welcome,
First off, have very low expectations, and plan on doing a lot of work, and spending a lot of money (the work to money ratio is somewhat up to you).
The most important thing to think of when designing a cell is voltage. H2O will split at 1.23V but you will need a little more to compensate for resistence in other part of the cell (electrodes, wires ext) most people aim for 1.8-2V.
If your using the alternator which puts out around 14V you'll need to have multiple cells in series so that the voltage across each cell is with in reason (ex; 7 identical cells connected running at 14V will have a voltage drop of 2V across each) Most people use neutral plates, that are not connected to + or -, in between there pos and neg plates |+| |N| |N| |-| each gap works as a cell so that the voltage in this case is divided by 3.
With those PV cells you linked to, you could use four of them the get a 2V charge, and that would be a very different type of cell, as you don't need neutral plates.
From my past investigations PV cells don't put out enough juice to be bothered with. but let's look at those ones
lets do a little math; .5V x 3.5A=1.75watts per PV cell
x4 cells= 7 watts
A very efficient cell can make 7 milliliters per min per wat (MMW)
so that would give you 49 ml per min
Most people recommend around 1 lpm (litter per min) per Litter of displacement in you engine. Some say less so lets do the math for .5
So lets say you have a 4.0 (just a guess, and an easy number)
you need 2lpm and you get 49 ml per 4 PV cells (let's round that to 50ml)
2000ml/50ml= 40 sets of 4
So you would need at lest 160 of those cell to give you 2lpm at noon on a clear day.
I think it might be a better plan to charge a supplemental battery with cells, so you're saving up electricity when your parked and can use it at night
Look at "dry cells", keep thinking, and please don't buy some crappy kit
I=V/R so R=V/I and V=I*R
P=V*I
(I=Amps, V=volts, P= power in watts, R=resistance in ohms)