DIY a Trickle Charger

[kinyo]

The Trickle Charger (A DIY project)

Here's a pictorial of the trickle charger. I don't know how to link pics so I'll be attaching them on my succeding posts.

As its name implies, the trickle charger will keep the car battery in "charged" condition continuously by applying a very low charging current (trickle). It is ideal for cars that will not be in use for a long time where the car's standby current (due to alarm system, ECU, radio, etc) might eventually discharge the battery. If a blackout occurs, the trickle charging will stop but will resume when power is restored.

PIC 1 shows the whole thing that needs to be assembled. It consists of an AC Adapter (12V/500mA), a long cord, a resistor (1-ohm/5W, a 3-amp diode, and two alligator clips (red & black). Except for the orientation of the diode, the pic already shows the soldering work required. For the long cord, I used the usual household extension wire (black) coupled with a typical speaker wire (red/black) at the end to easily identify the polarity. The length of the cord is the distance from the nearest AC outlet to the car's battery and the wire size is similar to the adapter's original wire. A multitester is not absolutely necessary but it will make things easier.

[kinyo]

PIC 2 shows a closer look on the output circuit. The resistor and diode are in series with the POSITIVE side of the adapter's output. The cathode end of the diode (indicated by the silver band) MUST connect to the RED CLIP. The alligator clips are necessary for connecting to the battery.

[kinyo]

PIC 3 show a closer look of the AC Adapter. Its current rating is 500mA. It has a selector switches for output voltage and output polarity. The switch settings are not clearly shown, but it is set at 12V. Any 12V (300-500 mA) adapter will work for this project. The polarity switch of the adapter, if any, may be set at one position but IT MUST REMAIN IN THE SAME POSITION DURING USE. It is only important to identify the polarity at the end of the long wire.

[kinyo]

PIC 4 shows how the charger is connected to the car battery. Some battery terminals may require larger alligator clips than what I used for the project. Before connecting to the battery, make sure that the polarities are correct. The RED CLIP is the POSITIVE output of the charger and must connect to the positive terminal of the battery.

[kinyo]

PIC 5 shows 185mV across the 1-ohm resistor which means the charging current is 185mA. It is normal for the current to drop to 120mA when the battery has fully charged.

[kinyo]

PIC 6 shows the battery voltage at 12.52 volts while the charger is connected. It is normal for the battery to read 14.4 volts when fully charged with the trickle charger.

[kinyo]

PIC 7 shows the adapter pluggeg to an AC outlet. The LED indicator shows that its output is active.

I did not include a fuse in my charger but i would recommend one in series with the positive line (in series with the resistor and diode). A 500mA fuse should be good enough. The fuse will give protection when the charger is inadvertently connected in reverse to the car battery. Without the fuse, in my case, the adapter will be damaged. The internal diodes and transformer windings are the weakest link in the circuit.

Good luck DIYing. Post questions if you have any.

[ebbfolls]

Great idea, bro!

Just a couple of questions: How can a 12vdc output transformer charge a 12v battery to 14 V? I would have expected a 14VDC output transformer with a current limiting resistor (for undercharged batteries). Also, why the need for the diode when the output of the transformer is already fully rectified dc?

[kinyo]

ebbfolls,

1. The no-load voltage of the usual "unregulated" 12V dc adapter is near 16 volts and would, in theory, be capable of charging the battery to 16 volts. But the internal resistance of the adapter coupled with the voltage drop on the external diode limits the voltage down to 14 volts, which turns out to be a safe level.

2. The diode does two things. 1) It gives a voltage drop of 0.6 volts, hence the battery will not be abused with overvoltage, and 2) It prevents the battery from discharging onto the adapter when AC power is not present.

Although the internal circuitry of the adapter is already sufficient for the task, I decided to add the resistor for testing purposes. It serves as convenient test point for measuring charging current.

[ebbfolls]

Yes, indeed. Thanks for adding to my web-derived technical knowledge.

By the way, do you know by any chance what the reverse bias breakdown voltage of the 3 amp diode is?

[kinyo]

The diode is 1N5402, a 3A/200V diode. Its reverse breakdown voltage is therefore 200V. Admittedly, the ratings of this diode is overrated for the application. I decided to use it for its more rigid terminal as compared to a 1A diode, a similar reason for the resistor which is overrated at 5W.

Except for the physical durability issue, the more popular 1N4001 1A/50V diode would work just the same and so would a tiny 1/4W resistor without them getting damaged due to self-heating.

[ebbfolls]

Is there any chance the car's standby current could exceed 500 ma, thereby approaching the transformer's current sustaining capacity? What would happen if say somebody tripped the alarm, activating the lights and the horn?

[kinyo]

500mA is too much of a standby current. I doubt if any stock car (with alarm) would exceed this value. My car's standby current is less than 100 mA.

When the alarm is tripped, the battery will supply most of the current required by the loads (horn, lights). The adapter is not likely to exceed its 500mA capacity in this condition.

But if somehow the battery is loaded continuously (light was left on, persistent alarm), then this will eventually discharge the battery and the adapter will eventually experience an overload as the battery's voltage drop further. I guess this is another reason for a fuse.

[333tinggay]

I truly appreciate your post of DIY trickle charger. Here's some questions. Can I use an AC Power Adapter rated at 1000 MA * 12 VDC? How do I tell which wire of the AC Adapter is positive and which one is negative? One black wire has a white stripe on it, while the other one is solid black. Thank