In order for a zener diode (reversed) to work, the input voltage must be higher than the rated voltage of the diode. I use zener diodes to convert high volts low amps to low volts high amps. So if you hook up a 12v zener to a 17.2 volt 20 watt panel, the panel which puts out 17.2 volt and 1.16 amps (V*A=W) is going to convert down to 12v in the zener, and the joules of voltage convert to joules of amperage, so you then get 12v 1.6 amps. Zeners only work if the voltage going into them is higher than the diodes rated voltage. It should also be noted that the avalanche breakdown (the phenomenon utilized by zeners) cannot occur if your input voltage is lower than the zener's volt rating. If the voltage is lower than the zener's rating, the zener acts as a regular blocking diode (if placed in reverse, in series). You will want to include a regular schottky diode coming off the panel, as a zener placed in reverse will leak joules back into the panel overnight.
It is called a dc-dc step-up converter. It will convert your 6 volts off the panel to another voltage you specify. Just remember v*a=w and a=w/v. When you convert from one voltage to another, your amperage changes (It should also be noted, that most voltage converters are not electrically efficient, and will dissapate some of your watts in the form of heat, so you will loose amperage from that.
Also, when charging 12 volt deep cycle batteries, a 14.2 volt charge is applied. SO for a 6 volt battery, you would apply 7.1 volts. i.e., you need to find a way to increase the system voltage to at least 7.1 volts to charge your 6 volt battery.
Batteries are not entirely efficient when they charge. They actually dissapate roughly 1/3 of the amps going into them as heat. Therefore, you will need to divide your amps coming out of your panel by 1.3 and then use that value to find how many hours it takes to charge your batteries. I like to call this value T, even though I'm pretty sure that is not what it is called, i think T sounds nice. So your 0.5 amps is going to look more like 0.38 amps to the battery, and you are going to need to adjust charging times accordingly.
First, you need to know how many hours of rated output you are going to be getting from your panels. You can find this out by hooking up the panel directly to a multimeter in the early morning and wait till the volts and amps read similar to what your panel is rated (Take note of the time of day). Leave it hooked up to the meter, and watch it thoughout the day. There will be a point where your volts/amps will start to drop (Usually around 4 o clock, 5 o clock) Take note of the time of day. Then figure out how many hours of full sunlight you are getting that your panel is able to produce its rated output. (I usually estimate this as six hours)
Take the T value for your amps, and multiply it by your hours of full sunlight. This will give you Amp/hours OR Ah (E.G. 0.38a*6h=2.28Ah). This is the amount of energy available for you to charge the battery in a given day.
So your small panel (Ignoring the incorrect charging voltage) would take 2.1 days so charge your 5Ah batterry, given you were to provide it with .38 amps for 6 hours a day. That is totally fine, lead acid can be charged over a period of time. It is important however, that you _do not_ use a lead acid battery below 50% of its charge. If you run a lead acid out below this value, sulfation of the electrodes within the batteries will occur, and this ruins lead acid cells. So make sure when you get to using the thing, you don't run it down more than 50%. I wouldn't run it down more that 60% to stay on the safe side.
A cheap charge controller is also a good idea, however, I think they are mostly made for 12v pv systems. The sunforce 7A solar charge controller does a nice job, fteches a price for about $35 dollars on ebay, and I think is even a MPPT carger (Multiple point power tracking) It will charge your batteries until the cell (batt.) voltage increases beyond a certain value, and then shut off the charger to prevent over charging.
You can also run a load off the panel while charging the battery, given the panel puts off an excess wattage not being used by the charger, and your load does not exceed this excess wattage.
Hope this was a fun lesson in solar electric basics!
Until next time