I am want to make a "Solar" charger for my Boat Battery. It is a standard 12Vdc Car Battery and I don't care if it takes all week to charge the battery since the boat is only used on the weekends. Yes, I know they have them for $30, but that will take away all the fun!

I cobbled up all 8 of my "Last Years" Solar garden lights. I have tied all the Solar panels in series, and much to my surpise I have seen as much as 24Vdc in direct sunlight from the panels.

I figure I will need minimally a Diode inline to prevent reverse current through the panels. What else should I do, a voltage regulator, some kind of current limiting device or just hook it up? I am not sure what the total current output capabilities these panels have, or even how to figure that.

Here is a picture of what I am minially thinking.

Any thoughts, assistance, ideas are greatly appreciated!!!

 

I don't think you will need a regulator, but I would install the diode.
The power generated by solar garden lights solar panel will be miniscule.

I just took one outside to test in full sun. The panel is 7cm square.
I'm reading in excess of 3V. Direct across the ammeter gives in excess of 100mA.

My guess is just connect as many panels in series to give you a voltage greater that 12V. I would aim for 15 to 18 volts. I don't think you can damage your 12V car/marine battery. How long it will take to fully charge your battery will depend on what you use the battery to power. It may take months at 100mA to recharge your battery. Do some quick calculations.

To increase the power delivered you will have to wire a number of strings in parallel.

It's worth trying but I don't know how effective it will be.

On second thoughts, maybe what you are doing with all eight in series will be fine. I suppose all the energy generated still goes into the battery. This way the solar charger will still work with cloud cover.

Here are some figures for you to do some math. A 5W 12V panel spec is 15V @ 330mA, dimensions 12" x 12".

 

Well, those solar cells were made to charge two AA NiCd batteries in a day's time. Those usually have a rating somewhere in the area of 2300mAh, or 2.3AH. An auto battery might have a 50Ah capacity - so just on that premise, it would take 50AH/2.3AH = 21.7 days or over three weeks to charge the battery.

The really bad part is that while your battery charge is low, your battery will be disintegrating inside due to the chemical activity. Keeping the battery charged above 12.5v @ 25°C also keeps plate sulfation from forming. Heavily sulfated batteries will no longer accept or release a charge.

You really need to first get your battery charged back up after you've discharged it; then you could use the solar cells as battery maintainers. They just won't produce enough current (perhaps as much as 200mA on a sunny day) to do more than keep the battery maintained after it's charged.

Also, if it's really an automotive battery you have on the boat - it won't last long in deep cycle usage; the plates are far thinner than in a deep-cycle battery, and they will fall apart very quickly with such abuse. Auto batteries are designed to give brief, heavy bursts of current for starting, and then to be completely recharged immediately.

Deep-cycle batteries were designed to be discharged down to about 70% of capacity and then recharged. The deeper you discharge any lead-acid battery, the shorter the length of time it will be in service. Leaving a battery discharged for any length of time will also shorten its' service life. Charging batteries at higher currents than they are rated for is also bad for them; it causes heat in the battery, which increases the chemical activity, which shortens their life span.

 

I actually have both a Marine and a Car battery. After a "Whole" day on the water entertaining others and such, generally the batter voltage will be down to around 9Vdc -10Vdc. Upon which it won't be enough to make it a second day. So in the event that it is that far down, I generally will change to my "Spare". So my thought is to permanently mount the panels to the boat and just let it assist how every minimally it may.

Most of the time my 2A Trickel charger will be more than enough to charge the boat in less than one night. This made me hope of a time when I don't have to worry about plugging in the boat (unless it has an extremely good day).

So potentially eliminate a couple cells, to something like a total of 5, install the diode and hook up and leave like it is and don't worry about over charging, Correct?

 

After a "Whole" day on the water entertaining others and such, generally the batter voltage will be down to around 9Vdc -10Vdc.

9V-10V? You are killing your batteries via a quick and painful death. Many battery manufacturers suggest not discharging them below 10.5V, but even that is not a recipe for long life. At a minimum I would be switching batteries at 11.5V.

And honestly, about the solar panels, I'm not sure it will do much as a trickle charger even with 16 of the panels you are using (doubling the current).

 

I am curious to know what you are powering with the 12V marine battery?
Are you using the battery to start the motor or are you also running all your electronics, nav and lights? I don't know how come you are running down the battery in one day.

 

Mr Chips, I am both starting the motor as well as powering a stereo, running lights, interior lights, a Coleman Refridgerator, and sometimes charging Cell Phones or other things.

 

You need a generator or a really large solar array. Those weedy little solar cells are like trying to fill a swimming pool with an eye dropper.

 

I figure I will need minimally a Diode inline to prevent reverse current through the panels.

Yup, a diode to prevent reverse current is not optional.

What else should I do, a voltage regulator, some kind of current limiting device or just hook it up? I am not sure what the total current output capabilities these panels have, or even how to figure that.

You can estimate current from various back-of-the-napkin approaches but it'd be better to simply measure it with an inexpensive (<$5) multimeter. Chances are, the series current is well under the 100mA reported by MrChips (most solar light panels are not as large as his). That's enough current to top off or maintain a big battery but as others have told you, not enough to give any significant recharge. But the good news is that it's also not enough current to require circuitry to protect the battery from overcharge. It's just not likely you could overcharge at that low current.

FWIW, your series arrangement will be a reasonably efficient use of the panels. At peak charge, your battery is ~14.5V and with the diode you need over 15V from the panels to force any charging. PV panels are most efficient when they run at 70-90% of their open circuit voltage. You're below that percentage, but not too bad. Putting two series strings of 4 in parallel is about your only other configuration option. But that low voltage would give you very little charging except at full sun with a low battery. On the upside it might give you more current under those conditions, and more current would be needed to have any significant impact. I suppose that might be useful in some circumstances. But it would be doing nothing most of the time if your battery is fresh most of the time, as it should be.

 

Spend a little money now and save a lot as time goes on. Not letting your batteries discharge to 9.5V - 10.5V will make that investment last longer.Charging them properly will do the same. repeated charging of your battery at such low currents will also reduce the longevity of the battery. So for the price of a 12V car battery you could buy a 15W solar panel that could provide a more friendly charging system. It still does not reach the recommended charging current but will be up to 10X the current you are providing not and the included charge controller will offer the correct charging voltage and float voltage. Ideally two of these would be optimal.

15-Watt Solar Charging Kit

 

Photovoltaic solar panels are sensitive and can turn out an electrical current when exposed to nearly any daylight. Because of the solar energy support gives the valuable resource to the products.The intensity of the present created by the battery charger is maximized by optimizing the lean angle of the panel in relation to the sun. within the northern hemisphere panels ought to face true south whereas within the southern hemisphere panels ought to face true north. the subsequent chart indicates the angle on top of the horizon the panel ought to purpose for optimum potency.