This is only a preview of the July 2015 issue of Silicon Chip. You can view 35 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Build a Driveway Monitor, Pt.1":
Items relevant to "Install USB Charging Points In Your Car":
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E- LED
R
P MB
SEPCB
S
A
Ideal for . . .
• Cordless Power Tools
• Model Racing Cars
• Battery Appliances
• Electric Planes
• Just about anything
with Nicad or NIMH cells
INTELLIGENT CHARGER
for Nicad and NiMH Batteries
Cheap chargers supplied with the original equipment can – and often
do – damage the battery. Proper chargers are usually expensive. This
cheap and easy-to-build Nicad/NiMH Battery Charger is suitable for
automatically charging a wide range of batteries.
T
his ‘intelligent’ charger, controlled by a microprocessor, was designed for high-current and rapid-charge
charging applications such as required for cordless
power tools and model racing cars. These battery packs
are expensive and can be difficult to purchase (it’s usually cheaper to buy a new tool!). This charger uses the cell
manufacturer’s recommended charge method to safely and
quickly charge batteries.
dreds of charges and could potentially last many years.
The important part of that last statement is “properly
treated.” Batteries can be ruined by just one incorrect
charge.
Unfortunately the battery packs are fairly expensive to
replace, sometimes costing almost as much as the entire
drill kit, if in fact you can purchase the batteries separately
at all. Note that if you can solder, you can rebuild a pack
with tagged cells – don’t solder directly to batteries though!
Introduction
(See SILICON CHIP, December 2006). If you’re really keen,
Batteries for power tools and many other electrical prod- you can even upgrade the battery to the latest Lithium cells
ucts range from 2.4V to 24V and usually consist of Sub-C (October 2013) but a special charger would be needed.
Recently I found my 2-year-old 9.6V cordsize Nicad or NiMH cells. Properly treated,
these battery packs should be good for hunby PETER HAYLES less drill battery wouldn’t perform to its rated
60 Silicon Chip
siliconchip.com.au
capacity after charging.
I decided to repack the battery.
In selecting replacement cells, I
researched the manufacturer’s specifications on charging and discovered
that the battery charger that
came with the drill didn’t comply with these specifications.
The supplied battery charger is
a very simple device that applies
constant current to the battery
pack, with no cut-off, only a warning not to leave “on charge” for more
than 14 hours. As we will see, even
this is a recipe for disaster!
There is no charge termination
method used by the charger. During
the recharging process, once a battery
reach its full charge, the cells start to heat
up and the internal pressure builds up,
causing the battery to eventually rupture
or vent electrolyte.
Having paid good money for a new battery
pack, I decided to design a new charger that would not
damage the battery. A better battery charger would require
the charger to sense the condition of cells and charge accordingly. I soon realised that the simplest design would
be a one-chip design. I selected a PIC controller as it was
the smallest and cheapest device available at the time with
a suitable analog-to-digital converter.
Nicad/NiMH cell characteristics
Even if you don’t want to build this charger, you still
stand to gain something from this article by understanding how to get the most from your rechargeable batteries.
A cell is defined as a single vessel containing electrodes
and electrolyte for generating current. A battery consists
of two or more cells, usually connected in series to obtain
a higher voltage.
Nicad/NiMH cells are nominally rated at 1.2V for design
purposes although they normally develop about 1.25V.
Under full charge they require about 1.5V to 1.6V. They
can supply very high current and display a remarkably flat
discharge characteristic, ie, they maintain a relatively consistent 1.2V throughout discharge. The voltage then drops
quite suddenly, and they are almost completely discharged
at 0.8V. This is called the “knee” characteristic because of
the shape of the graph of voltage against time.
Rechargeable battery capacity is rated in mAh (milliampere-hours). The total capacity of a battery is defined
as “C”, that is it can supply C mA for 1 hour, or 2C for 30
minutes etc.
Charge rates can vary –
• from trickle chargers (to keep the battery ‘topped up’) of
3.3% of C to 5% of C
• from a standard charger (a slow current charge) of 10%
of C to 20% of C
• or from a fast charger of 50% of C to 100% of C.
• some ultra-fast chargers can go higher than 100% of C but
these are normally designed for a specific type of battery.
Slow charges are not meant to be continually applied, as
they will eventually overcharge the battery. Since Nicad/
NiMH batteries are about 66% efficient, the slow charge
siliconchip.com.au
The heart of the charger is this pre-assembled PCB which
makes construction a breeze! Basically, all you have
to do is put it in a box, connect power . . .
and connect your battery to be charged.
time is normally about 8-15 hours.
Fast charges, such as 100% of C,
should be terminated after about
1.5 hours, providing the battery is
flat to begin with. Once a battery
is fully charged, it produces gas,
creating a high internal pressure
and a sudden rise in temperature. The charger should be
switched to trickle charge
at this point or the battery
will begin to vent and
release its electrolyte.
My old battery was rated at C=1300mAh and my old
charger was rated at 400mA
(30% of C) so the charger should have
been switched off after about 4 hours, provided that
the battery was almost flat to begin with.
However there is no way of knowing if C was actually
1300mAh or if it had decreased a bit and once the a battery starts to deteriorate, I suspect this becomes a vicious
cycle and the battery deteriorates rapidly due to more and
more overcharging.
The “Memory Effect” myth
Possibly the biggest myth that exists particularly for
Nicad cells is the “memory effect”. The myth is that cells
have to be completely discharged - otherwise they develop
a sort of memory, and can only hold a partial charge from
there on.
Like all good stories, this one has a grain of truth in it!
The myth originated from the early days of satellites when
they were using solar cells to charge batteries and because
of the orbiting of the craft around the earth, the batteries
were subjected to precise charge/discharge cycles many
hundreds of times. The effect disappears when the battery
cycle is suddenly varied, and it is extremely difficult to
reproduce this effect even in a laboratory. In practice the
“memory effect” is not a significant problem in home usage.
While it may be OK to discharge individual cells to 0V,
it is certainly not recommended to discharge an entire
battery of cells. When the battery is discharged below
0.8V per cell, one of the cells is inevitably weaker than
the others, and goes to zero first. Then this cell begins to
be charged in reverse.
This is easily observable on any battery pack. This creates
a more common but less commonly known effect called
“voltage depression”. The battery performance is greatly
affected by the weakest cell, as the cells are all in series.
One other thing – batteries don’t like getting too hot or
cold; they do not take a full charge and they actually self
discharge (even under no load) much faster when over 40°
or below 0°. They can build up internal heat when working
and this can also cause temperatures inside to increase.
Particularly avoid leaving cordless tools inside a hot car
for this reason.
They also should be left to cool down for a while after
discharge before placing them on charge. Nicad/NiMH
July 2015 61
R1*
2.7W 1W
REG2 LM317T
1
AC
BR1
2
AC
+
REG1 7805CT
KBL407*
~
4
BAT–
ADJ
D
GND
~
4700mF
CON1
63V
G
D
+5V
100nF
ICSP
Vpp
Vdd
Vss
PGD
PGC
4
1
AN2/GP2
GP3/MCLR
G
50V
1
Vdd
BAT VOLTS
7 AN0/
GP0
4
6
5
IC1
PIC12F615
-I/SN
GP4
CLKIN/GP5
GP1/AN1
D
3
G
ADJ
Q2
2N7002K
1k
OUT
IN
ADJ
Q3
2N7002K
180W
A
ADJ
180W
A
KBL407
2N7002
LEDS
D
S
–
G
K
A
K
+~
~–
S
LM317T
7805
SC
Ó
2015
NICAD/ N I MH BATTERY CHARGER
~
K
G
~
l LED2
l
Q4
2N7002K
1k
+
LED1
D
R4*
2.7W 1W
OUT
IN
K
*RESISTORS R1– R4
MAY BE REPLACED
WITH 1W 3W
COMPONENTS
TO INCREASE THE
CURRENT RATING.
BRIDGE BR1 AND
DIODE D1 SHOULD
ALSO BE INCREASED
TO 8A DEVICES (EG
GBU806 BRIDGE
AND BY229 DIODE).
1k
REG5 LM317T
S
D1 1N5404*
A
R3*
2.7W 1W
REG4 LM317T
2
1k
OUT
IN
1k
5
Vss
8
CON2
R2*
2.7W 1W
REG3 LM317T
S
2
3
Q1
2N7002K
S
–
5.6k
1k
OUT
IN
3
BAT+
OUT
IN
1N5404
A
K
GND
IN
GND
OUT
OUT
ADJ
OUT
IN
Four regulators share the load, which is the battery charging current. A single PIC microcontroller takes care of all the
housekeeping, including monitoring the battery voltage to ensure it is not overcharged.
batteries do self-discharge too, as a rule of thumb a battery
will hold a full charge (with no load) for about a month or
two, although when they get old or hot, they might only
last a day.
So therefore:
• You should not discharge your battery before you recharge it,
• Don’t flatten your battery below 0.8V per cell,
• Don’t overcharge your battery beyond 100% of C, and
• Nicad/NiMH don’t like to get too hot nor too cold (0° to
40°C is ideal)
Nicad/NiMH charging
Common values for C for cordless tools and racing cars
are in the range from 500mAh to 3000mAh (mostly sub C
cells and AA cells). The first step is to determine what C is
for your cells. Inspect the cells or contact the manufacturer
to determine the cell part number.
In drills, the battery packs can often be easily disassembled. The value for C often forms some of the part
number. For my new battery the value for C was 1700mAh.
Note that the cell value for C is the same as the battery
value for C.
Usually the charge time required is as fast as possible,
between 1 and 2 hours. This does not harm the cells, in fact
they are designed for it. My battery was capable of taking
a fast charge of 100% of C, which equates to 1.7A (Some
can take up to a 2C rate). Each of the four regulators must
These two shots show how the PCB is secured to the diecast
box lid – it actually mounts on small threaded stand-offs
with countersunk head screws used from the top (lid) side.
The four regulators must be fitted with insulating washers
and bushes to prevent them shorting to the lid.
62 Silicon Chip
siliconchip.com.au
Battery Charging Algorithm
Start
MODE
LED display
(FL – flashing)
Test
Conduct self test if no battery
FL
FL
0 Standby
Wait for battery
OFF
OFF
1 Cool
Wait for V to stabilise (1min)
OFF
ON
2 Soft
20%C for 4 mins
100%C until - DV or time out
3 Fast
4. Trickle
4%C
5 Error - Alternate flashing
FL
ON
ON
ON
FL
OFF
FL
FL
If battery
removed
The algorithm flow-chart shows the steps the
microcontroller goes through to charge the battery. LED
codes are repeated on the front panel (see right)
therefore supply around 450mA for a charge rate of 1C.
This value should be good for most readers, and it doesn’t
really matter if it is a bit less than 100% of C, because the
charger will still detect a peak eventually anyway. However,
some readers will want to increase the maximum current,
and this is described a bit later on.
There are two recommended methods of detecting charge
termination, either using a temperature sensor in the battery pack or using a “negative delta V” cutoff system. The
temperature technique relies on detecting the sudden rise
in battery temperature to shut off the charge. There is nothing wrong with doing this but battery packs do not always
come with temperature sensors built in. Furthermore ones
that do usually don’t sense all of the cells. The negative
delta V system relies on the electrical characteristic that
the Nicad/NiMH battery voltage peaks and drops about
20mV per cell when fully charged. This charger in its basic
configuration will detect a peak of 40mV (per battery) from
10mm CSK
M3 SCREW
SILICONE
INSULATING
WASHER
INSULATING
BUSH
M3 NUT
6mm CSK
M3 SCREW
DIECAST CASE LID
PCB
LM317
6mm M3
REGULATOR
6mm
THREADED
STANDOFF M3 SCREW (TO-220)
Here’s how the four regulators are mounted using
insulating washers and bushes; also how the PCB mounts
to the case lid via four threaded stand-offs and screws.
siliconchip.com.au
(Above) same-size label which can also be used as a
template to get the LEDs emerging in the right place, as
seen below (before the label was fitted).
2V to 21.5V, thus will charge any battery pack in this range
(ie, 2-20 cells or 2.4V to 24V).
Another point to consider is the requirement to let a battery cool down. If the battery has just come off discharge
and is hot, it may take a minute or so for the charge to
begin to start. Additionally, new batteries may show false
peaks in the first four minutes of charge, as various cells
synchronize their charge state. For this reason the charger
starts with a slow “soft start” charge for four minutes to
allow the battery to cool and get past this point.
Normal operation of the charger is fairly straightforward:
the charger is switched on and both LEDs will flash once for
self test. The charger uses a threshold of 2V (open circuit
July 2015 63
voltage) to recognize that a battery has been connected. The
charger will progressively start and peak the battery. The
battery can be left on trickle charge indefinitely.
Powering the charger
The method of powering the charger depends on what
you want to charge – that is, the voltage and current rating
of the batteries.
As specified, the charger is intended for low-voltage
cordless power tools with batteries of, say, 9-18V. And as it
has a bridge rectifier built in, you can power it with either
AC or DC. Of course, the current rating of the transformer
or DC supply needs to equal or exceed the required charging current.
For batteries up to 7.2V (six cells) a 12VDC or 9VAC supply rated at 2A or so would be ideal (as you can see from
our photos, we used a perfectly good 12V/3A supply from
a perfectly bad laptop PC!).
For higher voltage batteries, you’ll need a higher-voltage
supply – say 24VDC or 15-16VAC for 12-14V batteries
(again, look at surplus laptop supplies – there are plenty
around with 16-18V output at 3-4A) but if you’re wanting
to charge a 24V battery, you’re going to need something
higher – say 30VDC or 24VAC.
It is strongly suggested that a “plugpack” supply or transformer be used; these keep the “bitey bits” out of harm’s
way, especially for beginners.
If you must (and you know what you are doing) a trans-
12mm
12mm
48mm
45mm
5mm
diam
14mm
65mm
ALL
UNMARKED
HOLES:
3MM CSK
12.5
mm
18mm
21mm
12.5
mm
25mm
Here’s a template to help you drill out the diecast case. It is
intended for a standard 117 x 92 x 55mm case.
former could be used and mounted inside a (much larger)
case with the PCB.
Operation
In this “opened out” shot, the lid-mounted PCB is at the top
with the output on the left and the socket for the AC or DC
supply on the right.
64 Silicon Chip
A constant-current supply is generated by several
parallel linear regulators and pulse-width-modulated by
a PIC12F615-I/SN microcontroller. The microcontroller
senses the battery voltage and internally uses an analogto-digital converter to read the battery voltage.
The microcontroller has its own 5V regulated supply
(delivered by a 7805 regulator) and displays the current
charging status on two LEDs.
Four LM317T regulators connected in parallel will each
maintain 1.25V between their OUT pin and ADJ pin. A 2.7Ω
SMD resistor in the output limits the current to a constant
1.8A, or about 450mA per regulator. These resistors also
help to ensure that the load current is spread evenly between the regulators.
A power diode in series with the output makes sure that
current can only flow in one direction; it will be reversebiased and therefore stop current if a charged battery is
connected with the circuit not powered.
A voltage divider, consisting of a 5.6kΩ and 1kΩ resistor
monitor the battery voltage, while ensuring that even with
a high-voltage battery (eg, 33V) the input to the PIC cannot
exceed 5V (the input limit).
From this point, virtually all circuit operation is controlled by the PIC. It monitors battery connection and if one
is present, waits for one minute for the voltage to stabilise
(which could be required if the battery is hot from hard
siliconchip.com.au
Increasing the charging current
As supplied, each regulator has a 2.7Ω SMD resistor in its
output. As well as limiting the charging current to ~450mA
per regulator (or 1.8A total), these resistors help to ensure
that the load is shared equally between the regulators.
If you have a higher-rated battery (eg, ~5000mAh, which
can handle higher charging currents), by lowering this to a
1Ω 3W SMD resistor (2512/6432 size), the total charging
current will approach 5A.
The bridge rectifier (BR1) would need to be changed
to, say, an 8A GBU806 (same pinout), as would D1 – a
BY229 is also rated at 8A.
We haven’t tried these changes, by the way, but the
output current is well within the regulators’ ratings.
The only rider on this is that dissipation will also increase,
so simple heatsinking to the case lid might not be enough.
At a minimum, we’d also add some heatsinking compound
to ensure as much heat as possible is removed. An alternative might be to use a “real” heatsink.
use). It then provides a “soft” charge for four minutes, followed by a fast charge. The fast charge terminates when the
microprocessor senses the “delta V” point of battery charge
or in worst case, if the charge time is exceeded.
It then enters a “trickle” charge state which is intended
to maintain the battery at full charge until it is used.
LEDs
Two LEDs, driven by the microcontroller, give a visual
indication of which mode the charger is in. These modes,
with LEDs on, off or flashing, are shown on the front panel
of the charger.
Construction
Because the PCB is supplied already built and tested (no
need to solder those pesky SMDs!) the only construction
required is to put the charger in a suitably drilled diecast
box and connect input and output wiring.
We used a diecast box not so much for its strength (though
it certainly has that!) but because the diecast box provides
heatsinking for the four LM317T regulators.
The only fiddly bit about this is that the drilling for these,
the two status LEDs and the four PCB mounting screws
must be pretty good! Use our diagram as a guide (or even
a template if your photocopier is accurate).
We mounted the PCB upside-down in the case, so that
it “hangs” from the lid rather than mounts on the bottom.
This is to allow the two status LEDs to poke through the
front panel. While using the lid does not give quite the
same heatsinking as using the box itself, it’s more than
adequate for the task.
All screws through the lid were countersunk so the label
could be glued flat on the panel. There’s not a lot of depth
available in the case with the large electrolytic capacitor
hanging down, so we used the shortest M3 countersunk
screws we could find (6mm). These go into tapped 6mm
stand-offs, with four more M3 screws (5mm pan or flat head,
this time) securing the PCB to the stand-offs.
If you find (as we did) that the stand-offs aren’t quite
long enough to accommodate both screws, you could use
10mm types or place a 3mm inner diameter washer each
side of the stand-offs to make them just that little bit longer.
siliconchip.com.au
And now for something
completely different . . .
Here’s something from the
past that you will enjoy
far into the future!
Radio, TV & Hobbies
April 1939-March 1965
Every article to enjoy
once again on DVD-ROM
ONLY
$
00
62
plus P&P
Only available from
SILICON CHIP
Order
online79
via
See page
siliconchip.com.au
of
this
issue
for
a
or call (02) 9939 3295
handy
order
form
9am-4pm
Mon-Fri
This remarkable archival collection
spans nearly three decades of Australia’s
own Radio & Hobbies and Radio, TV &
Hobbies magazines. Every article is
scanned into PDF format
ready to read and re-read at
your leisure on your home
computer (obviously, a
computer with a DVD-ROM is
required, along with Acrobat
Reader 6 or later (Acrobat Reader is
a free download from Adobe).
For history buffs, it’s worth its weight in
gold. For anyone with even the vaguest
interest in Australia’s radio and television
history (and much more) what could be
better? This is one DVD which you must
have in your collection!
The four LM317 regulators need to be insulated from the
diecast box in the usual way – our diagram shows how the
flat insulating washers and the round insulating bushes (one
set per regulator) ensure there is no shorting to the case.
Input and output leads depend a lot on your application. As we mentioned, we used a surplus laptop supply
so simply drilled a hole in one end of the box and used a
panel-mounting DC power socket which matched that on
the supply.
The output merely goes to some heavy-duty polarised
figure-8 cable with spade connectors on the far end (again,
because these suited our application). You might prefer to
use crocodile clips or some other plug/socket arrangement.
That’s up to you.
Input and output leads all screw into the same PCBmounted terminal block. The PCB is clearly labeled so you
shouldn’t be able to mix them up (did someone mention
Murphy?).
And that’s it! As we mentioned earlier, the PCB is tested
when assembled so it should work straight away.
SC
Wheredyageddit?
The pre-assembled and tested PH-00001 PCB comes
from Shapely Electronics Design (www.shapely.asia) and
sells for $50.00 inc GST, +P&H.
All other components – the diecast box, DC power socket,
standoffs, silicone insulators and grommets, etc, are commonly
available from electronics retailers.
Download the front panel from siliconchip.com.au
July 2015 65
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