A universal Ni-Cad and gel-cell battery charger for under $10.

Here is a quick project you can compete in under an hour.

A brief work about different types of chargers and the charging of rechargeable batteries.

There are many types of battery chargers, battery conditioners/chargers, Rapid Chargers, Over-night chargers and trickle chargers. There are also many different charging styles, constant -voltage, constant-current, pulse charging, and delta-peak charging. Without going into a long discussion of the differences between the charger types and their advantages and disadvantages, this charger is a cross between all of them!

It is important to keep your rechargeable batteries charged, but not overcharged if you want them to perform for you. You will real about the nicad "memory effect". While the memory effect may show after a long while of mistreating your batteries, two things will kill nicad and gel-cell batteries faster than you will see the memory effect. They are allowing the batteries to stay completely discharged for an extended period, and overcharging.

Rechargeable batteries work based on reversible chemical reactions. If the chemicals are lost, or the reaction goes too far, the reaction can not be reversed. Rechargeable batteries also "self discharge". This is why a fully charged nicad pack left used for a few week will be almost completely discharged. Not all nicads are equal. Some nicads have higher capacities or can tolerate different high rate charging, different nicads will self discharge at different rates. So a nicad left unused for a long time will completely self discharge and loose the ability to be completely charged. When nicads or gel-cells are overcharged they will vent electrolyte, and loose capacity. Even batteries left on trickle for a long time will loose capacity. I personally recommend using a low rate charger and topping off your rechargeable batteries every week or so.

Changer in use What makes a good universal battery charger?

The project

Here you see the parts needed to build the charger
Parts
  1. A universal ac adapter - $3.99 at Harbor Freight tools
  2. A 10 ohm resistor - 5 for $0.49 at Radio Shack 271-1101
  3. Two alligator clips - 12 for $2.99 at Radio Shack - 270-1545
  4. A T 1 3/4 LED - 2 for $0.99 at Radio Shack.

Build it

Bottom showing three screws To get started remove the three screws from the bottom and open the case.

Here is a picture and schematic of the charger before.

Schematic before

Circuit before The blue capacitor and right hand switch will be removed.

Steps

  1. Remove the capacitor and the polarity reversing switch.
  2. Enlarge the top center hole to 1/16.
  3. Take 1/2 of one of the resistor leads and put a jumper between the lower left two holes.
  4. Add the LED between the upper left two holes. The polarity of the LED is important. 1 lead may be longer than the other, or their may be a flat spot on the LED. The longer lead should go to the left hole, or the lead on the side of the flat should go the center hole.
  5. Add the 10 ohm resistor from the lower right hole to the upper center hole.
  6. Solder all in place.

Schematic After

Circuit After Here is what it looks like completed.

Remove the 9v battery snap and put the clip alligator clips on. The red clip goes on the wire with the white stripe.

Notes: