During a power outage, which occurs frequently in Florida, I would like to
use a battery to power some communication gear. Although the battery is 12v
....[snip]....

Although many have suggested you should use "deep cycle" (also called
"traction") batteries, be aware that the specifications for such batteries
are DIFFERENT in that they are rated to still be producing useable power
when their output voltage is somewhat LOWER than others. If your equipment
poops out before that lower voltage is reached, you won't get all the
output you've paid for. (My TenTek 540, for example, starts FM'ing at
about 11.5 volts, so the remaining output from a deep-cycle is totally
unuseable.)

The following is abstracted from a 3 Oct. 1992 post to
rec.radio.amateur.misc by Brian Kantor :

1. Automotive starting: formulated with thin pasted plates and designed
to supply high peak currents for brief periods of time while cranking
an engine. Designed to be discharged to more than perhaps 75% of
capacity and to be recharged immediately after discharge. Typically
discharged-rated at a 20-hour rate.

2. Traction (e.g., deep-cycle batteries): made with thick pasted plates
and with very rugged separators between the plates to make the battery
more immune to physical shock and vibration and to reduce the chance
of failure due to dendritic growth during recharging. Sold for use in
electric forklifts, golf carts, marine trolling motors, and RV power.
Designed to be discharged nearly fully each day and recharged each
night. Typically discharged-rated at a 5-hour rate.

3. Stationary: made with thick solid plates. Designed to be used as
standby power, supplying minimal power and kept in a state of nearly-
full ("float") charge until needed. Can take deep discharge. Because
of the solid-plate structure, they are bigger and heavier, but their
lifetime is much longer (10 years is not unusual). Typically
discharged-rated at a 10-hour rate.

Each type of battery has a specified voltage at which it is considered
to be completely discharged; if discharge continues below this voltage,
battery life may be considerably shortened, and repeated abuse of this
nature can result in a battery which cannot practically be recharged.
Each battery manufacturer specifies this voltage; in general, the final
voltages for the three general types of batteries a
Automotive: 1.75 volts per cell
Traction: 1.70 volts per cell
Stationary: 1.85 volts per cell

A typical traction-type cell shows the following voltages:
2.12 Fully charged, open circuit, at rest with no charge
or discharge for at least 12 hours
2.00 As soon as load is applied (internal voltage drop)
1.70 Fully discharged, under load
1.99 Fully discharged, open circuit
2.10 Beginning of charging after full discharge
2.35 70-80% charged; gassing begins
2.65 Fully charged

# The following assumes 12 volt negative-grounded "automobile" batteries as
# found in most cars, light trucks, and vans in North America.

Liquid-electrolyte lead-acid batteries can be recharged at any rate
exceeding internal- and surface-discharge rates and which does not cause
"excessive" gassing (liberation of oxygen, hydrogen, and steam).

In NON-float service, there are several simple chargers:
* A single-rate constant-current charger limits its charge rate to about
7% of the battery's ampere-hour capacity. Thus, for a 100 Ah battery,
the charger would supply about 7 amperes and must be able to supply
voltages between 12.6 and 16 volts over the duration of the charge.
Charging is complete when the battery reached 2.65-2.70 volts per cell.

* A simple taper charger is a constant-voltage source set to 2.8 volts
per cell with a series ballast (typically a resistor, but a choke or
the internal resistance of the supply can be used) which limits the
output current to 7% of capacity when charging is started at 2.1 volts
per cell. Again, charging is complete at 2.7 volts per cell.

* Trickle-charging (supplying 0.5-1 milliampere per ampere-hour capacity)
of a fully-charged battery can be done to keep it charged. Trickle
charging should be discontinued when it has continued for at least 24
hours and the battery has reached 2.25 volts per cell. Typically,
trickle chargers are set to run perhaps once a week. Because of their
thin plate construction, automotive-type batteries will deteriorate if
trickle-charged for more than perhaps six months. (However, using
pulsating rectified AC or superimposing a small AC current on pure DC
charging current increases battery life by up to 30%. It is postulated
that this reduces gassing, leads to more porous lower-resistance
plates, and lessens the tendency to form dendrites during charging.)

In FLOAT service, where the battery is in parallel with the mains supply,
the supply voltage must be set to 2.15-2.20 volts per cell. This charges
the battery and avoids excessive gassing, but does not serve to "freshen"
the cells--there is not enough gassing activity to move electrolyte
around and clear the beginning of deposits from the surfaces of the
plates. It is recommended that batteries in float service occasionally
(perhaps once a month) be charged to 2.65 volts per cell to freshen and
equalize the charges. In large installations, this is done by switching
parts of the battery banks out of service in rotation; in smaller systems
that can tolerate the voltage excursion (about 16 volts!), it can be done
by simply boosting the output of the mains supply.

Charging inevitably leads to some water loss due to gassing: 100 ampere-
hours of a gassing charge (2.4 or more volts per cell) causes a water
loss of about 1.2 ounces. Hydrocap Corp (975 NW 95th Street, Miami FL,
303-696-2504) makes replacement filler caps ($5-10 each) containing a
catalytic material which recondenses emitted steam and recombines hydrogen
and oxygen gasses back into pure water which then dribbles back into the
cell, greatly reducing the required maintenance.

For further information:
Smith, George. "Storage Batteries, including Operation, Charging,
Maintenance, and Repair". ISBN 273-43448-9, TK2941.S57, 1978.

Aguf, I.A. and M.A. Dasoyan. "The Lead Accumulator" (translated from
Russian byu S. Sathyanarayana). Calcutta, 1968.

Longrigg, Paul. "Rapid Charging of Lead-Acid Batteries for Electric
Vehicle Propulsion and Solar Energy Storage." DOE/NTIS 1981.

Darden, Bill ) battery FAQ's found on the WWW.

--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)