PV Gap

Josephine Said:

How would I make a circuit that recharges batteries?

We Answered:

Let's assume you are using batteries designated to be rechargable. While you can recharge ordinary drycells, it is not very effective.

UNIVERSAL PRINCIPLE:
In recharging a battery the universal principle is that you reverse the energy delivery process. When a battery powers a device it spends its energy doing so. When you recharge the reverse happens.

HOW TO IMPLEMENT UNIVERSAL PRINCIPLE:
Apply a slightly higher voltage to the battery than its terminal voltage (the voltage it is designed to operate at). NiCad single cells have a terminal voltage of 1.33 Volts. Lead-acid (automotive) batteries have a terminal voltage of 13.85 Volts (commonly called 12 Volts). So, to charge a car battery apply a voltage to the battery, with matching polarity (plus to +, minus to -) of maybe 16 volts.

HOW MUCH IS TOO MUCH:
Overcharging a battery will shorten its longevity. Stop charging a battery when its voltage has risen to maybe 5-10% above its terminal voltage as measured while still charging. You will need a cheap multimeter for this available at any hardware store. The cheaper the better since the more expensive models offer no advantages for this application.

YOUR CIRCUIT:
Determine the voltage required from the instructions above. Electrically string together in series your solar cells with a number such that you at least reach your required minimum voltage. Apply this voltage to the battery you want to recharge and let it sit in the sun for a day.