Solar charging two 12 volt batteries (parallel)...

[SKIN]

I have two boat batteries in parallel at 12 volts. I want to keep them charged using solar panel/s. Can some one point me in the direction I need to go? Where, what to buy?

[Ryland]

You did not give enough info.
We need to know the battery size and how much you are taking out of it every day, if you are not using it every day then how often are you and how long can you have it charge for, if you are charging it some other way and only need to do a float charge while they are in storage for 6 months or more then a solar panel that puts out 3-5% of the capacity of the battery should be fine as on average you should get around 4 sun hours per day, so a pair of batteries that gives you 300 amp hours at 12 volts should be ok with a 10-20 watt panel, so $50-$200 for something that will work, any larger and you would need a charge controller as well, to small and it's hardly going to keep up with the self discharge of the batteries.
You question is kind of like saying "I have a car, how much am I going to spend on gas for the next ten years?" when buying solar to charge a battery yes, you need to know the battery size and type, but mostly you need to know how much power you need to produce and that is based off of how much you used.

[SKIN]

Two boat deep cycle batteries. I don't know much more. I will have to check them for specifics. I know I need a charge controller. I know I need to do more then 15 watts. I use these batteries to start the engine. I also have another set of two batteries that I use for my stereo. I would like to be able to use the panels to charge them if I want to as well. The stereo batteries are pretty dead by the end of the day. But the engine batteries are still charged but not enough to start the engine. It needs to be 11.84 volts to start.

Is this enough info?

[MetroMPG]

Are you saying you go from all batteries "fully charged" to "unable to start your engine" in one day?

Sounds like the batteries are either too small (low capacity) or pretty worn out (low capacity) or the power draw of the stereo is pretty high. Can you estimate the power usage in watts of the stereo?

If you want your panel to be able to support the batteries at the same time you're using the stereo, you need to find a panel output at least equal to that total draw for the limited number of sun hours you have.

[SKIN]

No. Two sets of two batteries each are separate.

There are two scenarios here, lets separate them.

Usually I only go out on the boat on a Friday or Saturday. The engine requires 11.84 volts to start. Then an unknown draw to continue to run. By the end of the day, when I am ready to go back the engine will not have enough juice to start. The batteries have around 10 volts in them at that point. I need to have a solar panel to charge them back up to 11.84.

The stereo is a whole different matter. I run those batteries dead within a day. I want to be able to walk down to the boat and set them to charge using the solar panels on say Sunday (the whole day while I am not using the boat).

So the engine starting batteries need to be charged up to 11.84 the same day I am on the boat (while I am on the boat).

The stereo could be charged any other time (when I am not on the boat... Docked).

[Orweller]

A portable 12V Solar panel should be enough to keep a battery charged, or even to charge a battery given a number of days. However, the wattage you would need to charge a low battery to full (or close to it) in a day or less... Would be so large, that you would not be able to use a portable power system.
Most portable solar panels are under 20 watts.
What you are looking at here is a Watt Hour issue.
Hoe many watts are being used for how many hours.
For example. (A Watt = Amps * Voltage) I have a marine battery that has 85 Amp Hour Rating. The battery is 12V. So I have a capacity of 1020 Watt Hours of electricity available. (If you use 100 % of the charge, which you NEVER should!)
Lead Acid batteries, even "Deep cycle" batteries are damaged when discharged more than 50% of their capacity. You should try to never get them below 50% of their full charge.

If you test the battery's voltage when it is not being used, the below voltage indicates the remaining charge approximately. (Note that this varies slightly with the temperature, and the age of the battery... But we're talking 10ths of a volt here.. These numbers are for a good, but worn in battery.)

% Charge Voltage (Disconnected)
100 12.49
75 12.16
50 11.86
25 11.62
0 11.50

Note that if your voltage ever drops below 11.5 volts, your battery is worse than dead... It is permanently damaged. You should Google "Lead Acid Battery care" if you are interested in more info about how to take care of the lead acid battery.

So based on the voltages you gave, I'd say your problem is that your batteries are damaged. You should give some thought about what capacity battery (or an array of batteries) you need to provide the Watt Hours you are looking for for your power needs. You may want to install several batteries in parallel to get the capacity you need. (Be careful when doing this!)

You may have damaged your batteries by over draining them, especially if this has happened repeatedly.

How big of a solar panel do you need to charge your batteries? You need to charge what you use in Watt Hours. (Plus a minimal trickle drain.)
Here's the important part.
AH * V = WH

Let's assume you get a new 85 watt hour battery, and plan on using 50% of it's charge capacity so you don't kill it.

If you are charging 50% of an 85 AH battery that is 12V - you need 510 WH.
With a 20Watt Portable Solar panel this means you have to charge it for
25.5 Hours of Sunlight or about 3 summer days of reasonable sunlight strait.

If you want it done in 4 hours? You need (510 / 4) watts or about 127.5 watts. You can buy a 125 watt solar panel on amazon.com for about $700 right now. You'll want to get a charge controller with that for $30-50. But the size and weight of that panel would put it well outside of the "portable solutions" range.

I'm guessing the Watt Hours you would need for this would require a system that is bigger and more expensive than you would be interested in for this application.

Maybe you should put an alternator on your boat to keep the battery charged while you have the motor on.

For remote charging that kind of Watt Hour power... I think you'd want to get a generator and a battery charger. (Or a unit with both together.) Just check out the Amp Hour rating of the charger, and do some math to see how long it will take to charge.

Maybe the info above will help you make your decision on what to do.

I hope this helps.

Jon from Kalamazoo