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
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.
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
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
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.
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
Down and Dirty
Confused? Let’s take a quick, down and dirty approach.
a plugpack that’s nominally 12V and has a fairly hefty current rating – eg more
than 0.5 amps
a 5-ohm, 5-watt resistor in series and connect the plug pack to a flattish
battery, positive to positive and negative to negative.
an ammeter (eg a multimeter set to amps) in the circuit and switch on.
the current flowing is excessive (ie it’s close to the max rating of the
plugpack), use a resistor with a higher value.
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.
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
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
(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
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
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.
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.
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
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.