. The
Battery Saver - A Low-Voltage Cutoff Circuit
for Mobile and Battery Operation
Originally published in The
Printed Circuit, Newsletter of the Tallahassee Amateur
Radio Society, February 2013, page 17
[VISIT HERE]
Edited/Updated December 2023
.
Picture this. You’re parked in your car or truck way out in
'no-man’s
land’ operating on the 20 meter ham band and you've just finished up an
extended rare DX QSO and lost track of time. [A DX QSO is a
radio
chat to someone, particularly in another country] Your
vehicle
was off, needless to say, while running your radio rig, so when you go
to turn the ignition key, of course your engine
won't start! Your rig has just sapped the last bit of power
from
your
vehicle's battery, and to add insult to injury, you'll later
come
to find out you’ve permanently ruined your battery and now you'll have
to buy a new
one. At this point, you've also learned a valuable lesson
about
running a ham radio station from a stand-alone battery rather than the
one your vehicle relies on to turn the engine over with.
If you so happen to be one
of the many hams using lead-acid
batteries, whether they be the good old fashioned 'flooded' kind or the
more pricey ‘spill-proof’ type, then you’ll need to take extra special
care. Lead-acid batteries cannot be ‘run dry’ like their
small alkaline
cousins, but actually only have a useful voltage range of 10.5 to 12.6
volts! Once you’ve depleted the charge of lead-acid
cells below this range, the
electrolyte solution within them is often compromised and the internal
electrodes are often damaged. Sulfur crystals form on the
plate surfaces
and ions can no longer flow. Even worse, dangerous heating
can occur as
well as the expulsion of explosive hydrogen gas.
Aside from any extreme
issues that may arise, the proper charging and discharging of your
vehicle or ham shack's lead-acid batteries is crucial to their
lifespan,
and ultimately the state of your battery's health may determine your
station’s usefulness when
all else fails. Are you keeping a watchful eye?
This article offers a useful circuit designed to cut power to
your rig or other station gear if the voltage on your battery happens
to dip below a chosen preset. The "Battery
Saver"
is a Low-Voltage Threshold Detection Cutoff circuit, or basically a
battery power “kill switch,”
if you will, that uses only a handful of parts and can be customized to
suit. Yes, there are many devices on the market that can do
the
job,
some quite pricey, and the battery protection circuits designed for
lithium-ion batteries are outside of this article's scope [you can see
my solution to that HERE,] but what better
satisfaction is there than from
enjoying something useful you built yourself?
.
. Operation:
Couldn't
be easier, just plug it in! The circuit’s input
section
connects to the
battery [source or terminals] and the output section to your station
equipment. You can use Anderson PowerPoles for convenience.
The Battery
Saver is waiting to cut the power as soon as the supply voltage dips
below your
preset threshold. This safe threshold is typically understood
to be
10.5 volts on a lead-acid battery with a nominal load present.
Many claim that it should be 11.5 volts but do your own
research with the battery's manufacturer specifications.
. THE LM339 VERSION - - - .
As originally published, this version
uses 1 of the
4 comparators included in the LM339 IC and the entire circuit can fit
within an Altoids' mint tin. For an even smaller footprint
you
can experiment with an Op-amp version using a smaller 8-pin IC.
There are two
controls, a potentiometer sets the voltage cutoff threshold
and a DPDT (double pole, double throw) switch chooses operation
behavior. When the device
is first powered up, the switch has to be moved to "Active" mode.
When the voltage at the battery is above the threshold, the
relay
will
engage, allowing power to flow from the battery to your rig or
other
devices. The relay, will of course disengage if the voltage
goes
too low.
In Active mode, if a low-voltage detection has caused power to be
cut to the output but the voltage has risen above the
threshold
again, the relay will re-engage and your equipment
will be given power again. This is great for stations using
'alternative' power-generation or in instances when alternate charged
batteries are
switched in to take over for a dying one. The second mode is
called "Battery Save"
where less current is used to run the relay during operation, and as a
manual check, once
a low-voltage cutoff has occurred, the circuit and your equipment will
remain ‘off’ until the option switch is moved back over to "Active"
mode and the input voltage rises above the threshold. This
can be
crucial for helping to protect sensitive equipment from brown-outs or
prohibiting the continued use of a battery that may have a functional
issue.
Circuit Overview:
A
relay is used to control the circuit path between your battery and
equipment via its contacts. A comparator is used to weigh a
sample of
the supply voltage against a reference voltage. When a
proportional sample
voltage which is provided by a resistor dividing network remains higher
than
the reference voltage which is set by a Zener diode, the comparator
yields a logic-level output that is used to switch on a series of
transistors, causing the relay coil to energize and its contacts to
engage. Or more simply put, when the sample voltage drops
below the reference voltage the signal to the transistors is dropped
and the relay is shut off, causing the connection between the battery
and your equipment to be severed. Your battery is then safe
from
depletion and your equipment is safe from undervoltage.
Circuit Detail: D1
protects the circuit from an accidental reverse-polarity connection.
A 1N4001 should provide 1 amp of protection, however, the
circuit's supply voltage will be reduced by at least 0.6 volts as
expected from the diode's voltage drop. A safety fuse should
always be used if possible on any non-self-contained
circuit, especially within the mobile environment. Blade type
fuses such as ATC and ATM are excellent choices for
portable operation as those can be
most likely borrowed from any vehicle if an emergency replacement is
needed in the field. The switch S1 sets the behavior of circuit
operation. In one position, one side of the switch bypasses
the current
limiting resistor/capacitor circuit from the relay and the other side
allows the
circuit to always feed from the battery source. In the second
position,
S1 allows the relay's current reduction circuit to function
and the Battery Saver's circuit
power is now controlled by the relay. When the relay opens
its contacts
the circuit is no longer allowed to function until S1 is manually moved
back to ‘Active’ mode. C1, a 1000 micro-Farad electrolytic
capacitor,
not only stabilizes any spurious source voltage fluctuations, but is
used
to keep the circuit 'alive' in the small instant that S1 is in
mid-swing. .
.
K1 of course, can be any relay that
will qualify within the circuit's operational parameters and should
safely handle any equipment load required from the battery with
contacts rated for current at the operational voltage. The
relay’s
coil is the main concern. The main battery cut-off bus lines
should also be
composed of a wire gauge or PCB trace width/thickness that
will
support the load current. In my 5 amp capable prototype, I
used a small PC-mounted relay that only consumes 50 milliamps at the
coil and can handle up to 10 amps through the contacts.
Though
not tested myself, I designed
the presented circuit to use a Bosch-style automotive relay.
The
relay
for automotive/mobile radio equipment operation (assuming one 100 watt
transceiver,) should handle up to 40 amps at the contacts and should
not draw more
than 150 milliamps at the coil.
You
should be able to provide adequate battery protection cut-off
for a small station with this circuit. The 'current reducer' circuit
(C4 and R5) works by allowing current to pass through the capacitor
with enough duration to engage the relay's contacts before the
capacitor saturates during its charge cycle and blocks continued DC
flow. R5 then allows just
enough current to pass as to support the continued magnetic latching
state of the relay as they typically
require as little as 10% power to maintain operation.
I’ve
found through a little experimentation that R5 needs to have around two
times the resistance as is measured across the relay's solenoid coil.
You should see at least a 1/2
reduction in relay operation current used with this implemented.
You can adjust the
values as needed by increasing C4's value if more time is needed to
engage the relay contacts and increase R5 to yield the most benefit,
but keep in
mind that as you reach the coil's limit for sustained contact, it may
fail in operation or fail to close, especially if there is significant
initial inrush current in the attached equipment. A
comparatively
tiny savings in relay current is going to be insignificant in most
applications with this implementation, but becomes more important for
smaller battery systems and as the Battery Saver may be left running
during time when equipment is not used, that savings may be realized.
In the LM339's
implementation, the
proportional sample battery voltage and the Zener reference voltage is
presented to high-impedance pins (4) and (5) respectively.
D2. A 5.5 volt Zener diode, through its Zener-effect
breakdown voltage, from
network R2/R1 allows a (somewhat) reliable 4.8 volts to be applied to
pin (5). Any arbitrary voltage presented to pin (5) is
acceptable as
long as it's below the lowest intended operating voltage of this
circuit. Zener diodes are perhaps the easiest way to create a
reference voltage for any comparator circuit. Check out IMSAI
Guy's 5-part Zener diode tutorial series on YouTube: .
.
The sample
voltage used for comparison is taken at the user-defined potentiometer
resistor network pad R1 and is also a smaller proportion to
the
supply voltage. In this
case, a 10k ohm potentiometer was used. R2 should be around
1/4
the
ohms value of R1 so that the voltage bias from the network should
remain more positive. A variable bench power supply is
suggest
when setting the
threshold in your constructed circuit. C2 and C3 are simply
used
to
condition the inputs of the comparator. LED D4 through R6
allows
you to
see an indication of the comparator output status. .
. Considerations:
The LM339 comes with four comparators in its package but only one is
used here.
With the remaining three sets you could add extra features
such as an over-voltage cut-off or indicator lights.
I would highly suggest using the other three comparators to
add
hysteresis to the threshold, one set to a half-volt above the cutt-off
threshold and the others to be used as a latch circuit, all to mitigate
unintentional relay chatter when the battery voltage is sitting at the
threshold. [See Below] Your choice of relay is determined by
the
current limit of
the transistor you're using and the current handling specs of the
relay's contacts. This circuit was intended for 6-18 volt
operation, but accommodations could be made for higher voltages and
currents, which is above the scope of this article. When
working
with 12 volt automotive-type battery systems, ALWAYS use fuses at your
(+) source points and
bond your circuit ground to the chassis or enclosure if metal is used.
Fire is not an option on this circuit. .
. . THE TRANSISTOR VERSION - - - .
The following updated version uses 8
transistors
operating together in a comparator circuit. There is less
space
advantage with this version but it has the advantage of being a better
teaching tool for how comparators work. There are a few
enhancements with this revision including only needing a POWER button
and an OFF (or kill) button to operate it along with the potentiometer
trimmer for adjusting the threshold voltage. Also, this
version
has a bit better immunity to the relay chatter (buzzing) that can
happen as the battery drains under normal operation and crosses over a
threshold that is too gradual.
Operation: There
are three
controls as mentioned, a potentiometer to set the voltage cutoff
threshold along with only two push buttons, one for On and the other
for Off. When the device
is first powered up, the POWER button has to be pressed and held until
the relay engages. This should happen relatively
instantaneously
if the input power is above the threshold. If above the
threshold
voltage, the relay will allow power to flow from the battery
to
your rig or
other
devices - as well as to the circuit itself. Releasing the
POWER
button should cause the circuit to stay operational. The
relay,
will of course disengage if the voltage
goes
too low below the threshold. If a low-voltage detection has
caused power to be
cut at the relay to the output but the voltage has again risen above
the
threshold, the circuit will remain disengaged in the OFF state only
until the POWER button has be pressed again. In this
revision, it
is assumed that a loss in power means a battery issue that needs
addressing before power is given again. If you wish to have
uninterruptable power you should add a UPS battery bank to your supply
line in this instance. Circuit Overview:
As in the previous circuit, an "automotive"
relay is used to control the circuit path between your battery and
equipment via its contacts. A comparator made from only
transistors is used to weigh a
sample of
the supply voltage against a reference voltage. When a
proportional sample
voltage which is provided by a resistor dividing network remains higher
than
the reference voltage which is set by a Zener diode, the comparator
yields an output signal that is used to switch on a series of
transistors, causing the relay coil to energize and its contacts to
engage. Or more simply put, when the sample voltage drops
below the reference voltage the signal to the transistors is dropped
and the relay is shut off, causing the connection between the battery
and your equipment to be severed. Your battery is then safe
from
depletion and your equipment is safe from damaging undervoltage. Circuit
Detail: Most
of the operational theory is the same as with the previous version but
this one is a bit of a different animal than its IC counterpart.
Positive power presented to relay (K1's) contacts from the
host
battery or generated power supply is fused at F1. 30 amps
should
be a good enough rating to handle most mobile radio equipment.
K1's contacts should be rated to handle more than your
required
power load. To activate the circuit, switch (S1), a standard
non-resistive pushbutton must be pushed and held for the required time
for K1 to swing. This may be quick enough to where a quick
tap on
the button is required. F2, a 10 amp fuse keeps anything like
a
transistor short from blowing the 30 amp fuse, but this power through
S1 is routed through a reverse-protection diode, (D1). The
entire
circuit should only draw around 14 ma with the relay off, 150-250 ma
when the relay is first engaging and only around 25 ma when the relay
is in contact mode, not requiring sustained coil momentum. IF
the
relay is not engaging or is disengaging when it's not supposed to, you
will have to investigate the specs of this relay (more to follow.)
When K1 is engaged, power is routed to the Out terminal to
power
your devices, LED D4 is lit from engaged power to indicate operation
and, through reverse-polarity diode (D2), power is fed to keep the
circuit going, forming an essential latch. The circuit and
relay
should only see the voltage drop of D2 in operation (0.7v typ.). .
.
The heart of this circuit is the comparator.
This one is a common design, even found in the 555 timer's
Threshold section and consists of a one-or-the-other configuration
where preferential switching is caused by an imbalance between each
symmetrical half. Q1+Q2 is a current mirror, providing
an exact current flow down to Q5 and Q6. Q5 and Q6 are a
Darlington pair that is switches on 'hard' when the threshold current
provided via R1 and R2 is applied to Q5's base. This causes
current to be drawn from Q1+Q2. The other side, Q3+Q4 and
Darlington pair Q7 and Q8 do the same thing, but each competing current
mirror has to also draw through R4, depending on which side sees more
current from each respective Darling switch pair, that side
'teeter-totters' and 'wins' causing current to flow from either Q2 or
Q3 exclusively. I just gave a pretty poor description but
there
are many YouTube videos on the subject.
One important design consideration here to mention is in
using
"matched" transistor pairs for the current mirrors. In this
circuit, it is not a crucial requirement but absolutely is if you use
the comparator of current mirror arrangement in other more analog
circuits. An easy way to find (more) matched transistors is
to
use one of those Arduino-based component testers found all over online
as they will give you the Beta and other information in which you can
use to grade your stock. Other than that, mismatched
transistors
shouldn't be an issue here.
Zener
diode (D5) provides the comparator's reference voltage when combined
with R7 to form a voltage divider network. "~730" ohms was
shown
in the schematic as that value seemed to work to provide 4.7 volts at
the base of Q7. The value of the Zener diode is NOT a crucial
value and you can use any value as the Threshold value set by R1 will
adjust for it. R7 should be high enough to limit reverse
current
potential through D5, but you can derive your own value by using a
temporary potentiometer in series with something like 500 ohms, and
adjust it for a good reference voltage that is anywhere from 2 to 7
volts, which will also depend on the Zener diode you use.
Zener
diodes are only used for making reference voltages so their
manufactured value rarely deviate from that range.
The Threshold potentiometer, R1 can be any value
from at
least 5k on up to 500k. R2 is used to limit current to the
base,
especially when R1 is biased towards +v. If you experiment
and
find that a certain setting just with a pot gets you a workable
cutt-off voltage threshold, then you can use just representative fixed
resistors and save the space. R6 is just the emitter current
limiter from the comparator Darlington switch sets.
As described for the previous circuit,
the relay
(K1) is current limited with C2 and R8. You CAN omit these
parts
if it gives you a better response from your relay. When first
activated, inrush current to K1's coil flows through C2 until it is
charged, allowing all the current needed by the coil to overcome any
kinetic contact movement limitation. Once the capacitor is
charged and restricts DC, current will by bypassed by R8, but will be
restricted. When a relay coil has drawn its contacts, it no
longer needs a lot of magnetic force to keep them engaged. So
less far less current is needed to hold things in place. This
may
prove a big save over time when you're using this on a smaller battery
with a QRP rig running. * The value of R8 should be at least
twice that of the measured resistance of the coil. To little
resistance and it will be a useless addition, too much and you risk
losing the contact when a sudden current draw is placed on your batter.
A 1k trimmer pot may be perfect for R8 and you can tweak it
in
action.
D3 is the standard kick-back
protection diode for any inductor loaded through a transistor.
If
you're having problems with K1 de-energizing after a while, you may try
adding another diode with the anode at the collector of Q10 and cathode
at ground. Q9 is another standard 3904 small signal NPN.
It's current is limited to 200 ma, so, through R10, Q10, a
2N4401
is highly suggested for switching in K1 as it will be rated to handle
up to 600 ma. C1 is a power filter/tank capacitor for the
circuit, keeping any sudden power inrushes from tripping the threshold.
If you get any excessive buzzing from the relay when moving
slowly through the threshold point, try omitting this as a test.
Finally, the pushbutton (S2) provides a tidy way to sever the
connection between the battery and the equipment as it sinks the
current at the base of Q9, causing the relay to disengage, shutting
down the entire circuit, or in other words, the "Kill" switch.
This should not be used if using a switch to hold the POWER
switch down in a fixed state of always 'on'. Setup:
Using a variable bench power supply, continuously hold down on the
POWER button or use a jumper to maintain the switch, adjust the power
supply to equal your desired cut-off, for example: 10 volts.
Connect a load to the output that is not crucial to being cut
on
and off rapidly, like 12-volt lighting and then slowly adjust the
Threshold pot (R1) until the comparator deactivates the relay (K1).
Switch off or unplug the attached load and see if K1 comes
back
on. If so, further adjust down R1 a bit so that the desired
threshold is set to consider some power fluctuation. Release
S1
and try again until the threshold is set.
Considerations:IF
you desire K1 to latch back on as soon as power is restored (as is an
option in the original circuit), use a SPST switch instead of a button.
Keep in mind that any fluctuation in supplied voltage due to
the
removal and re-introduction of any load may cause the relay to switch
wildly, probably causing damage to any attached load. Both
circuits presented here may have an issue with relay chatter (buzzing,
whining) at the threshold line! This is due to the analog
nature
of both types of comparators. With a digital or
microcontroller-based circuit, this is not an issue as once you reach a
certain number, action is going to happen in a controlled manner.
Nothing's perfect and these circuits
were
designed expressly for this article. I can’t
guarantee the optimal operation, and it's primarily
provided as a learning example. I've pondered a few
configurations
for the
'option switch' portion of the first circuit and felt to just keep it
simple. The transistor version is more for fun, but you could
probably build it after an apocalypse!
As mentioned, a nasty problem occurred as I was testing in
'Active' mode. When the voltage [being reduced] began to
cross
under the threshold the relay would chatter
severely with a resultant flickering in output! Many factors
contribute
to this effect including circuit supply voltage to the comparator being
subject to fluctuations influenced by changing load levels presented at
a continuously re-engaging relay contact. The clear solution
is
to add hysteresis to the comparator which can be done on the first
circuit if using an op-amp instead where it could be designed in, and
no doubt, the same could be done with the transistor version, if
applied to the Q5+Q6 and Q7+Q8 sections.
For the LM339 version, for the relay
chatter, it
makes a lot of sence to employ one of the three remaining unused
comparators on the IC, used in tandem where hysteresis can be had by
using the same Zener reference to feed both comparator, while adding a
resistor divider to the second comparator, fed off the Threshold pot,
so basically, one of the voltage sample inputs is reduced lower than
the other. It would have to be run through an exclusive-or
(XOR)
gate to only
activate the relay transistors when each comparator is in
agreement, or that part can be done with the last two onboard
comparators. Of course, the Saver circuit should rely on a
substantial capacitor to sustain operation for a small window.
Since the threshold chattering issue
doesn't seem to occur in 'Battery Save' mode it would be suggested to
only use this device in that setting and use ‘Active’ as a reset.
You can also use a delay-movement relay which has a
built-in
hysteresis which is designed to mitigate this. With that, if
you
choose to experiment with this circuit, feel
free to innovate and let me know! Would your version fit in a
mint tin?
I've used this circuit in many
operations since
this article was first written in 2013 [now 2024] and was not pleased
when it would 'sing' or relay chatter. If and when I revamp
this
circuit to fix that issue, I'll repost an updated schematic as well as
a Gerber file for sending off to a fab. for your own PCB's which would
make this an easy kit. This would also likely be an Arduino
or
ATTiny85
solution which will utilize their built-in stable voltage reference.
Or rather, since the microcontroller circuit would run from a
5
volt regulator, a reference voltage to its ADC will work.
Once
the
low-volt threshold has be crossed, a MOSFET would de-energize the relay
and the MCU would shut down according to set instructions rather than
analog gray zones. Once power is restored enough to
power the MCU again, the relay would not be re-energized until a higher
threshold was met. And there are more considerations as well,
but
I hope this circuit gives you a bit to tinker with. . Hey, the weekend is here.
Get your deep-cycle batteries charged up for some quality mobile QSO'ing and don't get
caught with your volts down!
Practical Electronics
Projects By Mike, K4ICY 
