Simple battery charger

Faller usually supplies a standard battery charger with the startsets. Nice to put some juice in the batteries when empty, but I have a few problems with it:

• It is not very clear how high the current is it delivers. It is a non-stabilized voltage source with a resistor (I think). That means that the current depends on the charge level of the batteries at that moment and on the number of cells connected in series.
• With a given battery voltage, the current is more or less constant. Meaning, a battery of 800mAh receives as much charging current as a battery of 150mAh
• Per charger you can charge only 1 car at a time.

I therefore have made for myself a very simple battery charger that works with an arbitrary dc-voltage of about 5V min.

The basis is an LM317 voltage regulator. The fun is that (with a resistor) you can very easily transform this component into a current source. The circuit looks like this: The BC557, LED and 68Ohm resistor can be ignored for the moment. Between pin 2 of the LM317 and the + of the battery are 5 resistors, 4 of them have a switch in series. These are simple dip-switches. The LM317 provides a constant voltage of 1.2V over the resistors. That means that the current through every resistor is exactly known. By switching on more or less resistors, you can set the required current. This works as long as the primary voltage is sufficient. The circuit itself uses about 2 Volts, including the 1.2V over the resistors. For a battery of 2.4V, of which the voltage in fully charged condition can run up to about 3V, you therefore require 5V min. A higher voltage is no problem. The difference is dissipated in heath by the LM317. That will be a problem only when you want to provide a high current at a high primary voltage. The LM317 will become hot then. This can be checked easily. As soon as you burn your fingers when touching it, you will have reached the limit. Don't worry too much, the LM317 is temperature protected. As soon as it thinks it is too hot, it shuts itself down.

The current delivered with each resistor is 1.2/R. With the help of that formula you can create a table that shows what the current is depending on the state of the switches. Choose the current you want, find a setting that comes closest, set, connect and ready. 0 = off, 1 = on

Be aware of the fact that the charger doesn't have any intelligence. You therefore have to check the charging time youself. The charging current of the battery usually is stated in combination with the capacity of the battery. If a battery of 500mAh charges with a current of 500mA, we call that 1C and the battery will be full in about an hour. Not completely correct because you have some loss, so you have to add a few percent to it. In principle you can keep on charging NiCd of NiMH cel with a current of 0.1C. When the battery is full, the charging energy is transformed into heath. Letting this go on endlessly is not wise, but a battery of which the charge status is unknown you can safely charge with 0.1C during 24 hours. You can then count on the battery to be fully charged. When you provide higher currents, then you must know what the charge status of the battery is and check the time. When you are sure the battery is empty, you can for example fairly safely charge 0.5C during a bit less than 2 hours. After that the battery in theory will be full for about 90% and you can use it again. If you want it completely full then you end with 0.1C during 1 or 2 hours. Your battery will be charged then in 3 or 4 hours in total. An issue is: you have to check settings and time yourself, but well, it is after all a very simple circuit to be used everywhere with any dc-voltage around.

Next to the 5 resistors and switches there is an additional resistor, transistor and LED. This is purely for the visual indication of the current source being active. As soon as there is a current running, the LED will be on. It therefore doesn't say anything about the charge status of the battery. The current through the LED is constant and independent of the charging current or supply voltage.

I myself have placed 3 of these circuits on a board. With this I can easily charge 3 cars at the same time, with a current that is suitable for each battery. I have made a cable with which I can connect the board to the 2-pole connector of the UCCI. The round plug receives a 7.5V supply from a small net supply, the charger takes this along with the second connector.

Original contribution in Dutch by Leon van Perlo