I have solar cell with peak volt=9V and I=300mA. I want to charge 6V acid lead battery 4.5Ah. I'm thinking to put L7809 voltage regulator to prevent spike in voltage of solar cell and at night time there will low voltage at solar panel.
1.Does voltage regulator prevent voltage back flow from battery to solar panel?
2.Do i use voltage regulator or not?(this will reduce the life of battery if not use? right)
You must advise battery type and specification.
eg lead acid or NiCd or MimH or LiIon or ...?
How many mAh capacity?
I have solar cell with peak volt=9V and I=300mA. I want to charge 6V battery. I'm thinking to put L7809 voltage regulator to prevent spike in voltage of solar cell and at night time there will low voltage at solar panel.
You will not harm the panel by charging a lower voltage panel. The battery will "clamp" the panel output to the battery voltage and supply whatever current it cam.
You should add a diode between panel and battery to prevent "backflow" when panel voltage is lower than battery voltage. You do NOT need a regulator for basic charging except if the battery is so small that it will not toilerate a 300 mA charge rate - which is unlikely. You MAY need a regulator to stop the battery from overcharging.
The panel will supply about 300 mA into the battery in full midday sun when pointed at the sun and less or much less when cloudy, not midday or not pointed directly at the sun. If the battery has capacity below about 2000 mAh it MAY be fully charged in less than one day. If 2000 mAh it will not fully charge in one day from "flat".
Where are you located? What use will be made of the battery (daily discharged fully or ...?) These factors affect the answer.
To charge the battery you need the current, not voltage (if you are not about lead-acid ones, see comment by @Andy-Aka below). Therefore you should be mainly interested in using current stabilizer.
Voltage regulator could be used as current stabilizer anyway (look in the PDF, you simply need to connect proper resistor as a fake load and insert the chip in series with the battery). Meanwhile you do not need to use 9V regulator for this. The regulator will consume all unwanted energy (as it usually does in voltage-controlling schematic also).
To prevent the back current in this solution you will simply add a diode (perhaps some Schottky's) in series.
If you will use L7809 as a voltage regulator, you could not prevent some back current, I'm afraid. Look at the internal schematics in PDF for details. You will lose the current through internal output resistors etc.
P.S. it will be hard to charge 6V battery if the cell gives less than 6V + regulator_dropout. Perhaps you should disassemble the battery into 3V or 1.5V cells if possible.
