The simplest way:
Get a voltmeter and a solar panel. Connect the panel to your battery and watch the voltage rise. When it gets near 14 volts your battery is charged. Disconnect your solar panel to keep it from overcharging your battery.
Your battery can be damaged if you charge it too fast, so don't get a panel that's too big for your battery. Consult the battery manufacturer's data to see how many amps it can handle, both while charging and discharging. If you happen to have a big panel and a little battery, there are tricks further along in this instructable.
Some solar panels are made just for the purpose of maintaining batteries in vehicles that are parked a long time. They don't go over 13.5 or 14 volts and don't ever produce enough current to damage the battery. These you can just hook to your battery and forget.
If you want to use more power you'll want to get a bigger, higher voltage panel than that.
Here I am running my truck's electrical system off a solar panel wedged in the windshield.
My alternater fell off the engine and the belt broke. I'm driving south so I just set up the panel and drove into the sun, peeking over the panel. Read more of that Haywire Mechanic Story or click on "step two" to read more battery/solar tricks.
Step 1: Voltmeter Human Supervision
If the voltage suddenly jumps to 17volts I suspect the jumper cables that carry the solar panel current to the battery have gotten disconnected.
The panel output voltage rises to 17 volts or so when it's not connected to a load. When current flows the voltage drops due to internal resistance in the panel.
If your battery voltage gets to 14 volts or if you hear a bubbling sound when you put your ear to your battery it means you're overcharging your battery. Disconnect your panel. You'll get to know your panel, battery, and loads, and maybe you'll disconnect at a lower voltage, like 13.5 volts.
This method works fine when camping and you can lay around watching to see how your battery is doing, or if your panels can't keep up with demand so you don't have to worry about overcharging.
Step 2: Overvoltage Disconnect
All it does is disconnect the panel from the battery when battery voltage goes over 14 volts.
Home Power Magazine Has published schematics for circuits that do the same thing.
You can still overcharge a battery through one of these units if your panel is big and your battery is small. The bubbling sound you hear in your overcharging battery is water (H2O) being broken down into hydrogen and Oxygen, which then bubble out to join the atmosphere. Eventually the battery will run out of water. Eventually the exposed parts of the battery's lead plates will oxidize and the battery won't take a charge, even if you add distilled water to it.
Step 3: Low Voltage Disconnect and Inverter
It takes in 12 volts DC from the battery and produces 110 volts AC that I can plug my sewing machine, laptop, etc into. You can get them for cheap at any truckstop or discount store now.
This model has a low-voltage disconnect feature which is a big help.
When battery voltage drops below 11 volts it beeps and turns off.
That protects your battery from over-discharging.
Sulfating and De-Sulfating
If you run your battery down further than 10.5 or 11 volts, or if you fail to charge it up again soon enough, sulfate crystals will form on the battery plates.
The sides of your battery will bulge from the crystal deposits on the plates, and your battery won't take a charge anymore. There are tricks to fix this. Look up "desulphate", "desulfate"(both spellings are ok), "pulse charger", "battery edta". According to one vendor of desulfating products, 75% of the lead-acid batteries removed from service can be rejuvenated with these techniques. The others are suffering from shorts, open circuits, and similarly less correctable flaws.
To measure your battery's capacity
Plug in an mechanical electric clock and a known load, like a 5 watt lightbulb. See how long it takes to run down the battery til it shuts off. For example: 5watts/12 volts = 0.42 amps. If it takes three hours to discharge your battery after it's fully charged, your capacity is 3 * 0.42 = 1.26 amp-hours. (not very much) A typical emergency-lighting gel-cell has 7 amp-hours capacity.
Step 4: Christmas Lights As Current Regulators
One way to do that is by putting an incandescent light bulb in series with your battery as shown here. Here I'm using christmas tree lights. Play with an ammeter and try putting multiple bulbs in series or parallel til you get the current limiting you want. Signal lights from cars can also work.
The clothes iron seen here is nothing but a big resistor, that can work also to reduce current to your battery. The small DC motor seen here is useful for testing scrap batteries. You can immediately feel how much current a battery can deliver.
Step 5: FREEDOM!
My batteries are a couple of 12 volt gel-cells scrapped by a hotel from their emergency lighting system. That's plenty to run my laptop, fluorescent work-light, or sewing machine.
Thanks to these simple tools I was able to do my work in a swamp.
In fact, here in a swamp I found a much better work environment than I'd had at our most prestigious technical university.
For some reason the buildings there were grossly overheated in the winter while the windows stood open and gigantic airconditioners ran at the same time, making a roaring sound. Sound crazy? it is.
Anyway, if you can charge a battery with a solar panel you can run away from the craziness to a paradise swamp like this and be happy.
The second photo is my boat set up with a bug net and chaise-longe folding chair for sleeping. The solar panel is across the bow of the canoe to charge the batteries during the day.