ZVEI information leaflet No. 3e
Edition December 2011
Requirements for electrolyte
and refilling water for lead acid batteries
1. General information
This leaflet contains requirements for electrolyte and refilling
water for lead acid batteries.
The electrolyte for lead-acid
accumulators is diluted sulfuric
acid with density values related
to type of construction of the
accumulator or as specified by
the battery manufacturer.
Diluted sulfuric acid is used as
filling acid for unfilled dry
charged cells or batteries.
Purified water used is for the
preparation of diluted sulfuric
acid and for refilling of cells or
batteries.
The purity of refilling water has
to meet higher requirements
than for filling electrolyte, because the impurities of the
operating electrolyte will be
increased by regular refilling of
water.
Electrolyte and water, not
according to the requirements
result in damages of the battery.
mixing concentrated sulfuric acid
or sulfuric acid with high density
of d > 1.30 kg/l and purified
water. Sulfuric acid in
concentrated form is colorless,
high etching liquid with a density
d = 1.84 kg/l.
2.2. Water and refilling water
Water (H2O) is an ingredient of
the electrolyte for accumulators.
Purified water is used for the
preparation of electrolyte for
accumulators and for refilling of
water loss caused by overcharging and evaporation in the
operating electrolyte.
2.3. Filling electrolyte
Diluted sulfuric acid used for first
filling of accumulators or for
replacement of electrolyte in the
case of contamination of the
operating electrolyte is called
filling electrolyte.
2.4. First filling
The first filling denotes the filling
with filling electrolyte which is
generally carried out by the
battery manufacturer or the user
in accordance with the
applicable manufacturer’s
instructions for start-up.
2. Definitions
2.1. Electrolyte
The electrolyte for lead-acid
accumulators is diluted sulfuric
acid ( H2SO4) with density
values as specified by the
battery manufacturer. Diluted
sulfuric acid is prepared by
2.5. Operating electrolyte
The operating electrolyte
denotes the electrolyte which is
used during operation of the
battery. The density values and
the degree of purity of the
operating electrolyte may
This leaflet was prepared by the Working Group Industrial Batteries of the
ZVEI – German Electrical and Electronic Manufacturer’s Association
deviate from the values of the
filling electrolyte.
2.6. Electrolyte density
The electrolyte density in kg/l
denotes a measurement for the
mass included in one unit
volume.
2.7. Nominal density
The nominal density of the
electrolyte denotes a value
specified by the battery
manufacturer related to the
nominal temperature, nominal
electrolyte level and the fully
charged state of the
accumulator.
In the case of lead-acid
accumulators the density is a
function of the state of charge.
Density of the electrolyte for
accumulators related to
analytical statements
Density values which are
measured at temperatures
deviating from the nominal
temperature are converted to the
nominal temperature of 25°C for
the analytical evaluation.
Analytical evaluation denotes
information for the analysis of
electrolytes.
Density of electrolyte for
accumulators related to the
use and type of construction
The nominal density of
electrolyte related to the use and
type of construction of the
accumulator is specified by the
manufacturer.
Measurement of the
electrolyte density
As a rule, the measurement of
the electrolyte density is carried
out by densimeters, the socalled siphon acidimeters, in
which areometers (hydrometers)
are used.
Electrolyte level
The electrolyte level denotes the
level of electrolyte in the
accumulator. The permissible
zone is indicated by electrolyte
level marks (minimum,
maximum)
2.8. Nominal temperature
The nominal temperature is a
specified value, used as a
reference of important
properties, such as the nominal
electrolyte density and the
nominal capacity of the
accumulators.
2.9. Electrolyte additives
Additives which are designated
as agents of improvement are
not permissible as they may
cause damage to the battery in
the longer term and thus
endanger the functional safety.
2.10. Impurities
In practical use, the electrolyte
can be contaminated which may
cause damage to the
accumulator or reduce its power.
The type and maximum permissible quantity of impurities
are specified in the following
tables.
3. Water use for topping up
and preparation of
electrolyte
The water shall meet the
physical requirements as shown
in Table 1 and the chemical
requirements as given in Table
2. Purified water in compliance
with the require-ments can be
prepared from tap water by
distillation or by ion exchange.
3.1. Physical requirements of
purified water
Appearance
clear,
colorless,
odorless, no
oil drips
pH value
5 to 7
electric
conductivity at
20°C
- freshly prepared
- up to being filled
into the cell
4. Storage of purified water
Water shall be stored in
appropriate vessels, such as
vessels made of glass, ebonite,
polyethylene, polypropylene or
other plastic materials. Hoses
should be made of PVC, rubber
or polyethylene.
The dissolution of metal ions out
of metallic vessels is liable to
occur. Vessels made of metal,
shall therefore, not be used.
It is recommended that purified
water should always be stored in
airtight vessels because carbon
dioxide (CO2) which is absorbed
from the air, is increasing the
electric conductivity of water.
< 10 μS/cm
< 30 μS/cm
Table 1
3.2. Chemical requirements
of purified water
The purified water shall not
exceed the limit values given in
Table 2.
No.
Impurities
mg/l max.
1
evaporation residue
10
2
oxidable organic substances
calculated as KMnO4
20
3
metals of the hydrogen sulfide group
(Pb, Sb, As, Sn, Bi, Cu, Cd )
each element individually
all together
0.1
0.5
4
Metals of the ammonium sulfide group
(Fe, Co, Ni, Cu, Cr)
each element individually
all together
0.1
0.5
5
Halogens calculated as chloride
0.5
6
Nitrogen as nitrate
2.0
7
Nitrogen as e.g. Ammonia
40
Table 2
2/5
5. Preparation of electrolyte
for lead-acid accumulators
The electrolyte is prepared from
sulfuric acid of high
concentration by pouring it into
purified water in accordance to
point 3.
Note:
- The electrolyte shall only be
prepared by the battery
manufacturer or skilled
personnel.
- Concentrated and diluted
sulfuric acid has a highly
etching effect on human skin,
clothes and the appliance.
- Never add water by pouring it
into concentrated acid
- Note the safety data sheets.
6. Physical properties of
diluted sulfuric acid as
electrolyte
6.2. Correction of density
from measuring
temperature to nominal
temperature
specific gravity
dN
kg/l
correction factor
fd*)
kg/l per K
1.10
0.00050
1.15
0.00060
1.20
0.00070
1.30
0.00075
*) The correction factor refers to the
temperature range from 0°C to 55°C
Table 3
6.3. Dependence of the
specific gravity on the
contents of sulphuric
acid at 25°C
specific
gravity at
25°C
kg/l
1.100
%
mol/l
mass
concentration
g/l
15.18
1.704
166.98
1.110
16.45
1.863
182.60
1.120
17.80
2.034
199.36
1.130
19.15
2.208
216.40
1.140
20.47
2.381
233.36
1.150
21.81
2.558
250.70
dn = dT + fd (T – Tn)
1.160
23.11
2.735
268.07
meaning
1.170
24.39
2.911
285.36
1.180
25.63
3.086
302.43
1.190
26.90
3.266
320.11
6.1. Dependence of specific
gravity on temperature
The acid densities which occur
at the measuring temperature
and deviate from the nominal
density shall be converted to the
acid densities at nominal
temperatures.
dn specific gravity at nominal
temperature Tn
mass portion
amount of substances
1.200
28.12
3.443
337.44
dT specific gravity at measuring
temperature T
1.210
29.34
3.622
355.01
1.220
30.55
3.803
372.71
fd correction factor according to
Table 3
1.230
31.78
3.989
390.89
1.240
32.98
4.173
408.95
T measuring temperature
1.250
34.18
4.360
427.25
1.260
35.40
4.551
446.04
1.270
36.60
4.743
464.82
1.280
37.81
4.938
483.97
1.290
38.93
5.124
502.20
1.300
40.10
5.319
521.30
Tn nominal temperature
Table 4
6.4. Dependence of the
specific gravity on the
state of discharge
The specific gravity decreases
during discharge of an
accumulator, the permissible
limit values are given by the
battery manufacturer for the
various applications.
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7. Requirements of sulfuric
acid used as electrolyte
7.1. Impurities of sulfuric acid
of higher concentration
degrees
The purity of higher
concentrated sulfuric acid shall
be so, that after the dilution with
water to filling electrolyte
densities of < 1.30 kg/l the
values specified in Table 5 are
not exceeded.
Impurities
mg/l
max.
1
metals of platinum group
0.05
2
rhenium
0.1
3
copper
0.5
4
other metals of the hydrogen sulfide group other than
lead, e.g. arsenic, antimony, bismuth, tin, selenium,
tellurium
individually
all together
1.0
2.0
5
manganese, chromium, titanium, nickel
individually
0.2
6
iron
30
7
other metals of the ammonium sulfide group other than
aluminum and zinc, e.g. cobalt
individually
all together
1.0
2.0
Cons
No.
8
halogens calculated as chloride
5
9
nitrogen as nitrate
10
10
nitrogen as e.g. ammonia
50
11
volatile organic acids calculated as acetic acid
20
12
oxidable organic substances calculated as KMnO4
consumption
30
13
annealing residue
250
Table 5 - Permissible impurities of diluted sulfuric acid as filling electrolyte
for lead-acid batteries in the density range ≤ 1.30 kg/l
7.2. Impurities of filling acid
and operating acid
The sulfuric acid used for filling
lead-acid batteries shall be clear
and colorless.
The impurities included in the
acid shall not exceed any value
quoted in Table 5.
For operating electrolyte, the
maximum values of Table 6 shall
apply.
2)
Impurities
mg/l
max.
1
metals of platinum group
n.m.
1)
2
rhenium
n.m.
1)
3
copper
n.m.
1)
4
tellurium and selenium
individually
5
other metals of the hydrogen sulfide group other than
lead and antimony, e.g. arsenic, bismuth
individually
all together
3
6
6
Antimony
a) stationary cells with Plantè plates or flat plates
b) stationary cells with tubular plates and traction cells
3
10
7
manganese, chromium, titanium, nickel
individually
0.2
8
iron
100
9
other metals of the ammonium sulfide group other than
aluminum and zinc e.g. cobalt
individually
all together
1.0
2.0
10
halogens calculated as chloride
a) stationary cells
b) traction cells
50
500
11
nitrogen in the form of nitrate
10
12
nitrogen in other form e.g. ammonia
50
13
volatile organic acids calculated as acetic acid
30
14
oxidable organic substances calculated as KMnO4
consumption
50
Cons
No.
1.0
Table 6 - Permissible impurity of diluted acid and operating acid
for lead-acid batteries in the density range ≤ 1.30 kg/l
1) nm = not measurable / These metals remain deposited virtually completely on the
negative electrode. These harmful substances effect a high self-discharge
4/5
2) It is not possible to specify valid limit values for metals in general. The levels of
impurities which are harmful to the batteries depend strongly on other parameters as
type, age and operating conditions of the cell.
8. Storage of electrolyte
Electrolyte which is intended to
be stored shall be placed in
vessels which are appropriately
marked and shall be resistant to
chemical corrosion (e.g.
polyethylene, polypropylene or
similar plastic material).
9. Remedy in the event of
damage due to electrolyte
10. Additional instructions for
safe handling
Where parts of skin, eyes or
mucous membranes have been
exposed to the harmful effect of
the electrolyte, immediate
actions shall be taken in all
these cases by rinsing the
affected parts with plenty of
water. Additionally medical care
is required.
For additional instructions refer
to ZVEI Information leaflets
• Instructions for safe handling
of lead-acid accumulators (leadacid batteries)
• Safety data sheet on
accumulator acid (diluted
sulphuric acid)
Appliances, installations and
clothes may be cleaned by using
neutralization agents and by
rinsing them out with water.
Soda solutions of a
concentration of 5% (sodium
carbonate) or solid soda can be
used in order to reduce the
affect of electrolyte on
appliances, the human body and
clothes.
Diatomaceous earth is
especially suited absorbing
spilled electrolyte.
Editor:
ZVEI – Zentralverband Elektrotechnik- und Elektronikindustrie e. V.
Fachverband Batterien
Lyoner Straße 9
60528 Frankfurt
Fon.: +49 69 6302-283
Fax: +49 69 6302-362
Mail: batterien@zvei.org
www.zvei.org
© ZVEI 2011
In spite of all due care, however, we cannot accept any liability
that the information is complete or correct or up to date.
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