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[aprssig] Backup power

Bruce Gordon bgordon at ltronics.com
Mon Dec 11 06:18:14 UTC 2006

I have been involved with designing and installing standby battery systems
for many years. during this time, I have had the help of several experts in
battery design and have cleaned up the wreckage of too many systems that
used the "rules of thumb" recently suggested.

For a reliable installation:

Use only sealed "gel cell"  or SLA batteries designed for float or standby
service.  Flooded batteries require "equalization" to stir the electrolyte,
water replacement and better ventilation to minimize corrosion.  (all should
have some ventilation.)

Temperature compensate the charging voltage if the battery temperature
varies by more than  10 deg. F.  Get the voltage vs. temperature curve for
the specific battery you will use from the manufacturer.  This may take some
digging.  Be insistent!  Most battery designers agree that departure from
the ideal curve by more that 50 mV either way AT THE BATTERY TERMINALS on a
12V battery will cut the service life in half!  Isolation diodes will ruin
the regulation unless remote sensing is used.  A typical charging curve
varies from 15.3V at 0 deg F to 13.3V at 120 deg F.

Provide a cutoff to limit the discharge voltage.  The limit is commonly
around 10.5V for a 12 volt system.  Again, get the data from the
manufacturer.  Even a single "run flat" can seriously damage a battery.

Provide a current limit that prevents overheating of the charging supply
under low battery and high line conditions.  Use a two stage charger only if
the time to recover the last 10 percent of capacity is critical.  Never use
a 3 stage charger as "equalization" is unnecessary and can be damaging to
SLA cells.

If more than one battery in parallel is needed, use only batteries of the
same type from the same manufacturer and preferably from the same lot to
minimize unbalance and circulating current.

Results?  How about our comm van, stored outside, which still has almost 90%
capacity after more than 7 years of continuous charging and use and no
maintainence using slightly modified Astron power supplies (not the Astron
mod).   Prior to that, ordinary chargers and three sets of deep cycle marine
batteries lasted less than 2 years each, even with constant attention.

Bruce Gordon N6OLT

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