Superpositioned: Tag battery

Charging batteries with Solar energy

Matthew — Mon, 10 Apr 2006 08:25:00 -0700

Depending on the application, charging batteries can be complex process. Charging methods range from constant voltage to pulsed and random charging. Once power is being delivered back into the battery, you have to know when to stop charging!

Once a battery is fully charged, the charging current has to be dissipated somehow. The result is the generation of heat and gasses both of which are bad for batteries. The essence of good charging is to be able to detect when the reconstitution of the active chemicals is complete and to stop the charging process before any damage is done.

Typically, common household batteries are charged with a current that is kept constant and relieved when the batteries reach a predetermined potential. However, solar cells typically generate a constant voltage of 0.5V and a varying current that depends on the amount of collected light. As such, a consant voltage charging model is easier to implement. I found two respectable tutorials on building your own charger:

AA Battery Solar Charger
Clean Power’s Solar battery project charger

The first solution uses a diode to stop the batteries from discharging when there is no sunlight. I highly reccomend including this protection. Unfortunately, neither project implements a charge limit. You have to remove the batteries and test their charge with a multimeter. A shunt regulator is the simplest way to regulate the upper limit.

Powering LEDs costs mere pennies

Matthew — Tue, 07 Feb 2006 14:07:00 -0800

Lately the hoopla concerning LED lighting has been overwhelming. Everyone claims this costs mere pennies to power. I decided to put a new twist on a classic science experiment to prove that LEDs do cost pennies to power. Literally.

Creating a battery from pennies

In order to turn pennies into batteries, another electrode and an electrolyte are needed. In this case, dimes (zinc) are used as the positive electrodes and salt water is used an electrolyte. Copper wire, galvanized nails, and lemon juice are also popular and cheaper solutions. Such a battery produces a differential of about 0.5 volts.

Finding ample power for an LED

Unfortunately, this battery is not enough to light an LED. In order to string eight of these cells in series, an ice cube tray is used. Metal paperclips hang the pennies and dimes into the electrolyte banks. Because the paperclips are conductive, the eight cells are automatically connected in series forming a more powerful battery. This provides a differential of about two volts.

As you may notice, 0.5 volts * 8 != 2 volts. Not all of the banks produced a reliable voltage. In fact, one bank seemed to be working against me.

Lighting a LED with pennies

Generally, LEDs require a resistor to prevent excessive current flow from blowing them out. This project does not require a resistor because the battery simply cannot provide that amount of current.

Connecting the short end of the LED to the penny and the long end to the dime lights up the LED! Everything works as planned. The penny batteries provide about 110 micro-amps of current in series. At two volts, this is only about 220 micro-watts of power!

It does in fact ‘cost’ pennies to power an LED.

Project Photo Gallery

Low-Cost battery monitor chip

Matthew — Wed, 25 Jan 2006 20:53:00 -0800

The MCP100-XXX family of integrated circuits is designed to monitor a voltage source for drops below a hard-coded voltage. The chips are intended to reset a microcontroller whenever the voltage drops. However, the output can be directly routed to an LED (low-battery indicator) or sent to an input pin of a microcontroller for monitoring as described in Electronic Design.

The program simply monitors RB0. If there’s a logic 0 on that input, the program flashes pin 17 (LED), indicating to the user that the battery voltage is below 3.15 V dc.

Block Diagram of MCP100 chip

Check out the MCP100/101 datasheet for details. In small quanties, these chips can be had for 33 cents.