[aprssig] Charge controllers

Ron Stordahl ron.stordahl at digikey.com
Fri May 6 14:15:51 EDT 2005

I ran a node consisting of an old Micor 40 watt mobile transceiver plus 
MFJ1270C tnc with TheNetX1J4 firmware where the power supply consisted 
of a car battery and a 5 amp fully automatic battery charger, the last 
two items bought at KMart.  It worked for about 5 years.  It was the 
battery charger that gave out.  I never expected it to last that long, 
so I was not disappointed.

Ron, N5IN

Bruce Gordon wrote:

>Hello all,
>I agree with a lot that has been said lately about floating batteries but
>find it incomplete and wrong on a couple of points.  I believe that the
>original inquiry was for a repeater yet none of the comments address
>compensation for temperature variations typical of repeater sites and only
>one mentioned variation of float voltage depending on the battery type.  All
>lead-acid batteries are NOT the same and all need variable float voltage to
>get the best life if their temperature
>varies more than about 25 deg F.  I have never seen a commercial charging
>system at a telco or comm site that didn't have temperature compensation.
>Car charging systems do too. Many cheap chargers don't.
>Many charging arrangements will work for a little while but the following
>steps can give trouble free performance and 80% capacity after 10 years or
>more.  Values are for 12V lead acid batteries.
>1. Float the battery at a voltage recommended by the manufacturer.  Don't
>guess.  Data is available from most reputable suppliers if you are
>persistant.  This may vary from 13.5 to 14.3 at 68 deg. F.
>2. Use a thermistor or other temperature sensor to vary the float voltage
>with temperature.  A typical sealed battery that recommends 13.8V at 68 F
>needs 15.3V at -4 F and 13.25 at 120 F.  Both Solar Electric and Absolyte
>state that battery life will be cut in half with an error of only **50 mV**
>from the recommended value.  Don't guess if you want your batteries to last.
>If the temperature varies rapidly day to night, a temperature sensor on the
>battery case will track better than one inside the controller.
>3. Provide the charger with a constant current limit (not foldback) to limit
>its dissipation when recovering from substantial battery discharge.
>Calculate the dissipation at high line and an 11V battery. For a "50 amp"
>(ICAS) Astron, this is 27A for example.
>4. Put in a fuse or breaker with a rating at or just above the current limit
>between the power supply and the battery in case the regulator fails.
>5. If the power supply cannot stand battery voltage on its output with the
>input turned off, put a diode, preferably  schottkey, between the supply and
>the battery.  Set the float voltage on the battery side.  All recent Astron
>linear supplies and many others are designed to take this reverse voltage
>and do not need an extra diode.
>6. Put a low voltage cut off circuit between the battery and its load so the
>batteries are never discharged below about 10.5V (check specs on your
>batteries).  I used automotive relays before but new FETs can do the job
>without the 60-100 mA drain of the relay coil.
>7. For liquid electrolyte batteries (most deep cycle) not in vehicles, a
>timer that raises the float voltage about 10 percent for a half hour each
>month, a so called equalization charge, will stir the electrolyte to prevent
>8. The charger should be capable of  supplying at least 1.5 times the
>average equipment demand.  A higher rating will give faster recovery after a
>long power outage but small, repetitive discharges to handle short, peak
>transmitter loads have little affect on life.
>I don't recommend the Astron battery charging modification.  It gives very
>slow recovery and a temperature variation mildly opposite that which is
>I no longer use anything but sealed batteries designed for standby or solar
>electric service.  To me, the dependable service life and freedom from
>corrosion , gas and spillage hazards of these batteries is more than worth
>their higher initial cost.
>Bruce Gordon N6OLT
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