Warning: this solar panel 12 V battery charger (designed for wet open or closed lead acid batteries) does not switch off automatically when the 12 Volt battery (40-80 Ah) is completely charged. So in fact you need to “use” the energy stored in the battery every day. On the other hand: the charger keeps the battery properly charged by the solar panel and due to the maximal voltage set by the zener the charge current falls back to a very low safe value (10-50 mA) when the battery is fully charged. The circuit is in fact a “heavy” (high current) constant voltage source, able to handle 7 or 8 Ampere charge current. The circuit is not high tech but sturdy and made for harsh conditions. The 2N3055 transistors have to have the same Beta (Hfe), so select them on their beta with a transistor tester. When they are not identical in their Hfe/Beta, one of them gets the full current load (the other with a lower Hfe/Beta a limited load) and can burn out, leaving all the regulation to the other one. This is, by the way, a worst case scenario, so it will not happen frequently. With the “normal” diodes, in series with the 13 Volt (0,5 Watt) zener diode, the output voltage can be “trimmed” to the charge voltage that the wet lead acid battery needs to be charged properly, often approx. 13,5-14,5 Volt (read the datasheet from your battery). Add or remove more normal silicon diodes in the row to get to the right charge voltage, each diode adds or subtracts 0,7-0,8 Volt to/from the output voltage. I tested the circuit in bright sunlight with a 100 Watt solar panel and found that 7 normal silicondiodes (1N4007) + a 13 Volt 500 mW zener were necessary to reach the 14,4 Volt that I needed for my closed 12 V lead acid battery. More electronics in my books on the Lulu website, author Ko Tilman.
Post time: Feb-08-2017