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CD-ROM Aktief 1995 #3
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BATTERY.008
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1991-07-21
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Internal Resistance in Lead Acid Batteries
by
Robert G. Hester
This article, by a Home Power reader, is the type of feedback that
we are hoping to share in this magazine. While the approach is quite
technical, it does demonstrate a simple technique for actually measuring
the internal resistance of the batteries you are using. By keeping
track of our batteries' internal resistance we can be informed on
their condition and reliability. Rich
The internal resistance (Ri) gets its name from the fact that it
is located inside the case of the battery and is a characteristic
of the battery itself. This resistance is a function of the chemical
reaction taking place in the lead-acid battery. Ri is a necessity,
an unavoidable evil; any power dissipated here does no useful work.
In solar applications, the power dissipated in (Ri) represents wasted
solar panel time.
If the useful load Ri is a very large wattage inverter, then the
voltage drop caused by the battery's internal resistance Ri may be
large enough to reduce the voltage at the battery terminals (Eb)
below the operating point of the inverter. When several hundreds
of amps are demanded from the battery, its internal resistance may
reduce its operating voltage to an unacceptable level.
The internal resistance of a battery pack may be controlled by the
system user by paralleling more batteries into the pack. Doubling
the number of batteries reduces the pack's resistance by half, each
time the number of batteries is doubled. The internal resistance
of the batteries forces us to increase the size of the battery pack
to handle large surge loads.
Operation of lead-acid batteries at low states of charge should be
avoided, as Ri increases as the batteries are discharged. Car battery
manufacturers get high cold cranking amps (reducing Ri) by close
plate spacing and reasonably high specific gravity. Also, the depth
of discharge in car systems is limited in normal operation.
The internal resistance (Ri) is equal to the change in battery voltage
when a load is applied, divided by the change in battery current
due to the application of this load:
INSERT ILLUSTRATION
SIMPLIFIED SCHEMATIC FOR TESTING Ri
A test was made to determine what kind of value Ri might have with
the author's limited resources of batteries and test instruments.
Two Trojan T105 lead-acid batteries (205 Amp-hrs.) were connected
in series (for 12 volts) and charged by 40 watt and 7 watt solar
panels connected in parallel to operate an emergency Amateur Radio
Station.
A digital Voltmeter having a one tenth volt resolution was used to
measure the voltage change. A 300 watt Heart inverter was used to
power a 100 watt light bulb as the test load. A 1 CP tail light
bulb was used as a fixed load. The test load was calculated to be
9.16 Amps, the fixed load is 1 amp. (estimated).
The test load was turned on to take the "surface charge" off of the
battery. After this the load was applied and the voltage dropped
almost instantaneously from 12.4 volts to 12.2 volts, then leveled
off at 12.1 volts after several seconds.
The behavior of the batteries under load is our concern. The total
voltage change was 12.4 - 12.1 = 0.3 volts. The total current change
was 9.16 amps.
INSERT EQUATION
If a 1,500 watt inverter had been the load the change of current
would be 1,500 watts divided by 0.9 inverter efficiency equals 1,660
watts divided by 12 volts equals 138 amps. The voltage drop across
Ri (0.0327 ohms) equals 4.5 volts. Therefore the inverter would
receive 12.1 - 4.5 = 7.6 volts and would not operate at all. The
internal resistance of the battery pack is important. This battery
pack is obviously too small to effectively source a 1,500 watt inverter.
The fact that a fast decrease in voltage was followed by a slow decrease
indicates that the equivalent circuit shown was perhaps too simple.
We are probably seeing the effects of the mobility of the ions that
make up the electrolyte. These ions of hydrogen, oxygen, and sulphate
(H2, O2, SO4) must migrate to the battery's plates in order to participate
in the chemical reaction. The O2 (oxygen) ion has 16 times the weight
of H2 (Hydrogen) and has an equal but opposite charge.
INSERT ILLUSTRATION
THE REVISED EQUIVALENT CIRCUIT
The addition of Capacitor (C) in Parallel with R1 creates a time
constant that was estimated at 3 seconds. Ri = R1 + R2
INSERT FORMULAS
In electrical terms this 275 Farad capacity (in terms of electrical
capacitance) of the battery pack is a remarkable value as few Farad
capacitors exist. This is the electrical analogy, however.
These simple circuits allow determination of the actual internal
resistance of our own batteries. Record the data generated from
your tests and compare it to later tests at varying temperatures
and states of charge. By keeping a careful eye on our battery's
performance we can detect weakening and possible battery failure
long before it actually happens. If a battery pack shows a dramatic
increase in internal resistance it is time to run an equalizing charge.
If the internal resistance continues to rise in spite of repeated
equalizing charges, then it's time to look for a good deal in new
batteries.
I would like the following information from various battery manufacturers
regarding their batteries.
1. A detailed description of the time constants encountered after
the application of a load. How many are there? What are their magnitudes?
2. Is the variation of the internal resistance an inverse linear
relation to the state of charge? Probably yes.
3. A chart of internal resistance as a function of temperature at
several states of charge. Probably goes up as the temperature goes
down; but is it a linear relation? ((The lead-acid battery's internal
resistance certainly does rise under the following conditions: 1)
low temperatures (below 45íF.), 2) at low states of charge (below
15% SOC, & 3) high states of charge (above 90% SOC. --Rich)).
4. A plot of the dynamic (AC) internal resistance seen by a load
which has high frequency components. (ie. an inverter load that pulses
at a high frequency rate as when powering inductive loads). A plot
of Ri (Internal Resistance vs. Load AC Frequency) would be helpful.
This is of interest to Ham Radio Operators who power single sideband
transmitters where the load varies at the frequency and amplitude
of the human voice.
In my personal station a Kenwood TS-130SE 100 watt output high frequency
transceiver is powered by stored solar energy. The voice load components
on this transmitter interfere with the operation of a Heart HF-300X
inverter used to power lights. This should have been predictable,
but it wasn't. More battery data is needed by battery users than
just Ampere-hours. (( It is possible that this interference is due
to RF getting into the inverter's logic, rather than changes in the
battery due to loading.--Rich))
Robert G. Hester may be written concerning this information at Box
226, Pearblossom, CA 93553.