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Zero Cost Trickle Charger

As simple or as complex as you like - a trickle charger that'll save you having a flat battery in rarely-driven cars

by Julian Edgar

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If you're the sort of person who isn't into hoarding junk, read no further. And if you're the sort of person who immediately prepares a new parts shopping list when reading about a project of this sort, again read no further.

However, if you like picking junk up cheaply (or at no cost at all) and aren't afraid of putting together some bits and pieces, this is for you!

So why the preamble? Is this some sort of complicated, weird project using hard to get parts? Not at all! But by the same token, if you have to go out to buy all the bits, it won't be worth making.

So take a look around...

Lots of car enthusiasts have more than one car. The kept-mostly-in-the-garage car might be a full-on show car, it might be a classic of yesteryear, or it might simply be a car that you drive rarely.

In this household – three cars and two people – the rarely driven car is my turbo Maxima V6. In the lucky event that I have a sequence of new cars to test, poor old Max might be driven only a few times in a month  - and then mostly on short trips to the post office. The outcome is that more often than not, it’s got a flat battery. And that’s with a near-new battery in the battery box in the boot!

I’ve already got a battery charger but it’s not really the kind of thing I want to leave connected semi-permanently. For one it’s a mains-powered charger with short battery leads – and often the Maxima is parked outside. And outside weather and 240V battery chargers don’t really go together well. Secondly, I often have another requirement for that battery charger, and so I don’t want it tied up semi-permanently on the Maxima. And finally – and it sounds trivial - but I have a healthy respect for the hydrogen gas released during battery charging and I always feel a bit vulnerable pulling on and off those bare alligator clips – especially when the charger doesn’t have an on/off switch on its main body.

Maybe some of these things apply to you as well. 

So what was needed was a low-cost trickle charger, one that could be left connected for long periods to just keep the battery topped-up. (A trickle charger is not the sort that you connect when you want to boost a flat battery into life in a few hours; nope, it’s the type that is connected for a few days or more at a time.) It would be good if it had a long low voltage lead and a readily accessible on/off switch. A quick and easy connection method to the car would also facilitate its use.


The Charger

In its most bare-bones form, to build the charger all that you’ll need is a 12V unregulated plugpack (“wallwart” in the US) and a resistor.

Yup, that’s it.

When all is said and done, charging a battery is like water flowing down a slope. The difference in battery and charging voltages is the steepness of the slope. In this case, where we want to just trickle-charge the battery, the slope can be so shallow that just an – er – trickle slowly flows down it.

Click for larger image

Most unregulated 12V plugpacks produce about 15-17V when off-load. They’re designed so that when the electrical load is placed on them, the voltage drops to closer to the nominal rating. And just such a plugpack is the basis of this design. So take a look around your junk pile and se if you can dig out a nominally 12V plugpack. The higher the current rating, the better. In my case I found a 12VDC, 500mA (ie half an amp) plugpack. It came from the tip shop (see Driving Emotion : Shopping for Rubbish) and so cost me probably 20 cents.

If you use a multimeter to work out which is the positive lead and which is the negative (that is, after you cut off the original phone charger plug - or whatever plug is on it) you could connect the plugpack straight to the car battery. The plugpack’s output would fall (maybe to 12 or 13V) and the difference between this voltage and the car battery voltage would determine how much current passed. (Think of it like the slope and the water trickling down it.)

And if the car battery voltage is – say – 12V and the plugpack’s loaded output voltage is – say – 13V, it would be sweeeeeeet. But what if you connected the charger (cos it’s a charger now, not just a plugpack!) to a battery that was very flat? Then the slope would be too steep and perhaps too much current would flow, damaging the charger.
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Getting around this problem is easy, though. What you need is a resistor – and obstacle to slow the amount of water that can flow down the slope, even if it’s steep. The obstacle is called a resistor, with a 5-ohm resistor about right in many applications.

The placing of the resistor in the current path will slow the flow, including decreasing the current as the battery voltage rises. But that’s what’s wanted – it is a trickle-charger, after all. The resistor will have to dissipate some power, so pick a 5 or 10-watt design. This is one of the few items that you might have to buy – but a 5-ohm, 5-watt resistor will cost only 30 cents from Jaycar Electronics...

Down and Dirty

Confused? Let’s take a quick, down and dirty approach.

  1. Find a plugpack that’s nominally 12V and has a fairly hefty current rating – eg more than 0.5 amps
  2. Insert a 5-ohm, 5-watt resistor in series and connect the plug pack to a flattish battery, positive to positive and negative to negative.
  3. Insert an ammeter (eg a multimeter set to amps) in the circuit and switch on.
  4. If the current flowing is excessive (ie it’s close to the max rating of the plugpack), use a resistor with a higher value.
  5. If the current flowing is too low (ie it’s way less than the max rating of the plugpack), use a resistor with a lower value.
If the resistor gets hot after a while, increase the wattage of the resistor.


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The above is all that’s necessary, but in my own trickle charger I added some extras. Firstly, I mounted the resistor and a switch in a box. This worked out pretty well, because the cord already attached to the plugpack wasn’t long enough, and by placing a box on the end of it and adding another cord on the other side, the overall length got greater. Then I added a 0.5 amp fuse and its associated holder.

Click for larger image

Next I got real – er – flash and added a green blinking LED and its associated 560 ohm resistor, so that I could tell at a glance when the charger was on.

(The LED has another important use as well. Given that it won’t be uncommon for people to inadvertently drive off in the car that’s still attached to the trickle charger, the flashing LED makes it much more obvious that the charger’s still there. Especially if you place the box in the middle of the windscreen!) 

Then I found that the LED would flash when the charger wasn’t switched on but when it was connected to the battery. So I added a diode - because the current flow is so low, even a 1amp diode is fine. This diode also has another function – well it has if it’s placed close to the battery. We’ll cover what it does in a moment.

The Plug-In Connection

One of the aspects I dislike about traditional chargers is the croc clips connections to the battery. (I once read of a battery blowing up in a man’s face as he disconnected battery charger clips. He was VERY lucky not to lose his eyes.)  So what I wanted was a quick, clean, safe and easy plug-in connection.

Looking through my assortment of salvaged plugs and sockets, I initially thought the most appropriate was a 6.35mm headphone jack and a guitar amp plug and cord (another tip shop buy). It was only a few minutes’ work to wire the socket to the battery, mount it through the wall of the battery box, and then wire the jack to the charger, making sure that the correct polarity was maintained through these connections.

However, as some readers have pointed out, the use of a headphone jack is not a good idea. Why not? Well, when it’s inserted, it momentarily shorts the tip of the plug to earth – not what’s wanted. So instead it’s much better to use a two-pin DC-style plug, or a plug of the sort that you chopped off the plugpack.

And remember the diode mentioned above? Put this in between the socket and the battery, rather than in the charger box. Placing the diode there prevents the possibility of a shortcircuit occurring in an accident – the diode allows current to only flow into the battery, rather than out of it.


Click for larger image

The design shown here charges a battery (that has a no-charge voltage of 12V) at about 350 milliamps. That more than makes up for the slow battery flattening which is a result of the alarm (and its flashing LED) being on, with the charger able to be left connected for long periods.

(Given that your plugpack is not likely to be identical to the one used here, you should always measure battery voltage over the first few days of charging. If it rises too high – ie the battery is being overcharged – you should switch off the charger when that starts to occur.)

My out of pocket cost for the charger was zero - I had every component close to hand, with most having been salvaged from discards. But look, even if I’d had to buy the box and LED and resistors brand new, it still would have been a 5-dollar job all-up. Going out and buying every single component mentioned in this story? It’s not worth it. But keep your eyes open and you’ll be amazed how many plugpacks and old bits and pieces turn up.

More Current?

If you have a plugpack that’s capable of flowing a lot more current, just reduce the value of the resistor and measure that the actual current flow into a flat battery, making sure that the current isn’t greater than the plugpack rating.

As you go up in current flow, the charger becomes less a trickle charger and more a full-on charger – which has got implications for leaving it connected to the battery for a long time.

But still, you can make it what you want it to be.

It Doesn’t Work?

If you follow the instructions and find that you can’t measure any current flow, the output voltage of the plugpack may be too low. To fully charge a 12V battery you’re going to need a charging voltage of around 14V – more than a nominally 12V plugpack will develop when under load. However, where we just want to trickle charge a battery to keep it around 12-13V, most 12V plugpacks will work fine.

But if you find it doesn’t, you might need to use a different plugpack with a higher output voltage.

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