From: brian@ucsd.edu (Brian Kantor)
Newsgroups: rec.radio.amateur.misc
Subject: Re: Safety of auto battery for power?
Date: 3 Oct 92 13:27:12 GMT
Organization: The Avant-Garde of the Now, Ltd.
NNTP-Posting-Host: ucsd.edu

[reposted from a few months ago.  No, this does NOT belong in the FAQ]

I've been doing some research on lead-acid batteries with an eye towards
using them to provide power for our ham radio repeater site.

Our site is difficult to get to, and the commercial AC mains power goes
away at times.  Everything in the site runs off a nominal 12 volts DC.
During idle periods, the equipment may only draw a few amperes, but
most of the transmitters can draw up to 10 to 15 amps each.  A maximum
drain of 100 amps isn't out of the question, although it would probably
be only for a few minutes at a time.  Some systems (such as the digital
communications equipment) key on and off quite regularly, with perhaps
as much as a 50% duty cycle, whilst others may not key for hours and
then stay on for as long as an hour or two (voice repeaters during
drivetime).  We do not want there to be any interruption of power when
the mains fail.  We don't believe that most of the outages are of a
duration that a generator will be necessary - a few hours is
sufficient.

It is clear that a good solution to our problem is a bank of lead-acid
batteries capable of supplying the peak current, floating across a
supply that can recharge them and supply the standby and perhaps one or
two transmitter's demand.

Ok, that's the problem.  Here's what I've found.

Lead-Acid batteries commonly available today can be roughly grouped into
three categories by construction and intended use:
	1. Automotive starting
	2. Traction
	3. Stationary

Automotive starting batteries are formulated with thin pasted plates and
are designed to supply high peak currents for brief periods of time
whilst cranking an engine.  They are not expected to be discharged to
more than perhaps 75% of capacity, and are expected to be recharged 
immediately after discharge.  If used in deep-discharge or float service
they will not last long.  (I.e., the capacity of the battery will
diminish fairly quickly.  While it will still act as a battery, it will
not be able to supply its rated capacity soon after being placed in the
wrong kind of service.)

Traction batteries are made with thick pasted plates and have 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.  These batteries are sold for
use in electric forklifts, golf carts, marine trolling motors, and RV
power.  They are designed to be discharged nearly fully each day, and
recharged each night.  Because there is some tradeoff in battery life
by using the pasted plate construction to keep the size and weight of
the battery down, they are not used in applications where extremely
long life is required.  The commonly-available Deep Cycle Marine
batteries are of this general type.

Stationary batteries are made with thick solid plates.  They are
designed to be used as standby power, supplying minimal power and kept
in a state of nearly full charge until needed.  They can take deep
discharge.  Because of the solid plate structure, they are bigger and
heavier, but their lifetime is much longer.  One source suggests that
10 years is not unusual.  Some photovoltaic storage batteries (for
solar-powered homes and such) are of this type.

The best battery for our application is the Stationary battery, but they
are not commonly available.  Much more readily obtained are the Marine/RV
batteries, at about $50 apiece.

Charging and discharging these batteries is a big question.  I posted a
query to the net and received about a dozen replies, most of which
contradicted each other in one or more points.  However, there is some
consistency in the information available in our library, and I'll try to
summarize it below.

Note that all the voltages given below are for batteries at working
temperature - typically 80F (27C).

DISCHARGE:

Batteries are rated at an Amp-Hour capacity at a specific rate.
For traction type batteries, this is typically a five hour rate, so a
fully-charged 100 Ah traction battery in good condition can supply 20
amps for 5 hours before it is exhausted.  Stationary batteries are
usually rated at a 10 hour rate, and automotive (if rated in Ah at all)
are given for a 20 hour rate.  The discharge curve is NOT linear; if
you double the current drain, you will get less than half the time.
Similarly, if you halve the drain, you will get more than twice the time.

Each type of battery has a specified voltage at which it is considered
completely discharged.  If discharge continues below this voltage, the
battery life may be considerably shortened, and repeated abuse of this
kind can result in a battery which cannot practically be recharged.
Each battery manufacturer specifies this voltage; in general, the final
voltage for the three general types of batteries are

	automotive	1.75 v per cell
	traction	1.70
	stationary	1.85

Thus a typical 12 volt marine battery with 6 cells should not be
discharged below about 10.2 volts.

Another way of looking at it is that no cell should be discharged more
than about .3 v below its full-charge rest voltage.

A typical cell will show the following voltages:

fully charged, open circuit, at rest with no
charge/discharge for at least 12 hours				2.12 v/cell

As soon as load is applied (internal v-drop)			2.00

fully discharged, under load					1.70

fully discharged, open circuit					1.99

beginning of charging						2.10

70% to 80% charge (gassing begins)				2.35

full charge							2.65


CHARGING:

Liquid-electrolyte lead-acid batteries can be recharged at any rate
that exceeds 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 Ah capacity of the battery; for a 100 Ah battery, it
would charge at a rate of 7 amperes.  Since the battery will start at
about 2.1 v per cell, and finish at about 2.7 v per cell, the charger
must be able to vary its voltage over this range.  For a "12 volt"
battery with 6 cells, the charger will need to supply between 12.6 and
16 volts over the duration of the charge.  Charging is complete when the
battery reaches 2.65 to 2.7 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) that limits the output
current to 7%C when the battery is started charging at 2.1 v/cell.
Again, charging is complete when 2.7v/cell is reached.

Trickle-charging of a fully-charged battery can be done to keep it
charged.  This is done by supplying .5 to 1 mA per Ah capacity.  Trickle
charging should be discontinued when it has continued for at least 24
hours and the battery has reached 2.25 v/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.

An interesting research result was that using pulsating rectified AC or
superimposing a small AC current on pure DC charging current increased
battery life by up to 30%.  Apparently the mechanism is that is reduces
gassing and leads to a more porous lower-resistance plate, 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 to 2.20 v/cell.  This
will charge 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 v/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, it can be
done by simply boosting the output of the mains supply.

Charging inevitably leads to some water loss due to gassing; 100Ah of
a gassing charge (2.4v or more per cell) will yield about 1.2 oz of
water loss.  Hydrocap Corp [975 NW 95th St, Miami Fla, (305)696-2504]
makes a replacement filler cap that contains a catalytic material
that recondenses emitted steam, and recombines the hydrogen and oxygen
gasses into pure water that then dibbles back into the cell, greatly
reducing the required maintenance.  With the available flame arrestor
option, they sound ideal for unattended battery systems, and should
greatly reduce the danger of fire and explosion from liberated
hydrogen.  They're about $5-$10 per cell.

To read further:

Smith, George. Storage Batteries, including operation, charging,
maintenance, and repair.  ISBN 273 43448 9, TK2941.S57  1968

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

Longrigg, Paul.  Rapid charging of lead-acid batteries for electric
vehicle propulsion and solar energy storage.  DOE/NTIS 1981.

Aren't libraries wonderful?
	- Brian