Dual Battery
Lifesaver
by
Nicholas
Vinen
This small board provides an easy way to protect rechargeable batteries
from being completely drained if a device is accidentally left switched on.
It can work with devices that run from a single battery, or two separate
batteries. Both thresholds are fully adjustable, and it can handle several
amps per battery, drawing just a few microamps when off.
E
arly next year we will be
publishing a Battery Vintage
Radio Power Supply project.
One of the authors suggested that the
low-battery cut-out section of the circuit could be useful on its own, and
we had to agree with him.
So, ahead of the full project we have
produced a separate PCB which contains just that portion of the circuitry.
It can be used with just about any
device powered by 3.6-15V DC at up
to 5A per output. Typically, it is configured so that both outputs are cut
off if either falls below its individual
voltage threshold.
However, it can also be reconfigured
only to cut the outputs off if both fall
below the threshold, or you can build
a slightly simpler version for use with
a single battery.
No heatsinking is necessary as the
MOSFETs used for switching have minimal dissipation, around 100mW at 5A.
It has provision for an optional
onboard power indicator LED, and
also provides for an SPST (or similar) switch to disable the outputs, so
that you can use a small, low-current
switch as a power switch.
We previously published a very
small single-battery Lifesaver in the
September 2014 issue, which has been
quite popular. Besides being small,
its other advantage is that it can
handle quite a bit of current;
20A or more.
However, it used quite a few
SMDs and was a bit tricky to
build, tricky to set up and had
a limited adjustment range
once built.
This version uses all through-hole
parts and so is nice and easy to build,
and not all that much bigger despite
being able to handle two batteries.
16
This one is also straightforward to set
up, with a single trimpot allowing the
cut-out voltage to be adjusted over a
wide range for each channel.
Circuit description
MOSFETs Q1 (and Q2, if fitted)
connect the supplies at CON1 and
CON2 to the outputs at CON3
and CON4 when switched on.
They are switched off, disconnecting
the outputs, if either (or both) supply
voltages are below defined thresholds.
When switched off, either via the
switch S1 or due to a low battery
voltage, the circuit only draws about
10µA from the higher voltage battery
and about 2µA from the other.
Presumably, you would notice the
device has switched off and either
recharge the cells or swap them for
fresh ones.
But if for some reason you forget and
leave the device switched on, it would
be several months before this minimal
current drain could damage the
cells. That’s why this
circuit was designed
with a low quiescent
current in mind.
When the power
switch (S1) is
Shown here
mounted on four
insulating pillars, the Dual
Battery Lifesaver uses
all through-hole components so
is very easy to build.
closed, current can flow from whichever battery has a higher voltage,
through small signal diodes D1 and
D2 and then switch S1, into the input
of REG1.
This is an ultra-low-quiescentcurrent, low-dropout 3.3V linear
regulator. It powers micropower dual
comparator IC1 and also serves as a
voltage reference.
A fraction of this 3.3V reference
is fed to the two inverting inputs of
the comparators, at pins 2 and 6 of
IC1. The fraction that is applied to
those pins depends on the rotation of
trimpots VR1 and VR2. These set the
low-battery cut-out voltages, and they
can vary the voltage at those inputs
over the full range of 0-3.3V.
The actual battery voltages are
applied to the non-inverting inputs,
pins 3 and 5, after passing through
fixed resistive dividers. While these
two dividers use the same resistor
values, they are in different orders.
So around 1/3 of the CON1 voltage is
applied to pin 3 of IC1a, while about
2/3 of the CON2 voltage is applied to
pin 5 of IC1b.
In combination with the nominally 3.3V reference and trimpots
VR1 and VR2, you can set the
switch-on voltage thresholds to
anywhere from 0-10V for the
CON1 battery, and 0-4.5V for
the CON2 battery.
Those ranges suit Li-ion,
LiPo or LiFePO 4 batteries
with one or two cells in series, respectively.
You can easily change these ranges by
changing the dividing resistor values.
We suggest that you try to keep the total
resistance around 3.3M ; lower values
will increase the quiescent current, and
significantly different values will alter
Practical Electronics | November | 2021
SC DUAL
Dual
Battery
Lifesaver
BATTERY
LIFESAVER
Fig.1: the Dual Battery Lifesaver is built around micropower comparator IC1
and micropower regulator REG1, which supplies IC1 and also acts as the voltage
reference. IC1 compares fixed fractions of the battery voltage(s) with the voltages
at the pot wipers, and if the battery voltages are high enough, it switches on
transistors Q3 and Q4, which in turn switch on MOSFETs Q1 and Q2.
the hystersis percentage (as described
below). Table 1 shows some possible
combinations for other voltage ranges.
Hysteresis is provided by 10M
feedback resistors between the comparator outputs and non-inverting
inputs. This has been arranged so that
the hysteresis is a fixed percentage of
the voltage.
The source impedance for the noninverting inputs is 687.5k in both
cases (1M ||2.2M ). This forms a
divider with the 10M feedback resistor, giving a hysteresis percentage of
687.5k ÷ 10M = 6.875%.
So for low-battery cut-out voltages
of, say, 3.3V and 6.6V, that would give
you switch-on voltages 6.875% higher,
or 3.525V and 7.05V respectively.
The resulting hysteresis voltages are
around 0.23V and 0.45V.
When both batteries are above their
switch-on voltages, output pins 1 and
7 of IC1 are high, at 3.3V. Therefore,
the base-emitter junctions of NPN
transistors are forward-biased and
so both conduct, pulling the gates
of MOSFETs Q1 and/or Q2 low and
lighting LED1 (as long as LK1 is in the
position shown).
If either battery falls below its
switch-off voltage, the corresponding
transistor switches off and thus Q1 and
Q2 switch off.
The high base resistors for Q3 and
Q4 (2.2M ) are chosen because if one
battery voltage is low but the other is
high, current will still flow from the
corresponding comparator output and
this will increase the current drawn
from the higher voltage battery (usually the one connected to CON1).
The 2.2M base resistors are the
highest practical values to minimise
this, and determine the minimum
value for LED’s current-limiting resistor as 12k . That means that LED1 has
to be a high-brightness type.
If LK1 is moved to the alternative
position and LK2 is fitted, rather than
being connected collector-to-emitter,
Q3 and Q4 are in parallel, collector-tocollector. In that case, if either battery
voltage is above the defined threshold,
the associated NPN transistor will pull
the MOSFET gates low, and so both
outputs will be connected to the inputs.
On/off switch
If you don’t need a power switch on
the supply, you can simply place a
shorting block on CON5. CON5 is provided as a convenient way to switch
power on and off, and you only need
an SPST switch that hardly has to
handle any current.
But with S1 off, there will still be a
small quiescent current drawn from
the two batteries due to the resistive
dividers which remain connected.
This is around 1µA for every 3.3V.
That should mean the batteries last for
around a year with the device switched
off via S1.
If you need to reduce the battery
drain further when off, you will instead need to use a DPST or DPDT
Features & specifications
• Two input/output pairs
• Individual low-battery cut-out voltage settings
• Passes through 3.6-15V at up to 5A per output
• Both outputs switch off if either (or optionally both) voltage falls below its threshold
• Fixed 6.875% hysteresis
• Quiescent current when off: around 10µA from the higher voltage battery and 2µA from the other
Practical Electronics | November | 2021
17
Parts list – Dual Battery Lifesaver
1 double-sided PCB coded 11111202, 70 x 32mm, available from the PE PCB Service
4 2-way terminal blocks, 5.08mm pitch (CON1-CON4)
1 2-pin header or polarised header (CON5)
1 4-pin header (LK1,LK2)
3 shorting blocks/jumper shunts (CON5,LK1,LK2)
1 SPST panel-mount switch (S1; optional)
4 tapped spacers (for mounting the board)
8 M3 x 6mm panhead machine screws (for mounting the board)
Semiconductors
1 MCP6542-E/P dual micropower comparator, DIP-8 (IC1)
[element14, RS, Digi-Key, Mouser]
1 S-812C33AY-B2-U micropower low-dropout regulator, TO-92 (REG1)
[Digi-Key, Mouser]
2 IPP80P03P4L04 P-channel logic-level MOSFETs, TO-220 (Q1,Q2)
[SILICON CHIP Online Shop Cat SC4318 or element14, RS, Digi-Key, Mouser]
2 BC547 100mA NPN transistors, TO-92 (Q3,Q4)
1 high-brightness LED (LED1)
2 1N4148 small signal diodes (D1,D2)
Capacitors
2 1µF 50V multi-layer ceramic
Resistors (all 1/4W 1% metal film, unless otherwise indicated)
2 10MΩ
4 2.2MΩ
2 1MΩ
1 100kΩ
1 12kΩ
2 1MΩ mini horizontal trimpots (VR1,VR2) [eg, element14 108244]
switch to cut the battery connections
to CON1 and CON2. That switch will
need to handle the full load current
for each battery.
Note that the batteries may still
suffer from a small amount of selfdischarge, so it’s still a good idea
to check and charge them every six
months or so.
Construction
The Dual Battery Lifesaver is built on a
double-sided PCB coded 11111202 . It
measures 70 × 32mm and is available
from the PE PCB Service. Refer now to
Fig.2, the PCB overlay diagram, which
shows where all the parts go.
As you read the following instructions, keep in mind that if you are
using the device with a single battery, you can omit D1, D2, Q2, CON2,
CON4, VR2 and some of the resistors
– see Fig.3. You will need to add a
couple of wire links, shown in red,
which you might be able to make from
component lead off-cuts.
Start by fitting all the resistors.
While you can determine the value of
a resistor by reading its colour bands,
it’s best to use a DMM set to measure
ohms to verify this, as some colours
can look like other colours under certain types of light.
If you are happy with the 0-10V
adjustment range for the battery connected to CON1 and 0-4.5V for CON2,
use 2.2M resistors for RU1 and RL2,
and 1M resistors for RL1 and RU2,
as shown in Fig.1. Otherwise, refer to
Table 1 to determine the best resistor
values to use.
With all the resistors in place, follow
with the two small diodes, D1 and D2.
These must be oriented with their cathode stripes facing as shown in Fig.2.
Then fit comparator IC1. Make sure
its pin 1 notch and dot go towards the
top of the board, as shown. We don’t
recommend that you use a socket for
reliability reasons, although you could
if you wanted to.
Next, fit switch header CON5. You
can use a regular or polarised header,
or just solder a couple of wires to the
PCB. If you want the supply always to
be on, you can either place a shorting
block on CON5 or solder a small wire
link in its place.
The next step is to fit small signal
transistors Q3 and Q4. They are the
same type; ensure their flat faces lie
as shown in the overlay diagram, and
bend their leads out gently to fit the
pad patterns. Follow with regulator
REG1, which is in a similar package
to those transistors, then install the
two ceramic capacitors where shown.
Now mount the two trimpots, which
are the same value. Follow with the
four terminal blocks. Make sure that
their wire entry holes face towards
the outside of the module, and note
that the side-by-side blocks are spaced
apart and so should not be dovetailed;
mount them individually.
Next, fit the two TO-220 devices,
which mount vertically. Ensure that
their metal tabs are oriented as shown.
You could crank their leads so that their
tabs are flush with the PCB edges, allowing heatsinks to be fitted later, but
their dissipation should be low enough
that heatsinks are not necessary.
All that’s left is to solder the four-pin
header shared by links LK1 and LK2 in
place, followed by LED1. How you do
this depends on what your plans are.
If you don’t need an external poweron LED indicator, you can simply push
it right down (with its longer lead on
the side marked ‘A’, opposite the flat
on the lens) and solder it in place.
If you want it to be externally visible, depending on how you will be
mounting the board, you may be able
to mount it on long leads and have it
project out the lid of the device.
Or you could chassis-mount the LED
using a bezel. You could then either
solder flying leads from its leads to
the PCB pads, or solder a 2-pin header
(regular or polarised) onto the PCB and
then solder leads to the LED with a
plug or plugs at the other end.
Fig.2: the PCB has been kept as small as possible while still being
easy to build, handling a decent amount of current and providing for
easy wire attachment and mounting. Assembly is straightforward
but make sure that the IC, terminal blocks, MOSFETs, diodes and
LED are correctly orientated. Use the component overlay above
in conjunction with the same-size photo at right to assist you in
component placement. Note that the values of RL1, RL2, RU1 and
RU2 need to be chosen from the table overleaf.
18
Practical Electronics | November | 2021
Fig.3: the same PCB can be fitted with fewer components if you only have one
battery to protect, as above. Again, the two resistors shown in red need to be
selected from Table 1. You will also need to add two wire links, shown in red.
Testing and adjustment
It’s best to test and adjust the Dual
Battery Lifesaver using a variable DC
bench supply; ideally one with current
limiting. The following instructions
assume that you used the resistor
values shown in Fig.1. If you changed
them, you might need to alter the suggested voltages.
Place one shorting block on CON5
and another across the middle two
pins of LK1/LK2.
Start by setting VR1 and VR2 at
their maximum settings. If you’ve
built the two-battery version, bridge
the positive inputs together (you
don’t need to bridge the negative
terminals as they are connected on
the PCB). Now set your bench supply
to around 4V and the current limit
to a low value, then switch it off and
wire up either input (CON1 or CON2)
to the supply.
Switch the supply on and watch
LED1. It should not light yet, and the
current drawn from the supply should
be low (under 1mA). If it’s significantly
higher than that, you could have a
board fault, so switch off and check for
short circuits and incorrectly located
or oriented components.
If all is well, wind the voltage up
to about 8V, then rotate VR1 anticlockwise until LED1 lights up. Then
reduce the supply voltage slightly and
check that LED1 switches off.
Now rotate VR1 and VR2 fully
anti-clockwise, set the supply voltage
to your desired cut-out voltage for
whichever of the two is lower, then
rotate either VR1 or VR2 clockwise
slowly until LED1 switches off. Then
Voltage
range
Upper
resistor
Lower
resistor
0-4.5V
1.0M
2.2M
0-5.25V
1.2M
1.8M
0-6.3V
1.5M
1.5M
0-7.8V
1.8M
1.2M
0-10V
2.2M
1.0M
0-12.3V
2.4M
820k
0-15V
2.7M
680k
Table 1 – suggested resistor pairs for
various cut-out voltage ranges.
increase the supply voltage to your
other desired cut-out voltage; LED1
should switch back on. Rotate the
other trimpot slowly clockwise until
the unit switches off.
You have now set both battery cutout thresholds. If you want both outputs to switch off whenever either battery voltage drops below the threshold
you’ve set, the unit is now complete.
If you only want it to switch off when
both batteries fall below their respective limits, remove the jumper from
LK1/LK2 and insert two jumpers on
the 4-pin header side-by-side.
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