If a battery acted as a simple “storage tank” for pure electrical energy, we could dispense with the effects of temperature on operation in a few words.
The lead-acid industrial battery, the type commonly in use for the last 90 years, is much more complex than a fuel tank, however.
These batteries are built of electrochemical cells that use a chemical reaction to create electricity on demand. The ability of an electrochemical cell to produce electricity does depend on the rate of the reaction, which in turn depends on the quantities of reactants (of appropriate composition), and temperature.
The Comfort Zone
Today’s batteries are optimized and rated for operation in a “shirtsleeve” environment that corresponds to an internal battery temperature of 80 degrees F (internal battery temperature is gauged by the internal temperature of the cell electrolyte). Operation above and below that point is possible, of course, but there are tradeoffs that become increasingly important the greater the temperature difference from the optimum.
Bring a battery down to 32 degrees F and it provides only 65% of its rated capacity. Operate in a freezer at ?10 degrees F, and a 1000 ampere Hour (AH) battery does no better than a 400 AH battery would at 80 degrees F
Operate a battery at temperatures higher than 80 degrees F? Higher temperatures do enhance output, but take a heavy toll on the cell plates. One commonly held estimate: for every 15 degrees increment above 80 degrees F, halve the expected battery life. Battery manufacturers consider 110 degrees F an operational maximum, and put 120 degrees F operation in the “warranty–voiding abuse” category.
Wait for Cool–down?
There is wisdom in the “cool–down” time commonly built into the charging and operation schedule of batteries used by large electric lift truck fleets. Cool down time is not trivial: an average lift truck battery at rest can take more than an 8–hr shift to shed the 10 to 15 degree F it gained just during operation. Use a battery for more than one shift (one 80% discharge/recharge cycle) during each 24–hour day, and you do so risking a considerable sacrifice in battery life.
Charging the battery also causes an increase in temperature, generally the older a battery gets, the higher the temperature rise. This occurs with aging for a number of reasons. Internal resistance of the cells increase as internal corrosion and deterioration take their toll. Voltage does not rise as high in older cells as they charge, allowing charger currents to remain at a higher level (and add more heat) for a longer portion of the charge cycle.
Advanced Chargers for Old Batteries
As a battery heats up to the limit of its operational range, cell voltages on charge actually drop, rather than rise. This in turn results in the charger current edging higher, raising the battery temperature, and depressing voltages further. Some relief from this vicious feedback cycle can be had if the charger used on the older battery is controlled by a microprocessor programmed with “intelligent” charging algorithms. Such a controller will sense the dip in battery voltage that signals heating, and will cut the charging period short–preventing the wear and tear of prolonged high–temperature charging.
Fresh, not Frozen
Freezing of the battery electrolyte (acid/water solution) is another possible consequence of very low temperature battery operation. Manufacturers do not design their cells to accommodate the expansion that occurs when a water solution turns to ice. Internal damage may occur, and visible splitting of the cell top and jar can result. Cracks will cause electrolyte leakage that will render the battery unusable (without repairs) even if it shows unchanged electrical behavior. Do not attempt to charge a battery with frozen electrolyte.
A fully charged battery with electrolyte of appropriate strength (approximately 1.275 specific gravity) will not freeze at temperatures of ?40 degrees F. The electrolyte of a discharged battery contains a lower concentration of acid, and can freeze in the vicinity of 0 degrees. Be aware that if water has just been added to the cells, and the battery has not been charged to mix the electrolyte, this water can freeze at 32 F. As mentioned above, a large led–acid battery takes a while to change temperature, so brief exposures to very cold temperatures may have little noticeable effect.
About those Electric Car…
The dramatic effect of temperature on battery performance should be popping up in media discussions of the new electric car concepts. It should, but it isn’t. Those of us who live in parts of the country where we must operate highway vehicles at subzero temperatures may have to start asking the auto makers if they intend to address the cold–weather battery problem.
For more information, contact Arcon Equipment Inc. (440) 232-1422.
