Retired ME here with limited circuits background so please be kind. I have a 3 watt 9 volt solar panel that i am using to keep the charge on 6 cell D sized battery pack for a trail camera. My d batteries are rechargable and have a voltage of 1.3 something volts. All batteries are in series so total voltage of 8.? I believe i neet to install a diode to prevent backflow when the panel is not putting out power. What size diode do i need? Where to get. And where should it be installed. Thanks for your help.

 

A 1N400x where x is 1-7 is fine. Put the anode to the + output of the solar panel and the cathode to the battery.

What chemistry is the battery? Likely NiCd or NiMH at that voltage. Your 6 cell battery should charge to a peak of 8.4V

Both of those can be harmed by overcharging, so, personally, I would not simply put the solar power directly to them without a charging circuit. But you might be safe since the diode will lose you 0.6V giving you a max of 8.4V.

More likely, it will not charge very effectively because the voltage is too low mist of the time.

An MPPT charger for your battery type would be far better. It could boost the lower voltage to get some charging at any voltage.

 

In most solar battery systems I've dissected, the unloaded panel voltage is roughly double the nominal battery voltage. This "over voltage" makes up for efficiency losses and ensures that some charging current flows even at less-than-full-sun.

Your panel rating MIGHT mean 3W at 9V. That would be good, because it means the unloaded voltage would be higher. I fear however that it means 3W at the MPPT, and 9V unloaded. If that's the case, it's going to be tough to get much charging current even at full sun.

Because you may be on the edge of voltage, I recommend a Schottky diode. It will drop a little less voltage than a traditional silicon diode and therefore allow for more current into the batteries.