almost identical so don’t get them mixed up; only the tiny
coded markings on the top of each set them apart.
Tack solder the central pin to the pad in each case, then
check that the other two pins are centred on their pads and
that all pins are in contact with the PCB surface. If not, re-
heat the initial solder joint and nudge the part into place.
Then solder the two remaining pins and add a little extra
solder to the first pin (or a bit of flux paste and heat it) to
ensure the fillet is good.
Next, solder IC1, IC2, Q1 and Q2. They are all in eight-
pin packages and must be oriented correctly. Identifying
pin 1 can be a bit tricky. For IC1 and IC2, you have to find
the chamfered edge, which is quite subtle. Pin 1 is on that
side. Q1 and Q2 have pin 1 marked by a much more clear
divot in the corner of the package. But you can also orient
IC1 and IC2 by matching the position of the markings up
to our photo.
In each case, make sure the device is positioned cor-
rectly and tack solder one pin, then as before, check the
locations of the other pads are correct and solder them
before refreshing the first joint. If you accidentally bridge
two pins with solder, use a little flux paste and some solder
wick to clean it up.
The only remaining SMD parts which are polarised are
diodes D1 and D2. Fit these now, ensuring the striped end
goes towards the top edge of the PCB, as shown in Fig.
and marked with ‘K’ on the PCB. Then solder the two
3W resistors in place. Follow with the remaining SMD
ceramic capacitors and chip resistors, as shown in the
overlay diagram.
For the two-pin devices, make sure that you apply the
soldering iron long enough so that the solder adheres to the
PCB and the component. Adding a little flux paste to the
PCB pads before positioning the part will make this easier.
There is a single component on the underside of the
board, a 10MΩ resistor positioned between CON1 and VR1.
Solder it in place, but use a minimal amount of solder, so
that you don’t plug the through-holes underneath. You can
add more solder later after CON1 and VR1 are in place.
All that’s left is to solder trimpot VR1 with the adjustment
screw oriented as shown, and a pin header for CON1. We
used a normal pin header, but a polarised header would be
a good idea if you’re going to use a plug to make connection
to the batteries so that it can’t be accidentally reversed. If
it is reversed, D1 and D2 should prevent damage but the
balancer won’t work!
Or you can solder the battery wires directly to these
three pads. They only need to be rated to handle 300mA
per board; medium-duty hookup wire should be more than
sufficient, even if paralleling multiple boards.
Testing and set-up
Connect your batteries in series, then connect the negative-
most terminal directly to the negative terminal on CON1.
Do not connect the junction of the two batteries to the
Balancer just yet.
Ensure that the total battery voltage is well above the
threshold and that they are reasonably close to being bal-
anced. You can ensure they are balanced by charging both
independently and then connecting them in parallel via
a low-value, high-power resistor (eg, 1Ω 5W) and leaving
them for a few hours. The voltage across the resistor should
drop to a very low level once their voltages equalise.
Now connect the most positive terminal to the positive
pin of CON1 via a 1kΩ resistor and check that LED1 lights
up. LEDs 2 and 3 should remain off. Measure the voltage
across the 1kΩ resistor. It should be under 20mV. If it’s under
5mV or over 20mV, disconnect the battery and check for
errors in your PCB assembly or battery wiring.
Assuming the voltage is within the specified range, re-
move or short out the 1kΩ test resistor and then connect
the junction of the two batteries to pin 2 of CON1. LED
and LED3 may light up. If so, rotate the adjustment screw
in VR1 until they are both off.
Now check that there is no balance current flowing by
measuring the voltage between TP1 and TP2, and between
TP3 and TP4. In each case, the reading should be zero. If
you get a non-zero reading between TP1 and TP2, current
is flowing through Q1a. And if there’s a voltage between
TP3 and TP4, current is flowing through Q2b.
Since you started out with balanced voltages, this should
not be the case, so adjust VR1 further until you get a zero
reading across both pairs of test points. Ideally, VR1 should
be adjusted to halfway between the point where the volt-
age starts to rise between one pair of test points, and the
point at which the voltage rises across the other pair of
test points. This ensures the balancing will be, for lack of
a better word, ‘balanced’!
The maximum reading you should get between one pair
of test points should be 8.8V. Any more than that and you
risk the resistor dissipation rating being exceeded. In this
case, disconnect the batteries and change the 10MΩ resis-
tor right next to VR1 on the top side of the board with a
slightly lower value (eg, 9.1MΩ or 8.2MΩ) to reduce the
current limit.
If that doesn’t fix it then it’s likely that the current-limiting
circuitry is not working so you should check for soldering
problems or faulty components.Parts list
2× 12V Battery Balancer
1 double-sided PCB, coded 14106181, 31.5 x 34.5mm
3 3-way right-angle or vertical pin header (CON1)Semiconductors
2 LT1495CS8 dual micropower op amps, SOIC-8 (IC1,IC2)
1 ZXRE330ASA-7 micropower 3.3V reference, SOT-
(REF1)
2 DMC3021LSDQ dual N-channel/P-channel power
MOSFETs, SOIC-8 (Q1,Q2)
1 2N7000 N-channel signal MOSFET, SOT-23 (Q3)
1 green LED, SMD 3216/1206 (LED1)
1 red LED, SMD 3216/1206 (LED2)
1 blue LED, SMD 3216/1206 (LED3)
2 S1G 1A schottky diodes or similar, DO-214AC (D1,D2)
Capacitors (all SMD 3216/1206 X7R ceramic)
2 100nF 50V (measure value before installing!)
1 10nF 50V (measure value before installing!)Resistors (all SMD 3216/1206 1%)
6 10MΩ (Code 1005)
1 6.8MΩ (Code 6804)
2 5.6MΩ (Code 5604)
1 2.2MΩ (Code 2204)
1 1MΩ (Code 1004)
2 390kΩ (Code 3903)
2 100kΩ (Code 1003)
2 10kΩ (Code 1002)
2 4.7kΩ (Code 4701)
1 1kΩ (Code 1001)
2 27Ω 3W (SMD 6331/2512)
[eg, TE Connectivity 352227RFT]
1 47mΩ [eg, Panasonic ERJ-L08KF47MV]
1 200kΩ 25-turn vertical trimpot (VR1)
For tips and tricks on soldering SMD components, refer to the
July 2010 EPE article: How to Solder Surface Mount Devices.