There are two major configurations for this board: you can make it
so that it only trickle-charges your cells, or you can add the ability
to fast-charge the cells as well. Fast-charging requires more parts,
is more complex, and your cells won't last quite as long. For many,
the benefit of getting your cells charged in just an hour or two is
worth this cost. (Trickle-only charging takes at least 10 hours.)
Because these two configurations are so different, the information
below will refer to fast-charge and trickle-charge circuits. Although
fast-charging circuits also do trickle-charging, assume that when I talk
of trickle-charging that I mean a trickle-only setup.
U1
U1 is an adjustable linear regulator. It's set up as a constant
current source to force charge back into the batteries.
The standard part here is an LM317T. You can buy these everywhere,
they're well understood, and they perform just fine for the purpose. The
only problem with 317s is that you must use a heat sink with them if
you're going to set up a fast charger.
There are several low-dropout (LDO) regulators which are more or
less compatible with the LM317, such as the LM1086. Because LDOs will
work with a smaller voltage drop across them, they can be set up so that
they dissipate less power. This means the heat sink area on the circuit
board is sufficient for most purposes. If you add an external heat sink
anyway, that will lower the regulator's temperature further, which may
help it to last longer. The downside is that LDOs are more expensive,
and you need to add C1 to stabilize the regulator.
LDOs are only valuable when you can set the power supply voltage
precisely. If there's more than a couple of volts dropped across the
regulator, you might as well use a standard regulator.
Optional? No. It's used in both fast- and trickle-charging
circuits.
U3
This is the MC3334x charge controller chip. Its job is to
monitor the charging process and to control the fast-charging
regulator to ensure that the battery pack is charged
safely.
There are two main variants of it, the '340 and the '342. The '342 is
a bit cheaper, but I recommend you get the '340 instead. The difference
in the '342 looks like it only makes sense for cold-weather charging
or other odd situations. For room temperature charging, the '340 is a
better chip.
Optional? Required if you want a fast-charging circuit.
Otherwise, leave it out.
C1 (tantalum)
This cap is there to stabilize the regulator if you're using an LDO
type like the LM1086. See your regulator's datasheet for the specifics,
but 10 µF works for most LDOs. It's not needed for standard linear
regulators like the LM317.
You definitely don't want an electrolytic cap here. The ESR of
electrolytics is too high for the purpose, and you don't want to put
an electrolytic near a heat source because that will shorten their life
dramatically. Tantalum caps are expensive, true, but if you want to save
the cost of this cap, use a regular regulator instead of an LDO so you
don't need C1 at all.
Optional? Required only if you use an LDO in U1.
Largest Part Size: 0.100" pin pitch dipped cap
C2, C3 (film, ceramic)
These are bypass capacitors for the
charge controller chip. Ceramics are fine here,
but you can use polyester film caps instead if you happen to have some
on hand. The circuit won't perform materially better with film caps
here, though.
Optional? If you use the charge controller chip, no.
Largest Part Size: 0.300" × 0.100"
LED
This LED indicates the charge state.
For fast-charge circuits, it's off when there is no power to
the circuit, blinking when fast-charging, and solidly on when
trickle-charging.
For trickle-only circuits, it turns on when there's external power
applied.
Optional? Yes. Leave it out if you don't want a charge
indicator.
R1
This is the trickle charge current limiting resistor in fast-charge
circuits:
R1 = (Vs - 1.4) / It
where:
Vs = Supply voltage to the battery board
It = Desired trickle charge current
The "1.4" represents the two diode drops from D2 and D3.
The trickle charge current shouldn't be more than 0.1C for NiMH
cells. (E.g. 70mA for 700mAh cells.) Lower is okay, especially if you
mainly just need trickle charging to maintain the charge level in the
cells.
Optional? Needed in fast-charge configuration only.
R2
This is the regulator's current setting resistor:
R2 = 1.25 / If
where:
If = Desired fast charge current
"1.25" is the voltage drop from the regulator's OUT to ADJ pins.
Although the voltage drop across R2 isn't precisely 1.25V when you use
R3 (the two make a voltage divider) it's close enough to
the real value when R3 is much larger than the value of R2.
The fast charge current shouldn't be more than 1.0C for NiMH
cells. (E.g. 700mA for 700mAh cells.) You may well want to go lower,
either to keep heat down or to make the cells last longer or because
your power supply has a lower current limit. I find that a charge
current of about 500mA is a good balance between speed and power for
750mAh cells.
Keep in mind, the lower the resistor value, the higher the current
and thus the higher the power dissipation in the resistor. For
resistors down in the 2 Ω range, you should use at least a 2W
resistor. Higher-wattage resistors usually have more surface area, so
they don't get as hot at any one point. I like to use 5W cement wirewound
resistors here. They're cheap, available everywhere, accurate enough,
and come in the right range of values. Good alternatives are silicone
wirewound and metal oxide.
For trickle-charge circuits, the value of this resistor will be up
in the tens of Ohms, so the power dissipation will be much lower than
in fast-charge circuits. 1/4W or 1/2W resistors will be sufficient,
depending on the resistor value and the charge current.
Optional? No.
R4, R5
These divide the battery voltage down to between 1 and 2V. The
charge controller uses this divided-down voltage
to sense the charge state of the batteries. Whenever the voltage at
the divider is outside this range, the charge controller will leave
fast-charging mode.
The simplest way to set this divider up is to use the
Configuration Calculator.
If you want to calculate your own resistor values, figure that your
cells are at about 1.55V each while fast-charging, and can be about
0.9V each when fully used up. Pick a division factor such that the full
voltage of the battery pack is divided down to just under 2V, and the
minimum voltage when divided by the same factor is still above 1V.
Beware that the battery pack will discharge through this divider.
NiMH cells self-discharge in about 30 days, so if the current through
the divider is about 10× lower than would be required to discharge
ideal cells in 30 days, the self-discharge rate will dominate the pack's
discharge time. Therefore, I suggest a maximum current of about 0.1mA for
AAA NiMH cells, and 0.3mA for AA NiMH cells. If you use the configuration
calculator, it gives divider values that obey these rules.
Optional? Needed in fast-charge configuration only.
R6, R7, R8
These parts are for setting the backup fast-charge termination
mode. Normally the charge controller can terminate fast-charging correctly
by sensing the pack's voltage, but there needs to be a backup method
in case something goes wrong with the cells. For instance, when a cell
starts getting weak, the stress of fast-charging can make it overheat.
If you don't want to think too much about this, I recommend that
you use the Configuration Calculator and select time-based backup
charge termination. The calculator will give you a reasonable R6-R8
configuration.
Temperature based fast charge termination is safer because a fault
in the battery pack is likely to cause overheating. If that happens,
you want the charger to turn off. With time-based termination, the
charger will eventually turn off, but only after the pack has been
too hot for too long. It's worth the effort to figure out how to set
up temperature-based fast-charge termination. Read through the MC3334x
datasheet to find out how to set these part values.
Optional? Needed in fast-charge configuration only.
RLED
This is the power indicator LED's current limiting resistor. It
can be a 5% carbon type; the exact value isn't at all critical.
RLED = (V+ - Vf) / If
where:
V+ is the power supply voltage, rail to rail
Vf is the LED's forward voltage drop
If is the desired current through the LED
1 mA gives enough brightness for a power indicator with most LEDs,
but some may require a bit more. You don't want it to be too bright,
or it's annoying. Typical values for RLED are 1 KΩ to 10 KΩ,
depending on the power supply voltage and the LED being used.
Optional? Leave this out if you aren't using a
charge indicator LED.
Cells
Arguably the single most important part you will buy for this
project. Although you can use NiCd cells with this charge circuit,
NiMH cells are a much better idea: they are better for the environment,
they have much higher energy density, and their disadvantages relative
to NiCd don't matter in this application. You can readily find NiMH AAAs
in the 700 mAh range and AAs in the 1800 mAh range. You can find higher
capacity types, but you'll save a lot of money by choosing the ones a
step or two back from the highest capacity types available.
All NiMH cells are not created equal. From the
links page, visit The Great
Battery Shootout for info on the best NiMH cells currently on
the market. The author recommends that you get your cells from Thomas Distributing, and I
second that recommendation. Yes, their web designer is apparently on acid,
but their service and prices are great.
For those of you who want to place orders to as few places as possible,
I have secured a deal with Mouser for their Sanyo 650mAh AAA cells for
$1.89 each. (Part# 639-HR-4U) Sanyo makes Kodak's NiMH cells, and Thomas
sells 750mAh Kodak AAAs for this same price. From my reading of these
two cells' respective datasheets, I think these are actually the same
cell, and are only rated differently because Kodak is using aggressive
consumer marketing, while Sanyo is selling to the OEM market so they
are giving conservative specs for engineers. Even if the Sanyo cell is
somewhat inferior, it's still a good product. Sanyo cells score right at
the top of the Shootout ratings I reference above. To get this price,
put "QT# 24120630WY" in the Comments field on the web order form, and
make the order attention to Sheila Williams.
BH
These are the battery holders.
A full AAA configuration is six 3×AAA battery holders giving
18 cells. You can also use 2× holders on the board. Using various
combinations of 3× and 2× holders, you can come up with many
different cell count arrangements.
There are no silkscreen outlines showing where the AA holders go,
to keep the silkscreen layer from being confusing. You just have to
find the holes that don't go with the AAA holder positions. A full
configuration is two 4× holders plus one 2× holder for 10
cells. The upside of using AA batteries is their capacity is almost three
times as high as for AAA's. The downsides are that you can't get as many
cells on the board so the output voltage is lower, and AA batteries are
larger in diameter so this isn't the best setup if the battery board
is in the case with the amp board. AAAs are still best for high voltage
and compact configurations.
Remember to put jumpers across any unused battery holder positions!
F1
This is a safety fuse. I have no specific reason to believe it's
necessary, but it's a power circuit, so I put it in as a matter of
course. What the heck, it's cheap and the space is there.
There are no surge currents in this circuit, so your fuse's value
should be between your highest charge current and the maximum output
current the regulator is capable of. This ensures that the fuse will
blow even when there's not a short circuit, but just an overcurrent
condition somewhere.
The board takes 5×20 metric fuses.
Resistor Sizes
The resistor pads on the battery board are 400 mils apart, sufficient
for up to 1/2W resistors in most resistor lines. However, 1/4W is
sufficient for all resistors except R2.
Setting the Power Supply Voltage
If the power supply voltage is too low, you won't be able to
fully charge your battery pack. If the voltage is too high, the
regulator has to drop the excess voltage, so it runs
hotter than it needs to. Ideally, you want to set things up so that
you have just barely enough voltage to charge the cells fully while not
allowing the regulator to drop out of regulation.
The Configuration Calculator asks you for your battery board
configuration and then gives you a minimal power supply voltage level. If
you don't care how the calculator works, just pick your power supply
voltage to be as close to the value it recommends without going under. If
you're curious, read on.
The key to picking the power supply voltage is to look at all the
voltage drops in the charge path. Figure that each cell will require
about 1.55V across it at peak during fast charging. Then add in all the
diode drops along the charge path and the 1.25V across R2. Whatever is
left between this and the power supply voltage is dropped across the
regulator. Depending on the output current, an LDO will require about
1V across it to maintain regulation. A standard regulator requires 2 to
3V across it at minimum. See your regulator's datasheet for details.
If you have the luxury of an adjustable bench supply, you can use it to
pick your power supply voltage experimentally. You simply keep tweaking
the voltage up as the battery pack charges until the current draw stays
steady. When the supply voltage is too low, the current from the regulator
drops below where it should be, so increasing the voltage increases the
current. When the voltage is high enough for the regulator to maintain
regulation, the current stops at the level set by R2. You will have to
keep tweaking it upward as the batteries charge. Whatever voltage you
end up with when the circuit goes into trickle charge mode is the minimum
voltage you should use to charge that battery board. You might make the
power supply put out about 1V more to provide a little headroom.
Suggested Configurations
12× AAA, 2.8 hour fast charge
This is a pretty conservative setup. The voltage you get is
sufficient for many setups, the charge time is pretty good, and it
uses very generic components and resistor values.
Supply: 24V
U1: LM317
R1: 1 KΩ
R2: 4.7 Ω
R4: 100 KΩ
R5: 10 KΩ
16× AAA, 2.8 hour fast charge
This is the same as the 12-cell configuration but with 16 cells,
which is the highest number you can charge with a 30V supply and a
standard regulator. It's useful if you need more voltage than the
12-cell configuration provides and you don't want to deal with the
problems that result from maxing out the battery board's 18-cell
capacity.
Supply: 30V
U1: LM317
R1: 1 KΩ
R2: 4.7 Ω
R4: 100 KΩ
R5: 7.5 KΩ
18× AAA, 1 hour fast charge
This is the fastest-charging, highest-voltage setup without
getting ludicrous. Use this when you must have high voltage and fast
charging. You'll have to take special care to keep heat under control.
Supply: 32V
U1: LM1086
R1: 1 KΩ
R2: 2 Ω
R4: 120 KΩ
R5: 8.2 KΩ