Genset Starting Education Module #2:
Engine Start Battery Performance Characteristics
William F Kaewert
SENS – Stored Energy Systems LLC
Revised October 2013
Batteries are chemical devices that behave differently from, and less predictably than, the electrical, electronic and mechanical systems most common in a genset.
Battery: definition and concepts
An electric storage battery is a group of electrochemical cells interconnected to supply DC current. The nominal1 voltage rating of the battery is determined by the number of cells connected in series. Lead-acid cells have a nominal voltage of 2.0 volts. Nickel-cadmium (Ni-Cd) cells have a nominal voltage of 1.2 volts. Thus, six lead-acid cells must be connected in series to create a 12-volt battery. Similarly, ten Ni-Cd cells are typically connected in series to create a 12-volt battery. Electrochemical reaction kinetics2 and diffusion characteristics3 cause a battery to deliver less total energy as discharge power rate increases. This means that the battery can deliver its total rated ampere hour (AH) capacity only if discharged slowly. When discharged quickly as during an engine start a battery can deliver only a fraction of its total capacity before the end voltage drops below a useful value. Typical minimum allowable battery voltages in engine starting applications are as shown in Table 1 below. It is essential that the battery be sized such that even under worst-case conditions its voltage stays above these values so the engine’s control computer remains powered.
Table 1: Typical minimum allowable DC voltages during engine cranking4
Battery performance degrades at cold temperatures
Performance of all batteries degrades at cold temperatures because of slower chemical reactions within the battery. For example, a lead-acid starting battery rated to deliver 1,805 CCA at 32 degrees F (0C) delivers only 1,444 CCA at 0 degrees F (-18C). Derating of a typical lead-acid battery is 1.1% per degree C or 0.6% per degree F.5 Practical solutions to compensate for cold temperature battery performance loss include oversizing and/or heating the battery. Although they also suffer performance loss when cold, Ni- Cd batteries remain operational at very low temperatures and derate less than lead-acid types as shown in Illustration 1 below.
- Oversizing the battery. Lead-acid starting batteries are relatively inexpensive, so oversizing the battery is simple and cost-effective. Ni-Cd batteries derate less at cold temperatures than lead-acid types do, but still need to be derated for cold. Consult the battery supplier for guidance on the correct battery to use for your starting duty and worst-case expected cold temperature.
- Using battery heaters. Heating the battery with thermostatically controlled, AC-powered blanket heaters can reduce or eliminate the need to derate the battery for cold temperatures. Blanket heaters are available for most standard sizes of lead-acid starting battery. Heating the batteries, however, places special demands on the charging system that can, unless applied properly, significantly accelerate battery failure. If battery blanket heaters are used, the battery charger must be equipped with a remote temperature compensation system, and the remote temperature compensation probe must be attached to the heated battery. Failing to use a remote temperature probe risks significant battery overcharge. Without a remote temperature probe charger, voltage would be elevated by a locally temperature-compensated charger to a level suited only for a cold battery, not a heated one. Applying elevated voltage to a warm battery would overcharge it enough to shorten its life by as much as 75%, giving only a few months’ life.6 Remote battery temperature compensation must be used whenever a battery heater is used.Battery heating systems are only available for standard sizes of lead-acid battery. Battery heaters not specifically fitted for the battery should never be used. As of this writing no purpose-built, UL-listed battery heating systems are available for Ni-Cd batteries. Heating systems not specifically intended for battery heating and not sized for the specific battery container are known to have caused premature battery failures, so homemade battery heating systems should not be attempted.
- Using a Ni-Cd battery. As shown in Illustration 1 below, pocket plate, PBE and fiber electrode type Ni- Cd batteries derate less with cold temperature than do lead-acid batteries. Although the derating curve of Ni-Cd batteries is shallower than for lead-acid, Ni-Cd batteries also suffer performance loss when cold.
Illustration 1: Different derating with temperature of different types of batteries7
Battery life reduction at high temperatures
Battery life decreases as the battery’s average yearly temperature increases above room temperature. Each eight degree C (14.4F) temperature increase above 25 degrees C (77F) cuts lead-acid battery life in half. Corresponding loss of Ni-Cd life is 18%. The chart below compares battery typical initial expected life to life expectations at different temperatures.
Table 2: Life derating versus temperature lead-acid and Ni-Cd batteries
Ni-Cd batteries are obviously a better choice for very high and very cold temperature applications. There is no life “credit” for operation at cold temperatures. Batteries exposed to extreme heat in summer and extreme cold in winter will lose life when hot, but not regain it when cold.
Summary of key points
- Batteries are chemical devices that behave differently from, and less predictably than, the electrical, electronic and mechanical systems most common in a genset.
- Battery performance degrades at cold temperatures. Lead-acid batteries suffer a steeper fall-off in performance with temperature than do nickel-cadmium batteries.
- When battery blanket heaters are used in cold climates, the battery charger must be equipped with a remote temperature compensation system, and the remote temperature compensation probe must be attached to the heated battery.
- Battery life is reduced at high temperatures. Lead-acid technology suffers worse life loss with temperature than does nickel-cadmium.
- “Nominal” voltage is the open-circuit voltage of a battery that is in a charged condition. During charging the charging source’s voltage must be higher than nominal to enable current to flow from the charger into the battery.
- The rate of an electrochemical process. Many things determine this rate, including temperature, specific gravity of electrolyte and quality of the interface between the battery’s electrode and electrolyte.
- The rate at which electrons move through the battery’s materials and across boundaries between materials.
- EGSA 100b, Recommended Practice for Engine Cranking Batteries Used with Engine Generator Sets.
- Rolls Surrette Battery.
- Refer to Illustration 3 in SENS Genset Starting Education Module #3: Solutions to Leading Causes of Battery Failure in Gensets. At 0 degrees C the output of a temperature compensated battery without remote temperature probe could be as high as 2.36 volts. Although this would be the correct charging value for the battery shown in the illustration, it would be 0.16 volts higher than the correct voltage of 2.20 volts/cell for a battery heated to 30 degrees C, or 86 F. Using a remote temperature compensation probe is essential to prevent overcharging when using a battery heater.
- EGSA Electrical Start Systems training course.
- Typical life under ideal conditions of temperature, charging, use, etc. Actual in-service life will be shorter under real-world conditions.
Copyright © 2012 William F Kaewert, SENS – Stored Energy Systems LLC