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[aprssig] Battery & Charger for APRS Station

Steve Noskowicz noskosteve at yahoo.com
Mon Apr 21 19:59:17 UTC 2008


--- John Habbinga <kc5zrq at gmail.com> wrote:

> I am installing a digipeater in a location that I want to have
> reliable battery backup power. ...snip...

Hi John,

   Looks like you already spent the money on your APRS digi backup, but here's
another 2 cents anyway.

  As far as I am familiar with lead acids, all the other remarks don't conflict
with what I know, but I'll add what I know in terms of basics.

   The correct term for lead acid batteries is "float" not trickle.  Float
implies a voltage in lead acid and trickle implies a current for nickel
chemistries.  The standard and correct float circuit is a linear voltage
regulated power supply, NOT a simple transformer and bridge.  The bridge type
is probably what Ctek is referring to in their ripple page....Shouldn't even be
seriously considered for floating - just use for charging, then stop it.
  From everything I've seen, the RESTED, open circuit voltage is acceptable for
measuring lead-acid charge state.  VERY, VERY roughly 11.8 empty, 12.3 full.

Some thoughts, that you may or may not already know.
  While there are some differences in the other posts (Ray Wells' 14.4 boost –
13.4 droop cycle), my experience is the following.  In the ideal, Floating is
done with a current limited, temperature compensated, regulated, constant DC
voltage of around 13.8.  I believe it is a positive tempCoef (hotter = higher)
and I mean pure DC.This is what is recommended by battery manufacturers and is
used in telecom and cellular systems. 
  Therefore, a fixed DC supply is not quite correct unless things are at a
constant temp and the supply output voltage is adjusted for that temp.  I do
NOT know what the effect is on life if you do not do the temperature
compensation correctly, but over charging an SLA is the killer no-no.  
Overcharging a non-sealed type makes the gases and removes water, changing the
spec-grav.
   The supply is connected to the battery and equipment.  The equipment
normally runs on the power supply/charger while the battery is simultaneously
float charged continuously.  Lead acid chemistry draws what it needs to stay
charged when fed the *correct* DC voltage.  Lithium happens to be similar.
  The charger must also have a current limit which limits the current going to
the battery.  Each battery has it's own safe maximum charging current.  Sealed
versions can tolerate much less charging current in general.  When charging,
the chemical reaction is being reversed.  When over charging, it is only
decomposing the water into H and O and generating heat and pressure( in the
sealed types).  Only extremely  small amounts of the gasses can be tolerated in
sealed types, therefore, floating is much more critical.
  I wonder about the "equalizing" mentioned by  Ray McKnight.  This is a
popular concept in the high end Li-Ion radio controlled aircraft batteries. 
They charge cells independently.   
   It seems to me that in the lead-acid case, this will over charge the lower
A-H capacity cells just a bit and use more water in order to full charge the
higher capacity cells (assuming series cells).  This means high-cap cells will
be cycled not quite as deep as the lower capacity cells.  Versus letting the
higher cap cells only reach not quite full charge.  If 95% charge is worse than
using water, then it must be good.  However, you can't expect to over-charge
SLAs, right?

   Depending on how the Ctek units turn on and how your battery voltage drops
under load, the radio/transmitter will run on the battery for a while, then on
the charger when it senses the voltage drop to its turn-on threshold.  In
either case, the battery will be supplying the rig for some time until the
charger senses a need to charge.  This looks like many mini cycles rather than
the classical constant float – then discharge only when power fails. 
Unfortunately, I can't comment on this mini-cycle mode vs. the classical pure
float mode.  I recommend asking Ctek – they obviously think it is better, or at
least want potential customers to think so. 

  Also, The float voltage should be what the manufacturer recommends for *that
specific* battery.  For longest life, It is advised not using any simple rule
of thumb float voltage for all lead acids.  There are variations due to
construction.
I can't comment on the relative advantage of the Ctek method, but perhaps you
can talk to them and get their information.  

   There are typos on the ctek page regarding ripple.  I wouldn't call the
transformer/bridge a "linear" charger; I'd call it cheap, consumer.   To me
"Linear" means linear regulator.  
   Is it 2 or 40 amps ripple at the 9.7 amp charge???  They obviously refer to
the bridge type which is junk.  How about comparing to the TempComp regulated,
current limited type?
   I don't think the 120 HZ. ripple of a bridge causes much RFI as they say,
but switch-mode can obviously be a killer if not tamed.  
   The yellow line looks red to me.

This either helps or confuses. Sorry if the latter.
73, Steve, K9DCI



-- 
73, Steve, K9DCI


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