Solar panels and controller for maintaining 12v battery

[Jamin_GX]

Howdy -

So, first let me apologize because I'm completely dense when it comes to this sort of thing.

I have a Brunton Solaris 12 panel I use for charging my electronics (GPS, iPod, etc).
Link:
http://store.bruntonoutdoor.com/port...olaris-reg-12/

Now I'm beginning to wonder if I could use this same solar panel for maintaining my vehicle's battery with a controller such as this:
http://store.bruntonoutdoor.com/port...es/controller/

I see that it says it works for everything over 10 watts (my panel is 12w) but it then continues to state that it "cuts in" at 13v. Well since my panel is only a 12 volt panel is this even feasible?

Simple question -- can I purchase this Brunton controller to use with my current solar panel to maintain my vehicle's battery when I'm out in the woods?


Thanks in advance.

[dwh]

Generally, when talking about solar and battery charging, "12v" is not an exact number. It's a range. A "12v nominal" solar panel generally puts out 15v or more. A "12v nominal" battery can range from 10.5v (dead) up to 13.5v (fully charged).

But while 13.5v is considered a full charge voltage - that's not really the whole story. "3 stage" solar controllers and smart chargers will push the battery until it gets to 14.4v (bulk stage), then drop back and hold it at 14.2v for a few hours to fully activate the chemistry in the electrolyte (absorb stage), then drop to 13.5v and say, "okay, NOW it's fully charged", and will then maintain the battery at 13.5v (float stage).

That controller you linked to, is the most simple sort of solar charge controller - it simply connects the solar panel to the battery whenever the battery gets down to 13v, and disconnects when the battery reaches 14.2v. It will get the battery "mostly" charged, but since it doesn't hold the battery at an elevated voltage long enough for the battery to fully absorb, the battery will mostly only reach around 90% charged (if that), rather than 100%.


Lead-acid batteries have what is called the "self-discharge rate". This is how much power the battery will lose if it's just sitting around. The rate is different for different batteries, and also an older battery will self-discharge at a higher rate than a newer battery.

In general, 10w or so of solar would be considered a "maintainer" - i.e., it's too small to do anything other than put out roughly what is needed to compensate for the self-discharge of a battery that is just sitting.

To know if 12w of solar would be enough to maintain the battery, you would need to know how much the battery self-discharges, plus how much parasitic load is on the battery, such as a car alarm, the clock in the radio and whatnot. If that totals up to more than 12w, then the solar panel would be fighting a losing battle. Since the solar panel wouldn't provide enough to ever get the battery up to 14.2v, the charge controller would just basically be leaving the panel connected to the battery all the time.

In that case, there would be no need for the controller. 5w and 10w solar battery "maintainers" usually DON'T use a controller since they are too small to ever overcharge a big ol' lead-acid battery. The main purpose of a charge controller, is to prevent overcharging. Connecting a 100w panel to a battery could easily overcharge it if there was no controller to disconnect the solar when the battery was full, but a 1a (12w/12v=1a) charge isn't enough to hurt anything even if it was left connected all day.

(That is, for a battery in a vehicle which is actually being used, and the occasional use of the solar panel. For even a 12w panel hooked up constantly to a battery (such as are used in a lot of electric gate systems), I WOULD want a charge controller, since in that situation, it would be "theoretically" possible to overcharge the battery even with a dinky little 1a charger.)


The next question is: Will that solar panel you linked to put out a high enough voltage to get the job done?

The solar panel has to put out a higher voltage than whatever it is charging (plus a little extra to overcome a sort of "electrical inertia"), that is why "12v nominal" solar panels that are intended to charge a "12v nominal" battery are setup to put out 15v or more.

The specs on the page you linked to seem a bit vague. Solar panels have all sorts of rating numbers, and the only thing that page says is "Max output: 12 watts (12volts / 800mA)". If that panel actually does put out 12v...well that's not enough to push a battery up to 14.2v, or even 13v.

Now, the first thing I notice is that the math ain't right. Watts / volts = amps.
12w / 12v = 1a
Not .8a.

However, if we do it like this:
12w / 15v = .8a

Okay, that works out. So my guess is that the panel is actually a "12v nominal", which actually puts out 15v. If so, then it could be used to charge a "12v nominal" battery.

The number we really need, is the "Vmp" (voltage at max power). That is the voltage that the solar panel is capable of under full load. A few minutes of Googling doesn't turn up the user manual, but I do find this on the Brunton support page:

http://www.bruntonoutdoor.com/suppor...Portable+Power (last question)

"Q. When I check the voltage of my panel with a volt meter I am getting 19 to 22 volts. I thought it was a 12 volt panel?
A. This is normal. You will find on an open circuit, when there is no load or device hooked to the panel, your voltage will read that high. Once a device is connected or the panel is loaded, the circuit is closed and your panels [sic] voltage will register at the expected 12 to 15 volt range."


So...if the Voc (voltage open circuit - i.e., no load) is 19v-22v, then the Vmp is likely to be in the 15v range.

Thus, that is a "12v nominal" solar panel, not a "12v" solar panel.
Which WOULD work to maintain a "12v nominal" battery.

But I would say you probably don't need the charge controller for a such a dinky little solar panel connected to a big ol' lead-acid auto battery.


In any case - even if it does put out 15v Vmp (I would want to confirm that for sure before I hooked it up), don't expect it to do much more than compensate for self-discharge + small parasitic load. It's just too small to do much more than that.

1 amp just ain't much in the "12v nominal" auto/marine battery world.

[Jamin_GX]

dwh -

Wow, amazing explanation and I thank you greatly for taking the time to go above and beyond to share all that. I'm sure I have the manual at home so I will definitely check there for the Vmp is really 15v or not. Otherwise I'll start calling Brunton to see if they can tell me.

At this time I'm just looking to find a way to keep the battery maintained enough so that I know it will start the vehicle. Load would be light (at least in my mind) as I'd only like turn the radio on for a bit each day for either some tunes while cooking or to try and get a station for a weather forecast. Also, I have more cig outlets in my vehicle for charging than the one that hooks to the solar panel so I was thinking that if I could get the solar panel to maintain the vehicle battery enough for it to start then I could use the multiple cig outlets to charge my GPS, iPod, etc all at once. However, I realize this may be asking too much from the battery.

Somewhere down the road I will likely invest in the larger 26w or maybe even 60w panels but I would rather use the little $ I have right now for gas and exploring first.

[dwh]

Not asking too much from the battery, but asking too much from the solar panel.

Figure that 12w panel can replace 1a per every hour of good strong sun. If you move it around to keep it aimed at the sun, you might get say 6 hours a day. So it's replacing up to 6 amp*hours a day.

It doesn't take much to drain 6ah out of the battery. That's a 2a load for 3 hours or a 4a load for 1.5 hours.

With a small solar panel, what usually ends up happening is not that the solar will keep up with the loads, but that it just helps mitigate the loads a bit to extend the time until the battery is drained.

For instance, if you have 4a of loads pulling from the battery, while you are supplying 1a from the solar panel, then the effective drain on the battery is only 3a instead of 4a so it'll take a little longer before the battery is drained.

The real concern is the battery type. "Cranking" batteries have thin lead plates designed to dump a lot of amperage in a hurry. But that large short amp dump is usually only 5% of the battery's capacity. Take it down 5% and then recharge it immediately, and it will do that for thousands of cycles.

That same battery, taken down to 50% capacity, might only last a couple of hundred cycles (if that) before it's ruined. Take it down to 20% and it might only do it 10 or 20 times (if that) before it's ruined.

Deep cycle batteries, have thick lead plates designed not to dump a whole lot in a short time, but to dump less over a longer time (though they can still dump a respectable amount of power in a short time). That sort of battery can be taken down to 50% (or some, to 20%) for thousands of cycles.


I personally, am never in favor of depending on the cranking battery to supply aux loads and then (hopefully) have it still start the vehicle. It might - but there is an equal chance it won't, and that's bad news out in the boonies.

Having an aux battery for aux loads is just about the only way to be sure the cranking battery will be up to the task when needed.

[dzzz]

"Maintaining" a battery normally means just protecting against self discharge. A 5-10W panel is enough for that. There are little charge controller kits that attach through the cigarette lighter. The Bruton controller you linked would work, but require more time setting up.

If you drive most days and don't have a fridge you might not have a problem. When you replace your starting battery you probably want to get something better like the Sears Platinum many people here buy . That will tolerate more discharge than a regular starting battery.

[LandCruiserPhil]

I like the Burton products and use a Solaris 25 tri-fold for charging my electronic. But for keeping my batteries charged I use a BatteryMinder SCC015 for the past year plus with excellent results.

[Jamin_GX]

Gentlemen -

I appreciate all your insight. What I'm realizing is that I may be ready for a dual battery setup sooner than I thought. With this info I'm thinking I'll refrain from charging toys and equipment via the vehicle and instead continue to use my solar panel until I can put together something a little more proper. Whether that be a larger solar panel, controller and new deep cycle battery ... or a dual battery setup we'll have to wait and see what the funds allow.