Part of a series of
Technical Bulletins produced to explain battery performance issues under
Australia's unique conditions.
High temperatures can have a devastating effect on the life of
lead-acid batteries and Australia has one of the hottest and harshest climates
on earth. While more batteries experience end-of-life failures in the
colder months, the real damage is done when the batteries are operating at
elevated temperatures. The damaging heat is generated from two primary
sources: -
- the ambient or environmental temperature, and
- the under bonnet temperatures of the vehicle
Modern vehicle manufacturers have within the past decade
increased engine operating temperatures in an endeavour to reduce harmful
exhaust emissions. The result of these changes, combined with high
environmental temperatures, are accelerating the damaging effects on battery
life as outlined below.
The key point of temperature measurement is how hot the
electrolyte (battery fluid) becomes due to the above primary sources of heat.
High temperatures can cause the following potentially damaging effects:
- increased gassing and resultant water loss
- increased grid corrosion / oxidation
- overcharging, and potential plate failure
- increased battery discharge capacity
- increased self discharge rate
The operation of a lead-acid battery is a chemical process,
and like all chemical processes, battery performance is temperature dependent.
The available capacity and maximum discharge current available, both fall at
lower temperatures and increase at higher temperatures. Capacity and
discharge current at -20°C is approximately half that obtained at +20°C.
1. Increased gassing and
resultant water loss:
As battery temperatures increase, the rate of gassing and
water loss in a battery is increased exponentially. The lead alloy used in
the plate grid construction also influences the rate of gassing of a battery.
As
gassing continues over time, the electrolyte level drops below the top of the
busbar and the top of the plate groups become exposed. This leads to major
grid corrosion and reduced battery life, which will be discussed in more detail
shortly. Because of gassing, batteries need to be topped up with
water and topped up more regularly under der extreme operating conditions.
The key point here is that the greater the
available volume of electrolyte above the busbars in a battery, the longer the
probable battery service life before the risk of exposure of the plates.
This is a very important design issue for battery manufacturers in Australia.
2. Increased grid corrosion /
oxidation:
The operation of batteries at elevated temperatures will
result in increased positive plate grid corrosion (oxidation),
and in extreme cases can bring about oxidation of the busbars in the battery.
Positive grid corrosion is a normal 'end-of-life' failure mode of batteries
which is greatly accelerated at elevated operating temperatures.
As mentioned previously, the failure of plate grids and
busbars is further increased if the electrolyte level drops below the top of the
busbar and the top of the plate groups due to gassing (water) losses. In
extreme cases, plate grids can expand and break at the top of the plate group,
causing battery failure due to short circuits.
The volume (reservoir) of electrolyte above the plate group
and busbar in a battery can provide improved operating life at high temperature
as more electrolyte is available to cover gassing losses before the risk of
exposure of the busbar.
3. Overcharging and potential
plate failure:
As the battery operating temperature increases, battery
internal resistance decreases and the charge rate acceptance of the battery
increases accordingly. That is, as temperature increases, the charging
voltage applied by the alternator to a battery must be reduced to avoid
overcharging. 
The
following curve indicates the variation of voltage with temperature that should
be applied to a lead-acid battery to adequate and correct charging.
At elevated under-bonnet temperatures, charging voltage
control can be difficult and inadequate, resulting in extended overcharging and
reduced battery life due to positive plate material degradation and loss, often
accompanied by softening of the negative plate material.
Different rates of air ventilation and the position of the
battery in the engine bay can lead to an internal material difference in
alternator and electrolyte temperatures which increases the rate of overcharging
the battery receives and the subsequent gassing.
4. Increased battery discharge
capacity:
At elevated temperatures, the discharge capacity
of a battery is increased, and the ability to deliver higher cranking current is
increased - as shown in the following graphs. In practical terms, this is
of no real advantage, except in the case of a worn or reduced capacity battery,
which may still function adequately due to the elevated temperatures.
This masks the fact the battery is nearing its end-of-life and
when the temperatures become colder and the engine requires higher cranking amps
to turn over due to the engine oil becoming viscous, the battery ultimately
fails suddenly.
5. Increased self discharge
rate:
All automotive lead-acid batteries slowly lose charge when not
in service. This is known as self-discharge. The rate of self-discharge
increases with increasing temperature. The time batteries can be allowed
to stand without recharging is reduced with increasing temperature of storage.
Batteries should be recharged when the Open Circuit Voltage (OCV) drops to 12.4
volts.
