bulk charging after the engine is started. What is important though is you get a controller that has a Float Function. The Coleman unit is incapable of doing that.

[Mike90250]

I think you are going to have to get the mfg's spec for the FLOAT voltage of the battery (Float service, not Cyclic service)

When you get that voltage spec, then I would get a programmable Morningstar controller, and program that voltage into it, and you will be all set.

But lets see what the Float service voltage needed, is.

[rug_burn]

I think you are going to have to get the mfg's spec for the FLOAT voltage of the battery (Float service, not Cyclic service)

When you get that voltage spec, then I would get a programmable Morningstar controller, and program that voltage into it, and you will be all set.

But lets see what the Float service voltage needed, is.

Yeah, you're right. I didn't know until I joined all you great guys (yeah, I'm gettin' a little choked up too, right now.... sniffle..) that aviation batteries use stronger acid(1.285 S.G vs 1.260S.G.) and thus the voltages are a little different. Back to the Gill Battery FAQ page...

The next thing is the Morning Star unit. I have two questions:
1. How much are they?
2. Can they be adapted for my battery switching unit?

I am just about to the point of simply doing the whole thing with a microprocessor anyway. The first cut would simply be a Ping-Pong style unit to keep the batteries all topped off, on at 13 and off at 14 volts for a regular battery, 12.8 and 13.4 or so for the airplane battery. Something like that.
I know it isn't the theoretical ideal, but it's way better than simply letting the batteries run dead, then charging them off my generator just enough to get the plane going, or jump starting it then hoping it got enough for next time.
I could then refine the design to include a PWM charging feature.
Another good thing about this would be the fact that I could use different charging levels for different battery chemistries, aviation vs. automotive, or even NiCad. I could even add a Lipo charge controller IC, which go for a buck or two at the IC level.

But I know, talk is cheap.. gotta get to work.