A couple of days ago, I replaced the failing PC925 with a new one. But I had bought a Harbor Freight, 1.5W solar panel with plans to see if it can keep the new one fresh for more than the four years of the last one. The first challenge was to make a battery controller. The direct noon voltage runs as high as 25V, way beyond the recommended maximum for the Odyssey batteries. So I rigged up a simple circuit: 14V Zener - wired on the solar cell side, it keeps the voltage from going too high. Measurements at noon indicated this particular Zener held it to 14.51-14.59 volts still safe. Schottky blocking diode - wired in series from the solar cell to the battery, this prevents a reverse current flow at night into the solar array. It also has a 0.6V forward drop to limit the charging voltage to 14.59-0.6 ~= 14.01V . . . again well within the 12V battery voltage range. So at lunch, I ran some jumpers directly from the battery out the trunk and with the lid closed: 12.85V - this is perfect and right at the Odyssey 100% SOC range. This PC925 is in excellent shape. 12.85 -> 12.91V - with the solar panel pointed face-on to the noon sun, we were adding 0.06V meaning it was handling the 'parasitic' load and providing some charge to the battery. Turning the array 180 degrees and the voltage dropped to 12.85V. 70-100ma - laying on the trunk where I plan to mount the array, 70ma, which handles the reported 30-38ma overhead load as well as charging current to replenish the night discharge. Pointed directly at the sun, the current increased to 100 ma. I am going forward with my permanent installation on the trunk with the wiring going to the 12V battery. I am fully confident there is no risk of overcharge and this unit seems to be well matched to the parasitic load of the car with enough power to handle the night time, discharge. Bob Wilson
You might look into a solar charge controller. There are some under$10 ones on the market. They will supply the correct voltages as the battery charges. They should also maintain it at a trickle charge level...be sure to check the specs before you buy.
Back in the mid-1990s when I was a US Army tanker in an M1A1-equipped Armor Battalion, there was an Army optional maintenance part that was an (about) 12 x 12 inch Lexan-encased solar trickle charger for the M1A1 battery bank. Our battalion chose not to expend funds for this item . The reason for this item: the M1A1 has a series of 24V batteries under armor on the left front hull sponson, and when the tanks sit (which is most of the time) in overwatch positions while their electronics are running, these batteries slowly drain down. The optional Lexan unit was supposed to be bolted flat (horizontally) on the left front hull just forward of the turret. The Armor fleet eventually went with APUs in the turret bustle racks, but I always wondered why not make the Lexan solar panels a mandatory add-on item... after all, sunlight is free and maintenance-free, and the battalion's yearly battery replacement bill was reliably high.
