bypassdiodessolar cell
We have a Solec 70W 12V mono-crystal solar panel. The junction box has one positive and one negative lead to connect to other panels or charge controller. It does not appear to have a bypass diode built in to the panel.
Can I install a 10A diode to provide some protection against shading, etc.?
And, if so, do I put it from the negative terminal to the positive terminal of the junction box?
When you connect the voltmeter between points A and B, you're connecting it directly across the battery, so if the solar panels put out less than the battery voltage the diode will be reverse biased and will, in effect, disconnect the battery from discharging into the solar panels and you'll be measuring only the battery voltage.
On the other hand, if the voltage from the solar panels is higher than the battery voltage and the drop across the diode, the solar panels will force current into the battery, charging it.
However, because the impedance of the battery is so low, it'll drag the voltage of the solar panels down close to the battery voltage, even though the solar panels will be continually pumping current into the battery.
As time goes by and the battery becomes more fully charged, you'll notice that its voltage will rise, but never to your solar panels' open-circuit full-sun value, because the battery chemistry won't allow it.
The proper way to monitor your battery's charging is to measure the voltage across it and the current into it, and never let either rise above the manufacturer's recommendations.
So my guess is that only 0.3 amps will flow, but when will the bypass diode in the PV junction box carry current? Where are tipping points?
The situation is far worse than you imagine.
The maximum available current from a series string of cells is the current available from the lowest current cell. A lightly shaded cell will reduce panel output. A heavily shaded cell will almost stop panel output - and as this happens reverse potential from all the unshaded cells develops across the shaded cell so that it contributes nothing. If bypass diodes are fitted current then flows in an appropriate diode around the shaded cell, and also nearby cells if the diode spans several cells.
Accordingly, panels and cells within a panel which are connected in series need to be matched as closely as possible both in performance and in received isolation (light) levels. A 1000 W/m2 : 200 W/m2 imbalance such as you suggest can be fatally bad and should be avoided by design to the maximum extent possible. Bypass diodes should be fitted to prevent large reverse voltages occurring across shaded cells as this can destroy whole installations, or worse.
Bypass diodes are connected in reverse polarity to a cell's output when it is acting as a current source. If the cell current is lower than the string current the voltage acting to drive current through the cell causes it to produce less output and then to reverse bias it completely. A bypass diode across the cell conducts when the cells reverse polarity exceeds the forward polarity of the diode. So none of a partially shaded cells output contributes to string current when the bypass diode is conducting and instead the diode reduces output by a diode drop, plus whatever voltage the shaded cell(s) no longer produce.
While this description may seem contrary to what you expect, you will find it well described in many online articles.
Many examples can be found here and here
The effect can be easily visualised using the diagram below. For the diode to conduct the usually -ve terminal of the PCV-cell must be positive relative to it's usual +ve output. No current is produced.
Diagram from Dale Marshall here
Note that in the absence of bypass diodes the full remaining string voltage will build up across the shaded PV-cell. In high voltage systems this may be in the 400-1000 V range. Cells with defects my have low isolation voltages and break down. In such cases fires can (& do) occur which destroy the cell concerned, often damage others, will usually write that panel off and may destroy the whole installation or even an the building it is mounted on.
Best Answer
Use a Schottky diode and put it in reverse accross the solar panel normal output voltage. You want the diode not to short out the solar panel in normal operation, but to provide a means to shunt current accross it when it is not producing its own voltage.
The problem is that solar cells are actually diodes with current going thru them in the reverse direction during normal operation. But, this only works when there is enough light on the cell to produce the current. When the cell is in the dark, think of it as basically a diode blocking the normal current flow. When a bunch of cells are in series, just a single one in the dark, and therefore acting like a reverse diode in series, effectively blocks the current from all the others. The Shottky diode in parallel with the cell but at opposite polarity allows the current from other cells to bypass a cell in the dark, minus the drop on the Schottky diode.