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Battery charger and automatic emergency power

(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

 

 

Battery

 

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 = 2 A. of continuous current, virtually independent of audio power setting.  At transmitting, the set is taking about 3 A. at no modulation, this to peak at 15 A. during SSB modulation. The ratio of peak voltage to average for 'normal' speech at my analyzer is at least a factor of five, which I take will also go for modulation current in SSB-mode. Total average current in SSB mode therefore may be estimated to:   

Itr = 3 +  15 / 5 A. = 6 A.

 

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. = 4 A.

 

For a total of 2 hours of operating time, the battery needs to have a total capacity of: 4 x 2 A.hrs = 8 A.hrs 

 

In general battery capacity is being specified at a temperature of 25 °C. At lower temperatures capacity is diminishing rapidly to 90% at 0 °C and 80 % at -10 °C. This should better be taken into account for your next polar circle expedition.

 

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.

 

 

Charger

 

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 8 A.hrs for a fresh device and about 10 A.hrs taking capacity loss over life-time into account. I was able to acquire a somewhat larger battery but it should be noted the calculated charger capacity is always sufficient to the application since this is based on the battery utilization, not on battery capacity.

   

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 0.1 A / A.hrs of capacity. Hereafter charging is stopped and the charger will switch over to become a voltage source of the same value. This type of charging will provide maximum battery life time.

 

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 %.

 

 

The circuit

 

Above considerations may appear to be rather complicated but 'translate' into a very simple circuit, as may be appreciated from figure 1.

 

 

                                                        Figure 1. Battery charger / emergency back-up

 

 

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 Watts) that should be 'handled' with some care as this is heating up quickly. For voltages in-between those presented, loaded values have to be interpolated. The highest and lowest voltages in the table also are the limiting values for this circuit.

 

 

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.

 

 

Emergency power

 

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. 10 A., since switching-over to emergency power may be requested right in the middle of a transmission. The current to the 'emergency relay' will be delivered by the external power source so will not drain your emergency battery.

 

 

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