Page 225 - Battery Reference Book

Maintenance-free lead-acid batteries 18f9

plates. Here the oxygen combines with the material Since a recombination electrolyte battery is still
of the plates to form lead sulphate, the same material a lead-acid battery, one would not expect to
produced during discharging. Further charging breaks find large differences between it and conventional
this down to lead, so the oxygen is returned to the flooded lead-acid batteries. However, there are
electrolyte. There is therefore a closed loop, since the some differences:
continual arrival of oxygen at the negative plate means 1 Charging This is best done by a charger which has
that it never reaches a state of gassing, so hydrogen control over the voltage such that it does not exceed
is never evolved. The principles of the closed loop 15V. With this type of control the commencing
system are outlined in Figure 18.4. With no water loss charge current can be as high as the conventional
and no gases permanently evolved, the battery can be electrolyte battery will accept at 15 V or below.
completely sealed and never needs topping up. Most mains chargers, if any control exists, can
In the event of severe charging abuse, hydrogen can control the current only, in which case the value
be produced. plus oxygen at a rate above the capacity of the current should not exceed 3A, as this is
of the battery to recombine it. In this situation, a valve the maximum value the recombination will accom-
on top of the battery will release the pressure safely modate with a 12V battery when it is approaching
and then reseal itself. a fully charged condition. If the charger has no con-
A good example of the application of recombinat- trol at all then 3 A (average) or 4A (MS) is the
ion electrolyte technology to automotive batteries is largest charger rating that should be used in order
the Chloride Automotive Ltd Torque Starter battery, to avoid raising the battery voltage too high. Even
introduced in 1983. Chloride claim that the battery is from the completely flat condition the charge is
a much safer product to handle, being totally sealed complete within 15 h.
with no free electrolyte to spill. However, under con- 2. Boost charging From what has been said so far
ditions of abuse. the generation of gas could be greater it can be seen that fast chargers or boost chargers
than the recombination process could cope with, and which do not control the voltage cannot be used
for this reason a small pressure valve has been built with recombination electrolyte batteries.
into the lid. 3. Jump starting This is carried out in exactly the
Excessive gassing occurs for only one reason-the same way as with any conventional battery. If the
voltage across the terminals is too high, because of two vehicles are allowed to run with the engines on
either faulty alternator regulation or mains charging at medium throttle and the jump leads connected, a flat
too high a setting. battery will normally accept charge at a very high
Figure 18.5 shows the improvement in energy den- rate. Provided the jump starting technique is the
sity from 30Akg in 1981 (non-RE) to 40A/kg in safe approved method, this form of boost charging
1983 (RE). is safer than using a mains boost charger no matter
what the types of battery involved.
4. Temperature A recombination electrolyte battery
1986 target conducts heat from its plates to the outside very
0
/ badly since there is only gaseous contact between
60 4 the container wall and the inner components. In
/ a flooded product the acid acts as the main heat
d transfer medium and is much more efficient at
50 / equalizing temperature differences. The net result is
/ that the recombination electrolyte battery requires a
/
longer time for the inner components to fd tQ the
outside temperature; overnight, for example. This
means that the battery appears to perform better
at low temperatures, because the inner components
may not actually be that cold.
5. Internal resistance The internal resistance of the
battery is lower mainly because of the properties
of the unique separator. This means fewer internal
losses when delivering high current; of course.
6. Voltage The voltage is inherently higher because
of the increased gravity of the acid, and this in con-
junction with the lower internal resistance means
I I I that the load voltage is also higher. Fewer internal
1970 1975 1980 1985 losses and a higher voltage mean that the starter
Figure 18.5 Cold cranking amps/kg battery weight (current at motor can utilize more of the energy available in
-1 8°C 30 s duration to 7.2 V) (Courtesy of Chloride Batteries) the battery.