Hi,

I am currently working on a solar project in school. I have an array of solar panels wired in parallel that at maximum rating will produce around 2 amps at 6 volts. The purpose of my project is to charge a small backup battery for my phone. The input rating on the battery is 5 volts at 800 milliamps. Now I want to overproduce amperage in order to makeup for changing environmental conditions, so that my battery will always be getting a minimum of around 1 amp. The problem is that if my panels do reach maximum output I don't want to fry my battery with 2 amps. I am looking for a way to have my solar panels output a constant level of amperage despite the fact that they may be producing more. Is there an effective way to make this happen? Thank you for any help!

 

To prevent excessive amperage in this case you would connect a resistor in series with the battery in the charging circuit. The resistor is the amperage regulator, more correctly an amperage limiter.

What will then happen is, the amperage will seek a happy medium where the number of volts dropped (lost if you insist) across (within) the resistor is equal to the number of amperes flowing times the resistance (Ohm's Law). The remaining volts are applied to the battery. Of course, for any charging to take place, the number of volts applied to the battery has to be greater than the battery's voltage at that moment in time. The battery's voltage will be at most 5 volts, less if the battery is quite discharged.

They make sophisticated charging circuits, sometimes custom tailored for the specific battery, that will apply close to the maximum allowable charging current from the start to the finishing up of the charging. A simple charging circuit, with perhaps just one resistor, cannot track the optimum charging current so it will deliver somewhat less than the maximum allowable charging current most of the time but never exceeding the maximum.

To limit the charging current to 800 milliamps at 5 volts, given a 6 volt supply, you need a resistor of at least 1.25 ohms. Run of the mill resistors have a plus or minus 20 percent tolerance. So that you will have at least 1.25 ohms you need to order a 1.5 ohm 1 watt resistor.

Since your charging source is 6 volts, you must find out (write the manufacturer for literature) how much charging current the battery will tolerate at more than 5 volts. As the battery charges up, the charging current will decrease which in turn means the voltage dropped in the resistor will be less putting more voltage across the battery. Let's say the battery can take 100 ma at 5.5 volts. To limit the current to 100 ma at 5.5 volts you also need to drop 0.5 volts in the resistor and you should order a 6.25 ohm resistor (resistor whose +-20% tolerance gives at least 5 ohms).

You may need to do more calculations to satisfy reasonable worst case devil's advocate situations.

Choose the greatest of the resistances mentioned above and what you calculate.

 

Actually, solar cells tend to be quite effective constant current sources, without need for a regulator. Running a series resistor will not be effective in reducing a current source output current. You would use a Parallel R to lower the output current.
You didn't say the type of battery. You likely would have to limit both the V and i of the charging circuit. If you are dealing with Lithium, you will need more sophisticated charger controls.