A 3w PV won't charge a dead battery or serrious depleted battery in a acceptable amount of time. For solar charging I found that 15v is a realistic number on 12v systems for calculating PV wattage requirements. A 12v PV will produce more than 15v, more like 21v open voltage but that's just not a useable number to use as it will cause you to under size the actual PV wattage requirement.
So that would be 7.5v on a 6v system. At 3w/7.5v=0.4ahr(amps per hour) that the PV will charge at. That's enough to maintain the state of charge in a fully charged battery without a CC(charge controller) and without overcharging the battery. A CC is nothing more than a voltage regulator. You'll need a CC on any PV greater than 5w or the battery will over charge if given enough recharging time.
At 0.4ahr then with a PV tracking the sun (move the PV once at 1pm to face west and once at dusk to face east) then we can plan on 8 solar collecting hours in full sun, cloudy then about 1/2 of the rated amp/hr, or about 3.2amps/day output on a 3w rated PV.
If you have a typical 6v starting battery at a 100ahr capiticy, then 100ahr/(3.2a/day)=31days/2= 16days to recharge a battery that's 50% discharged with a 3w rated PV.
And yes, you would need to put a 5a diode in one leg of the PV wire to keep the battery from discharging at night.
If you run a battery much past a 50% discharge rate then it will severely shorten the battery life.
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Today's Featured Article - Grain Threshing in the Early 40's - by Jerry D. Coleman. How many of you can sit there and say that you have plowed with a mule? Well I would say not many, but maybe a few. This story is about the day my Grandfather Brown (true name) decided along with my parents to purchase a new Ford tractor. It wasn't really new except to us. The year was about 1967 and my father found a good used Ford 601 tractor to use on the farm instead of "Bob", our old mule. Now my grandfather had had this mule since the mid 40's and he was getting some age on him. S
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