A solar panel is not a constant voltage, or constant current source. It can be thought of as a constant power source with maximum rated voltage and maximum rated current. The power is relative to the light hitting the panel, the voltage is maximum with no current, and drops as current is drawn from the panel.
If you are using a 10W panel, and it's in its full rated sun exposure, you'll get 10W out.
If you draw 1A in that situation, the voltage will be about 10V. If you draw two amps, the voltage will be about 5V.
If your battery is full, you probably aren't going to draw much current, so the voltage is higher.
If the battery is nearly empty, it will draw a lot of current, and it will cause the panel's voltage to drop.
In your specific case, what you're finding is that the panel can't provide full charging current all the time - whether that's due to less than full sun exposure, or a low-charge battery depends on the situation.
However, you can still use this system, even though the voltage is low. If you disconnect the battery and measure its voltage, then connect it to the charging system and measure the voltage at the battery, you'll find that the attached voltage is higher - the battery is accepting current from the system, and is charging. It isn't charging as fast as it could be, but that's due to the panel's limitations.
If you want to learn more about this, and what professional solar charging systems do in order to handle this effect, do a search for MPPT circuits - maximum power point tracking. The solar panel is most efficient at a certain voltage and current for a given sunlight input, and these circuits attempt to track that maximum point so you get as much power from the panel as possible.
Also, note that SLA batteries are very forgiving. It may be that you can eliminate the voltage regulator, and just use the diode in the circuit. This will increase the voltage at the battery since the regulator drops 1.5V-3V depending on load, and thus charging efficiency. Given that you're having a hard time keeping it charged, I'd expect the solar panel is unlikely to damage the battery, but check the panel's maximum current at 7.2V and see if the battery can accept a constant trickle charge of that rate.
You will probably need an intelligent charger (micro controller based with a custom program to handle inputs from V&A meters, solar charge controller, & a power regulator).
Common methods of determining the SOC of a 12V(nominal) battery aren't very accurate. Most often people use a generic table that shows V & SOC %. The problem with that data set is that it applies to a new/good battery and as a battery ages & deteriorates the internal battery resistance increases, charge capacity decreases, & charge rate decreases (i.e. the charge rate of an aged battery is slower at a specific voltage than a new battery)--so you can't rely on it as a reliable method to charge a battery with solar energy while the engine is off & power is being drawn from the battery.
If you use a micro controller based smart charger with a good program, you should be able to do the following to determine the maximum safe voltage to apply to the battery while the engine is off:
1. measure the battery V
2. measure the A draw of the system
3. To maintain the battery's current SOC, your power system must produce as much power as the system is consuming. Your program would calculate the optimal charge A & V to provide sufficient charge current to offset the power consumption rate.
4. To add a net charge increase in the battery, your computer program will have to provide a higher charge rate than the charge consumption rate (you can decide that rate or make it a user configurable parameter).
Note: The regulated charging system of a running vehicle provides sufficient power to charge the battery & maintain other electrical loads. Your smart charger will have to emulate the same power availability while the engine is off (which might mean you'll need quite a bit of PV surface area (depending on loads).
Best Answer
Because the solar panel puts so little power compared to the size of the battery, and lead-acid batteries are rather forgiving, all you need is a Schottky diode.
Connect the diode in series with the solar panel, cathode to battery +, anode to panel + output. Tie together the panel - and the battery -. That's all you need. The float charge level of the battery will be around 6.8 V, but the solar panel is too wussy to deliver enough current at that voltage to cause any harm. If you're really worried about overcharging, add a shunt regulator across the battery set to 7 V or so.