[aprssig] Battery & Charger for APRS Station

Ray McKnight shortsheep at worldnet.att.net
Mon Apr 21 02:15:37 EDT 2008

SLA (and non-sealed lead acid, they're basically the same thing)
realize their longest life when kept at or near 100% charge.  Caveat,
As long as they are NOT overcharged.  The best way to determine charge
State is specific gravity, but that isn't possible with an SLA, so
Chargers generally rely simply on voltage.  A healthy battery will
Be very close to the known, full charge voltage, after it's allowed
To rest, usually about 1 hr.  It's even better to have a temp sensor
To monitor battery temp to avoid excessive heating during charge, but
Usually this is only done with larger batteries being charged at or near
Maximum rate (30-50 amps).  Excessive heating may cause the batt to
Vent, a bad thing and a precursor to premature failure or at least

Since you do have a fairly constant drain on your battery, the charger
To overcome that PLUS the constant loss of voltage which occurs even
Any load, I believe usually around 10%/month.  Keep in mind, the hardest
part of any lead-acid battery charge is the last 10%, it requires far
Time and more energy to get it from 90 to 100%.  Thus, most folks
See that it's "close enough" and quit before 100% is achieved, which
To premature failure.  (since few ever bother to measure specific

Another trick you can use to get maximum life out of a lead-acid is to
perform an equalization charge about once per month.  Over the routine
Use, your cells will become slightly unbalanced in their charge and
"Zapping" the battery with 18+ volts while monitoring temperature will
equalize cells thus prolonging overall life, avoiding or slowing the
inevitable "dead cell" syndrome where one cell fails because it stops
accepting charge.  But few chargers have this mode and most users don't
any way to do it manually themselves.

If your battery is installed in a remote location this might be
Many sailors (particularly sailboats) use this as they rely heavily on
battery power underway and charging is very challenging.

One thing I'd be weary of is that you likely have no isolation of the
charger from the TNC/transmitter.  Be sure it's providing a clean DC
Voltage and not introducing any unwanted things like hum into the TX
Audio.  It might be fine during receive but TX could get into the
And give you problems.  Look at signals on a scope to see any unwanted
Or ripples.

It sounds like you're okay, just a few concerns to look at for 100%
of mind, the batt should be fine with that charger and last as long as

One last thing - those el'cheapo "float chargers" so popular and sold
By the zillions at Home Depot and Costco etc for $20 or less are sure
Death your battery, as they are completely stupid, they don't measure
ANYTHING even voltage, and simply continually spit voltage at your
Even when it finally reaches 100%.  BIG PROBLEM especially on RV's as
Seldom have any type of smart charging, it's simply brute force or
trickle" which will cook your batteries, that's why you replace them
2-3 years (also because you buy those $39 Costco "deep cycles" which
really and VERY cheap disposable crap batt's).

-----Original Message-----
From: aprssig-bounces at lists.tapr.org
[mailto:aprssig-bounces at lists.tapr.org] On Behalf Of John Habbinga
Sent: Sunday, April 20, 2008 10:42 PM
To: TAPR APRS Mailing List
Subject: [aprssig] Battery & Charger for APRS Station

I am installing a digipeater in a location that I want to have
reliable battery backup power.  For the last month or so I've been
testing out a setup where the radio and TNC run off of a 12 volt, 35
Ah sealed lead acid battery.

The battery is connected to the radio/TNC and it is connected to the
battery charger.

The charger is a CTEK Multi US 7000.  It is a multiple stage charger.
When it finishes charging it STOPS charging completely.  Every ten
minutes or so the charger tests the battery and if it needs to be
charged, then it adds current.  So it is not a 'trickle' charger.

My observation is that about once an hour the charger kicks in and
tops off the battery.  I don't get any interference from the charger
at all, even on HF frequencies.  You can find out more on the charger
I'm using at http://www.ctek.com/EN-US/Default.aspx

At 20+ charging cycles per day, am I prolonging the battery life by
frequently recharging when the battery is at such a shallow state of
discharge, or shortening it?

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