POWER CHARGER FOR NiCd BATTERIES

This design was a result of a work I did for a friend - he needed a power charger for his RC cars and I made one. But when I also have a few RC cars that needs charging, I though to make one too - but with all features avaiable. This ended up in the design You see here.

Following features was wanted:

   * 7.2V/8.4V/9.6V selectable to fit several battery packs.
   * High/Medium charging current.
   * Discharging.
   * A durable, compact design.
The result shows that all wishes on the list was implemented in the final product - without too much problems.

The design is based on standard components, rather easy to get. The heart of the charger is the MAX2003 controller chip. It has all logic needed to control all aspects of a NiCd/NiMH battery charger. It uses three control parameters to control the charging current: 1. Voltage (dV/dt sensing), 2. Temperature (necessary for NiMH), 3. Time (last resource to avoid overcharge in case of failure). Additional features in the chip are trickle-charge that activates after normal charge (keep batteries fresh when connected to charger for a long time) and discharge-before-charge (measures voltage during discharge to avoid draining the battery, making permanent damage). When discharging are done, normal charging starts.

The actual power control is done by a switching design (controlled by MAX2003), using a 100kHz switching frequency. An external MOSFET-transistor takes care of the actual power, together with necessary filter and protection components it delivers a neat current to the battery pack.
The discharge is much easier - here acts a logical controlled MOSFET as a plain switch, turning the load resistor (5ohm/20W) on and off.

To get heat off the compact design, a combination of heat sink and fan is used. The current control transistor, the discharge transistor and resistor and the current sensing resistors are all getting more or less hot. To get away with it, a heat sink is used for the transistors, the box wall (also an aluminium heat sink) is used for discharge resistor and the sensing resistors are placed in the air flow from the fan. The fan was taken from a 486 CPU cooler - it was the only one to fit the box in size and it runs fine up to 15V. The air is directed over the heat sink and all resistors that got hot.

The actual design of the charger is an implementation of the reference design by Maxim but modified to suite my needs.

To summatise - this is what I got:

   * Selectable voltage - 7.2V/8.4V/9.6V
   * Two charging currents - 2.7A/6A
   * Discharge-before-charge
   * Connects to car battery (12V) or other source (12-15V/6.5A)
   * Compact design - fan cooled
   * LED's at the front shows all settings

Interior of the charger - all cabels goes in at back side, all controls are at the front.
Front panel with all indicators and controls. The number of LED's makes it easier to see the current/voltage settings in a busy enviroment where small switches can be hard to see.
Another interior shot.
The 5V stabilizer for the control chip. To left is the filer choke and in front the relay that switches between high/low current charging.
The heart of the system - MAX2003 from Maxim. This little chip controls everything in the charger. The left transistor at the heat sink switches charging current, the right one controls the discharge.
Fan cooling - not only for overclocked CPU's... here it helps to keep the heat down in the small box. It is situated so it blows fresh air through the box, over the heat sink and the sensing resistors. The fan was taken from a 486 CPU cooler.
Discharging resistor (big yellow thing) and it's control transistor. This allows discharge-before-charge to condition the batteries.


Finally - the drawing for anyone else that wants to make one. The switchable current aren't included in this version, but when updated I will change it.
Click here to get it


Back to Electronics