The solar cells will provide some current when illuminated. This can be applied to charge the battery. It doesn't matter if something else is connected to the battery at the same time that is drawing current, you still get to use the power from the solar cells effectively. In fact, it's more efficient to power the end load directly than to first store the energy in a battery and later have the battery power the load. The point of the battery is to be able to use the power when it's needed, not just when the solar cells can provide it.
For example, let's say the solar cells can produce 1A in full sunlight, and this 1A can be applied to charging the battery. Now let's say a load is turned on that draws 400mA. The 1A from the solar cell will be split so that 400mA drives the load, leaving 600mA left to charge the battery. This is perfectly fine. If the load were more than the solar cell current, this would still work. Let's say the load takes 1.5A. 1A of that would come straight from the solar cell, and 500mA from the battery.
In other words, there is nothing wrong with the solar cell current relieving some of the load on the battery as apposed to charging the battery and then having the battery supply the load current. As I said before, that is actually more efficient since there is some loss in storing and the retreiving energy from the battery.
Knowing when the battery is full and what to do about it is a separate topic. Some battery types are quite fussy about how they are charged. For those you will need a active circuit to monitor the battery voltage and possibly temperature to decide what current to charge it with or what voltage to hold it at.
Lead-acid batteries are pretty forgiving in this area and don't have a problem with continuous charging even when full. The simplest solution is to arrange and size the solar cells such that they can only produce the maximum float voltage at the maximum current the battery can take when full. Then you can just connect the solar cells accross the battery and be done with it. This will charge the battery much more slowly than it is capable of when it is low, but is a simple setup guaranteed not to damage it.
If the solar panel size or cost is important, you need to use the energy from it more efficiently. In that case I'd use a switching power supply driven by a microcontroller that takes into account what current the solar cells can put out, the battery voltage, and possibly the battery temperature to decide what current, if any, to charge it with. This gets complicated. You have to read the battery datasheet carefully and implement the charging regime accordingly.
Best Answer
Light from the sun contains a range of wavelengths ranging from UV to IR.
Typical PV (photovoltaic) materials in common use work best at visible light wavelengths and "waste" energy at either extreme. Less commonly used and usually more expensive materials are available that can be optimised to work best at either UV or IR wavelengths. By combining cells that work most efficiently at various wavelengths it is possible to make cells that produce more energy per area from typical sunlight.
Light reaching the earth's surface is reduced in amplitude overall by passing through the earth's atmosphere. Far UC / UV-C / short wavelength UV is essentially completely absorbed, UV-B A (longer wavelength) is very significantly absorbed and UV-A (longer again wavelength) is somewhat absorbed.
Triple junction cells are optimised to work across the full solar spectrum. The abikity to deal with UV-C is wasted for terrestial PV panels and triple junction cells are usually targeted at space applications. Accordingly, no cost is spared in their manufacture and optimisation and they are both not vastly more useful than double junction cells with a somewhat enhanced UV and IR coverage, and much more expensive.
Air effects on solar insolation are standardised for test purposes - the Air Mass 1.5 = AM1.5 conditions usually being quoted. This is the light that results when solar insolation passes through the atmosphere such thata 1.5 x as much mass of air is passed through as is the case at true noon when the sun falls vertically. ie at about 45 degrees. (See figure at end of this post)
Wikipedia Am1.5 et al
If mounting area is not limited and weight is not a crucial factor then using cells which optimise energy output long term per total "net present value" $ spent is usually best. If space is constrained (yacht, backpack, limited roof area etc) or if windage matters (hurricanes, roofrack, ...) or weight (roof loading, back pack) or all of these are utterly crucial (across Australia solar challenge ...) then highest possible efficiency per area may matter. But in most cases just buying the reasonably most efficient commodity PV cells is good enough.
As well as efficiency uou may care about lifetime. Crystalline silicon PV laminated in glass usually comes with a 20+ year still running at > 80% original output guarantee or promise, and actual lifetimes of over 30 years are achieved by competently built units. Early CIGS and CdTe cells had limited lifetimes (as little as a few years) and while vastly longer lifetimes are now being claimed by some manufacturers you may wish to do due diligence on the probability of their claims being true in any given case.
Monocrystalline silicon laminated on low iron glass can provide whole panel efficiencies of over 15% with cell efficiencies of over 17% commonly advertised. While some people may advertise 20%+ efficiencies for some technologies in some cases you are generally into expert care and buyer beware situations.
Looking on ebay for monocrystalline silicon and working out offered efficiencies (17%+) and cost should give you as good a deal as most available.
Example only:
A quick look on ebay shows this offering as typical of an apparently OK deal. They say:
US made 125x125 Monocrystalline Solar Cells 2.8 Watt Mono 5x5 Photovoltaic wafer
Monocrystalline Photovoltaic Solar Cells Made in USA, by BP Solar, Frederick, Maryland
All cells, completely new, mostly in factory packs of 50 or 25 cells per pack. The price listed is for one 5"x5" solar cell not the whole pack. You can purchase any qty that you need. (10, 50, 100, 1000, we got you covered)
125mm x 125mm
2.8 Watts
0.5V
Efficiency upto 17.75 %
Wikipedia multijunction PV
Triple junction amorphous on stainless steel. BUT still only 13% total from here
40% in 2010 !!! from here - buying them may be another matter.
Atmospheric effects on optical transmission can be modelled as if the atmosphere is concentrated in approximately the lower 9 km.(From above Wikipedia ref)