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(published in Electron #4, 2001) Introduction The following situation probably is easily
recognizable: you quickly like to drill a small hole somewhere around the
house and the battery of your battery electric drill / screw driver is flat.
This is the problem with many battery operated accessories that you are not
operating too frequently. After charging and applying the tool, you forget
about this until the next time and the cycle will start all over again, even
if you promise yourself this will not happen any more. Same thing is going on for the batteries you
are using for your field experiments. Since you have to plan these operations
somewhat better and longer beforehand you remember to charge the battery and
if the last time was not too long ago, you may really start-off with full
available power. Otherwise you end-up buying a new battery since these
systems tend to deteriorate over time when being left unattended for too
long. Some time ago I decided to end this recurring
nuisance and go for an more definitive solution. This solution currently is
in operation for over ten years with the same battery, proving the approach
really works. That is why I decided to also show this simple system to fellow
radio-amateurs for them to end comparable problems. In the approach I decided the following
considerations should be taken into account: -
should be simple and reliable -
battery plus charger should be easily transportable (light weight) and
reasonably weather proof, and so: -
should generate low additional heat during charging -
requiring as little maintenance as possible -
ensuring no more flat battery because of low attendance Let's first take a good look at the battery. During field-tests and on holiday operations,
I want sufficient power to allow operation of a 'standard' transceiver for
about two hours. For this I take my TS440S as an example, that will generate
100 Watt of PEP in 50 Ohm when in SSB mode. According to the manufacturer
this transceiver is delivering optimal performance at 13,8 V. The receiver will take around Ir = Itr = 3 + 15 / Even the most notorious 'claimer of
attention' will in general take longer to listen than to speak. When taken
the listening periods as long as the transmitting time, the average current
drain for this set may be calculated as: Iav. = ( Ir
+ It ) / 2 = (6 + 2) / 2 A.
= For a total of 2 hours of operating time, the
battery needs to have a total capacity of: 4 x In general battery capacity is being
specified at a temperature of Note: Take care when operating batteries next
to (in the same enclosed space) as electric / electronic equipment. A
standard car battery is producing some vapor when being charged, that is
allowed to escape the battery enclosure the avoid internal pressure. This
vapor is mainly consisting of water vapor, but will also carry traces of
sulfuric acid. The latter will have strong corrosive effects that is
disastrous to all contacts, connectors etc. If possible stay away from this
type of battery for application in and near your expensive equipment. In the
car this battery is situated in a well vented cabinet especially for this purpose.
The battery we will be using should be of the
gas-tight and / or gel type, to avoid above problems. When utilizing the battery on average for two
hours every day during the holidays (a fair estimate), the charge should be
fully recovered during the other 22 hours. Since charging is not a totally
efficient process we will take this into account by estimating charging to be
a 75 % efficient process. Therefore the charger should be capable of
supplying a continuous charging current: IL = 8 / 22 / 0,75 = 0.49 Ampere. Note: The charge current has been calculated
with respect to the utilization of the battery. This could also be the
required total capacity of this battery which then should have a minimum
value of the same Many 'better' battery charger types will
measure the battery voltage during the charging process. When a certain value
is reached (around 13,8 Volt for a standard car battery), charging current
will be switched off to be replaced by a maintenance charge at a few percent
of maximum allowed charging current, to prevent self-discharge. This maintenance regime however is not to be
continued indefinitely since electrode structure will be damaged in de long
run ( after a few month). Higher quality chargers therefore will change over
to short bursts of maintenance charging about once every week. Still better chargers will be tuned to a
current / voltage regime, that is actually recommended by (gel-) battery
manufacturers. In this regime, the battery will be charged up to the voltage
as mentioned with a charging current of
maximum According to literature, the best charging
regime is to not supply a continuous DC current, but a pulsating current,
e.g. as being available directly from rectified mains transformer without
further smoothing. This charging regime will remove / prevent gas building up
around the electrodes. This regime is said to enlarge battery life up to 30
%. Above considerations may appear to be rather
complicated but 'translate' into a very simple circuit, as may be appreciated
from figure 1.
Figure 1. In figure 1 it is clear no buffer capacitors have
been applied; with the battery in position, system voltage is determined by
the battery and is 'stabilized' between 11 and 13,8 Volt. The battery is
being charged by current pulses directly at the rectifier bridge. Next 'remarkable' fact is the battery to be
connected directly to the rectified bridge, without any current-limiting
action. Background to this is this current-limiting action to burn additional
power (heat) because of the voltage across the current limiter; this we like
to avoid in closed cabinets. Since no current regulator is available, charge current is to a certain extend
depending on the mains voltage: -
too high mains voltage will not easily be around since life expectance
of all electrical appliances are depending on
this. Never-the-less high(er) charging currents
will never overload the circuit but will only shorten total
charging time, because of circuit dimensions. -
too low mains voltage, that is equally rare in our society, may take
the battery somewhat longer to get fully
charged, but will always deliver charging current under 'normal' mains
voltage tolerances . The mains transformer
is dimensioned such that this will take care of the current limiting action.
The transformer dimensions therefore should be around those specified and not
be too far off. In the following table a few combinations of loaded and
unloaded transformer voltages have been supplied. For this test, voltages
will be loaded by a 22 Ohm resistor of sufficient power (a few unloaded voltage loaded voltage (22 Ohm) tap at 13.0 V 11.0 V 1.8 V 15.5 V 13.0 V 4.3 V 18.0 V 14.5 V 6.8 V Although best solution is a
transformer with a voltage tap, an 'artificial' tap may be created by means
of resistors. These resistors are connected in series with each other and in
parallel to the transformer. The tap diode will be connected to the tap at
the resistors. In the diagram, R1 is connected to the top connection of the
transformer, R2 to the bottom connection. Both resistors are of a 1 Watt
type. unloaded voltage R1 R2 13.0 V 280 47 15.5 V 330 120 18.0 V 390 220 The circuit itself is straight forward. Potentiometer R1 will set the 'battery fully
charged' set-point at 13,8 Volt. This voltage has been selected because of
the TS440 specifications and also because 'battery full' specification by
various manufacturers is between 13,8 and 14,2 Volt. When the 'full' voltage
has been reached, comparator IC1 will trip and will light diode 'full'. At
the same time diode 'charge' will be extinguished and the charging regime
will be switched from 'current' to 'voltage', at a single rectified voltage.
This switching-over is accomplished with a relay that will not drain the
battery when the charger is not connected to the mains, since this circuit
will be switched of with no mains voltage present. The resistors around the input of the
comparator are taking care of some hysteresis to prevent this circuit from
continuously switching between 'charge' and 'full'. Tuning the circuit is simple and will start
with R1 in the lowest position: -
Use an accurate voltmeter to measure the battery voltage during
charging with this circuit. -
As soon as the battery voltage is 13,8 Volt, slowly turn R1 up until
the 'full' led is lighting up. Note: When turning too quickly through the
trip point, trip voltage will be too high, shortening battery life. Better
start over with R1 in the lowest position because of hysteresis. The second comparator is a 'luxury' addition
that will tell you when to stop discharging the battery (safe-guarding
against too deep discharging since this will also shorten battery life). This
addition almost comes 'for free' since two comparators are situated within
one housing anyway. Potentiometer R2 will be set to the 'battery low' trip
point at 11,0 Volt. This again a compromise value in a range of batteries,
specified at battery empty voltages between 10,5 and 11,2 Volt. Tuning is easiest with R2 at maximum, the
charger not connected to the mains and the battery removed. -
Connect and external power supply at the position of the battery, that
is tuned to 11,0 Volt by means of the same accurate voltmeter as above. -
Slowly turn R2 down until led 'empty' lights up This circuit needs no hysteresis so R2 may be
easily set to the accurate position. The relay at the extreme right-hand side may
come as a surprise. With this addition the function of 'automatic emergency
power' will be available to the shack, which is also the reason this charger
will become part of the 'standard' shack equipment, switching on and off with
main(s) power to the shack. This facility will ensure the battery to always
be fully charged, to be available when being required elsewhere and to
provided a firm reason the battery
system should be always connected to the mains supply. The main (DC) shack supply will be connected
to the 'external input' terminals with all further shack equipment to be
connected to the V+ and V- terminals. As soon as the 'main(s) shack supply'
is switched on, the relay will switch from emergency to regular power to all
equipment and the 'external' led will light up. Note this facility to also come in handy when
chasing for EMC sources. All mains connections may be switched off with your
shack equipment still operational. By successive switching on mains connected
equipment, your internal EMC source may be quickly located. Make sure the emergency relay will allow high
currents through the switching contacts, e.g. Finally This 'battery plus charger in one closed
cabinet' is currently in use for over ten years, still utilizing the same
battery from the beginning, without any maintenance so far. Regularly it is
following me at holiday trips and apart from field-power it also is serving
its purpose when the mains conditions 'elsewhere' are less reliable. This charger-cum-battery could also be a
(cheap) alternative to the regular supply of your transceiver and usually is
also more reliable. I should advise against using this system as
a (cheap) alternative to the experimenters power supply though. Since the
internal resistance of the battery is very low, a small mistake during an
experiment may lead to high damage to the equipment and maybe even to
yourself. Bob J. van Donselaar, on9cvd@veron.nl |
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