[aprssig] Charge controllers

Bruce Gordon bgordon at ltronics.com
Mon May 2 11:15:13 EDT 2005

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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