I am begging a project that I have been wanting to do for a while, finally have some free time to start it. Basically it is an intelligent solar charger system. Basically I want my charger to charge some batteries using solar power when the batteries are full i want the system to isolate the batteries and the power from the solar panels power be used to power a data logger. When the power from the solar panels drops too low the system uses the batteries to power the data while the solar panels use what ever power they have to charge the batteries.

So my question is this, where should I start when trying to design this power system.

 

It sounds like a fairly low power system.

The simplest approach is:
Fit a fuse inline with the battery before connecting anything else, to prevent damage in case of a fault. From now on that will be assumed to be part of the 'battery'.

Use a shunt regulator across the battery (basically like a large zener diode to limit the voltage to whatever the battery maker specifies).

Fit a diode in series with the solar panel to prevent the battery discharging through it when there is no sunlight. Some panels already have this internally.

Connect the panel across the battery & regulator.

Connect your data logger to the battery.
Done.

Any time there is surplus current from the solar panel (beyond what the data logger needs), the battery will be charged.

Any deficit of current will be supplied by the battery, up to full supply when there is no light.

When the battery is fully charged, the shunt regulator dissipates any excess power from the solar panel.

 

Its ok to use a solar panel as a back-up charging source for batteries, but only a back-up as there is usually not enough sunlight to do the job 100% of the time. That said, an AC-DC charger will be the primary charging device for your batteries. If you need to make your data logger portable (AC power un-available.), then another primary power source will be needed to keep the batteries charged; automobile will work.

Regards, DPW [ Spent years making heaters out of op-amps.]

 

Thanks for the info.

Duane,

I am considering the sun as the primary source of power since the datalogger is going to be sitting in the back country for a couple of weeks measuring the weather.

 

If you search on this forum, you will find a variety of projects where solar panels are used to power devices or charge batteries or both. The reliance on them keeping batteries charged is usually a disappointment however. Clouds, darkness and rain all conspire to make solar power a secondary power source at best.

Regards, DPW

 

Just some speculations: Assume logger draws 100mA, 24hr = 2.4Ahr; @ 6 hr sun, panel needs to supply 600mA hr, double for charging = 1.2A. Panel short ckt A =2. open ckt 20V to give approx 20W panel with effective loaded output @ 14V, so might need some boost for early & late hour to keep charging. Best to double again to 40W and 5 to 8 Ahr battery, still assuming 100mA load.

 

I have been monkeying, off and on, with a similar problem. So far, I have been using a photo-resistor to turn on a transistor that in turn drives a relay. When a cloud hits, the circuit shuts off. A capacitor does the duty of a battery so far and functions as a kicker for the extra startup current needed for the motor that is being driven when the sun returns. Once the motor is started, the solar panels are providing run current, so I figure I can charge some batteries with the startup current overhead that has already been provided by the panels (the capacitor is more for the brushless motor I am using. It does not awake properly, even when proper current is supplied. Experimenting indicates that for this motor, 2 more panels need to be added. With the circuit, I can skip adding the 2 extra panels).

You might be able to do something similar. I have been using the 1.8w car battery charging panels available at many stores. One panel can charge 10 ni-mh batteries in a few hours and they don't get hot to the touch.

Paul

 

Hi Talguy,

I don't have specific experience installing this type of system, but have recently been studying systems for homes that are "off-grid". The most successful use DC appliances. It sounds like your application is similar, though smaller. The off-grid home's power systems use a combination of solar, wind, and a back-up generator all to charge the batteries, which drive the house current.

It sounds like your system could work to drive the data logger. Stating the obvious...it will depend heavily on the load, amount of sun, number and output rate for the panels, and the size, condition, and charging rate of your batteries.

Do you have the technical data for the components you plan to use? The math to figure it all out is a little beyond my knowledge, but there are experts here that can tell you, based on that information, if your system has a chance of working.

Curious, though...what are you logging data for?