12 volt sealed lead acid batteries are very useful for many applications, but many charge controllers put a small drain on the battery. Once this drain has emptied the battery, it will be ruined if not immediately recharged. But with a charge regulator that does not put a drain on the battery, it can store for months and still hold a decent charge.
So I put together a simple shunt regulator to test out the concept. So far it has been working well. I can leave it in the garage for months at a time, and grab it for camping trips, etc.
You will need a heat sink big enough to dissipate the excess energy from the solar panel. The piece of sheet aluminum seems to be enough for the 5W solar panel I am using to charge a 12V 7AH SLA battery.
There are pros and cons to this design. Pros: simple, no vampire drain, inexpensive. Cons: it does not cut the battery off when it runs low, you need to watch that, you will need a big heat sink if you have a big solar panel.
I needed to build another one of these and decided to use a bipolar transistor. Even though the one I built using the above design has worked well for many years, my concern is that the Vgs turn on voltage may not be consistent from MOSFET to MOSFET. So I used a MJE3055T instead. The 3055 is a bit cheaper and the turn on is based on the .7 V turn on of a bipolar junction. It has worked well, and it ends up the wiring is exactly the same. Bipolar transistors require more current to drive, but if you use a 5W zener, you should be fine for most things the 3055 can handle.