Solar battery charging, solar IV curve

Question

I am thinking of making a solar battery charger for some NiMH batteries but am a bit confused as to how make sense of a solar panels IV curve in the application of battery charging.

1) For a direct connection of the battery to the solar cell, will the solar cell be forced to operate at the voltage of the battery?

2)For charging a battery, will higher currents reduce the charge time?

he maximum power point of the solar panel a point where power is maximum but this does not imply that the current is maximum at that point.

3) So if 1) and 2) are true then why is operation at the maximum power point desirable, if the current is not as high as it could be at the MPP? Wouldn't a battery with a lower voltage charge faster as the solar panel would be forced to operate at a point where the current was higher (left of the MPP)?

Answer 1

Connecting a battery to a PV cell without some kind of switcher to buck/boost or otherwise control the voltage of your panel will not make for the kind of results you're looking for.

If you just wire the battery and PV in parallel, the effect can be similar to putting an old battery in with a new battery; in short, the effect will be that current will flow from one source to the other and your battery might see the cell as a load, or vice versa.

As for point 2, more current would typically make for faster charging times, however, if we inspect the IV curve of a PV we see that as we draw more current, the voltage will begin to sag. Thus, we may run into the problem described above.

It is desirable to operate a PV at the MPP because solar can be quite an investment in certain cases. As such, we want to not necessarily draw the most current nor the most voltage, but the maximum amount of usable power with which to do work. Is MPP necessary for a battery charger? Debatable. I think the most important point is to make sure we're only loading the panel, and not loading the battery unintentionally. A SEPIC converter might be used in a battery charging scenario to get you where you need to be.

Answer 2

1) Yes. A pv panel connected directly to a battery will be "clamped" at the battery voltage.

2) Yes. More current at the same voltage means more "power".

3) No. At the maximum power point, there may be less current, but there is also more voltage and therefore more power. The trick is to convert the pv's higher-voltage power to the lower battery voltage using a switching dc-dc converter. Essentially, the extra volts are converted to extra amps. In other words, a dc-dc switch converter is an integral piece of an MPPT circuit. Without an efficient power converter, there is no gain.