Electronic – 480 VDC Series Battery Bank – Solar PV Charging Current Feedback Control Circuit

Battery University is a good starting point. A vast amount here Battery University

Particularly:

Charging Lead Acid

Based on the following, it would SOUND wise to charge fully for "a while" under trickle and then to open circuit the charger until Vbattery fell to approaching 2.1V/cell. A controller to implement this could be extremely simple.

Dangerous in isolation, but, they say:

• Most stationary batteries are kept on float charge. To reduce stress, the so-called hysteresis charge disconnects the float current when the battery is full. As the terminal voltage drops due to self-discharge, an occasional topping charge replenishes the lost energy. In essence, the battery is only “borrowed” from time to time for brief moments. This mode works well for installations that do not draw a load when on standby.

  Lead acid batteries must always be stored in a charged state. A topping charge should be applied every six months to prevent the voltage from dropping below 2.10V/cell. With AGM, these requirements can be somewhat relaxed.

  Measuring the open circuit voltage (OCV) while in storage provides a reliable indication as to the state-of-charge of the battery. A voltage of 2.10V at room temperature reveals a charge of about 90 percent. Such a battery is in good condition and needs only a brief full charge prior to use. If the voltage drops below 2.10V, the battery must be charged to prevent sulfation.

  Observe the storage temperature when measuring the open circuit voltage. A cool battery lowers the voltage slightly and a warm one increases it. Using OCV to estimate state-of-charge works best when the battery has rested for a few hours, because a charge or discharge agitates the battery and distorts the voltage.

Most of the time the panel current in the datasheet is measured as a short-circuit. I would suggest connecting the panel directly to the battery and measuring the current to at least get a baseline.

Next, measure the voltage at the panel terminals while charging. You might be surprised as it might be below the minimum voltage the regulator requires. You also have to consider the diode drop.

Maximum current reachable is not easily calculated by a formula, you have to measure it using a load or getting the manufacturer graphs.

Last but not least, without a schematic it's really hard to figure if everything is wired correctly.

Also, take a look at http://learn.adafruit.com/usb-dc-and-solar-lipoly-charger/design-notes it describes various solar panel limitations and charging approaches.