Page 380 - Battery Reference Book

Traction or motive power type 32/7

Table 32.3 Recharge voltage table for a typical Plante cell at 32.2 Traction or motive power
15°C
Traction or motive power lead-acid batteries are the
Cell power source used for every kind of electric vehicle
e.rn.6 from road transport to fork life trucks in industry.
In many applications electrically operated vehicles
2.70 Top of charge volts at 7% of A W10 are replacing vehicles with internal combustion
2.40 Minimum top of charge volts to recharge with good engines. Electric vehicles are cheaper to run because
gassing of the lower fuel cost, and because of their relative
2.25 Ideal for trickle charge float and minimum voltage simplicity they are easier to maintain than vehicles
for rechar,ge, but subject to problems such as equipped with internal combustion engines. They also
stratification present fewer environmental pollution problems. In an
2.20 Floating/trickle charge, but with need for freshening electric vehicle the motor and controller are virtually
charges maintenance-free and this is equally true of the battery
2.05 Open-circuit voltage for 1.210 electrolyte relative and charger. With the introduction of automatic battery
density topping-up devices and self-compensating battery
chargers, these once onerous battery and charger
maintenance routines have now become almost non-
It is satisfactory only if the amount of discharge is a existent. Present indications are that petrol and diesel
very small proportion of the ampere hour capacity of fuel costs will probably escalate more steeply than
the battery. electricity prices, and that the price differential will
continue to remain in favour of electricity. Some data,
supplied by Chloride Motive Power, on actual costs of
electric versus diesel vehicles, are givec in Tables 32.4
Float charging and 32.5. Both on vehicle maintenance and fuel cost,
Float charging is used where the connected load will therefore, electric vehicles are displacing conventional
not tolerate even a momentary interruption. A simple vehicles in many applications.
example of such a scheme would be a continuous The interior design of lead-acid batteries intended
process plant where even a millisecond failure of the for traction purposes, such as milk floats and electric
supply would result in the complete breakdown of a locomotives and fork lift trucks, differs from that of
manufacturing run, which could be extremely costly. stationary batteries because of their different service
To guard against such interruptions, the battery, load requiremens.
and charger are connected in parallel, the charger being
of the constant-voltage type, capable of delivering an 32.2.1 Selection of motive power batteries
output equivalent to the normal load demand plus the The duties the battery will be expected to perform
battery charging requirements. With the float system,
should a failure occur, the battery takes over the load can be broken down into a series of components, e.g.
vehicle range, hills to be negotiated or loads to be
with no interruption whatsoever. lifted in the case of a fork lift truck, each of which will
A constant voltage charger is a statically controlled demand a certain amount of discharge current from the
rectifier, normally employing transductor or thyristor battery.
devices to correct for changes in output. The trans- The amount of capacity available from a battery is
ductor can be regarded as an inductive choke, the a.c. dependent largely on the number and size of plates
impedance of which is varied by the degree of mag- in the cells. Clearly, battery manufacturers cannot
netic saturation produced by direct current in an auxil- dictate to the vehicle designer how much room must
iary winding. Thyristor control is achieved by variation be allowed for the battery, as thls is only one of very
in the firing angle of the a.c. waveform. many design considerations that must be taken into
The normal float level of a floating battery system account.
is equivalent to 2.25V/cell. This preset voltage is Normally, the battery maker is presented with a
held within usual limits of +2% irrespective of mains given space into which the battery must fit, and this
voltage variations of f6%, with load variations of space is a compromise between overall design require-
0-100% of the charger rating. Facilities are usually ments and the need for a battery of sufficient size to
incorporated for quick boost facilities for recharging make the vehicle a viable, economic proposition to
of the battery following a deep emergency discharge. the user.
The selection of the correct charger for a battery- In practice, therefore, the exact battery capacity is
operated standby power system is of vital importance, not usually calculated in advance for a particular duty.
and the advice of both battery and charger manufactur- Instead, the type of truck required is determined by the
ers should be Sought to ensure the most efficient and maximum loads, and one of the standard batteries for
properly matched combination. the truck is used. There is usually some limited choice