Battery Considerations
Lead-acid batteries reach peak efficiency at 90 degrees F (32 C) As ambient temperature drops, ampere output and recharging efficiency decline, dropping to 40 percent of rated output at 0 degree F (-18 degree C).
Cold Cranking Ampere (CCA) rating is the best yardstick by which to measure a battery’s capacity. It indicated the discharge rate (measured in amperes) a fully charged battery will maintain at 0 degrees F (-17.8 degree C) without terminal voltage dropping below 1.2 volts per cell. Table 4 shows the effects of colder temperatures on battery capacity.
Lead acid batteries are almost always used on gen sets due to their lower upfront cost and maintenance needs. However, nickel cadmium batteries are preferred in harsh environments or situations where they will likely be unused for lengthy periods. They also tolerate long overcharge intervals better than lead acid batteries, and offer a nearly constant voltage output throughout a discharge cycle.
Table 4 – Ambient Temperature vs. Battery Output
| Fahrenheit | Celsius | Percent of 80 F (27 C) Ampere hours output rating |
|---|---|---|
| 80 | 27 | 100 |
| 32 | 0 | 65 |
| 0 | -18 | 40 |
Charging Systems
Gen sets with engine mounted alternators cannot be relied upon to recharge batteries in standby situations. Charging systems must be used to maintain batteries while the unit is on standby, and fully recharge batteries if gen set cycle times are short.
Consider these factors when specifying a charging system:
- Select a system that recharges batteries quickly in the constant current mode, then automatically switches to maintain charge in a constant voltage mode. Units should be able to recharge completely dead batteries or those with no open circuit voltage. Select a system that will float and equalize, which maintains charge with minimal water loss in cells.
- Match charger capacity to the battery’s ampere-hour (AH) capacity. Charger output should be between C/5 and C/20, where C equals the battery’s AH capacity. For example, a 10 amp unit can charge batteries between 50 and 200 AH.
- Consider units that offer short circuit protection. This prevents damage to any DC powered controls, and allows engine cranking without disconnecting the charger.
- Specify diagnostic functions or alarms. An output voltage sensor is one of the best trouble indicators because it can detect power loss as well as overcharging problems. Consider a time delay on the voltage sensor to prevent false alarms caused by power drains from engine cranking.
- Make sure chargers can withstand high transient response loads. Input voltage range minimum of +\- 5 Hz are acceptable.
- Equipment should meet UL standards. Other industry standards to consider specifying: Transient Voltage Withstand Test per IEEE std. 472-1974 (ANSIc37.90A-1974). Other standards to assure performance, construction and safety: UL EGSMA BCES-1, NFPA-110 and NEMA PV-
We can assist you in identifying battery and charging needs for the installation you are specifying. Contact us for these or other specifications.
Assuring 10-Second Start Capability
When writing get set bid specifications that include 10-second starting capability, consider all the factors necessary to guarantee satisfactory performance of the installation.
The following requirements assure 10-second starting:
- Cranking batteries must be adequately sized and fully charged. A trickle-charge system can ensure a consistent charge. When an air starter is specified, the air system must supply the required air volume and maintain a 100-psi (689.5 kPa) minimum pressure.
- Combustion air temperature must be a minimum of 70 F (21 C).
- A jacket water heater to maintain a minimum of 90 degrees F (32 degrees C) jacket water temperature.
- A readily available supply of clean fuel.
- The generator rotating members must not exceed the rotating mass of the standard Caterpillar generator.
Minimize Variables
Fuel quality and ambient temperatures are two variables out of your control once a gen set is commissioned. However, you can specify fuel and temperature requirements to minimize the impact of these variables.
Fuel quality is not generally a cause of problems in North America, but minimum quality standards should be included in your specifications, as outlined under Construction Specifications Institute (CSI) 105.000. Caterpillar engine can be operated on a variety of fuels, including natural gas.
Preferred fuels include a variety of distillates, such as No. 1 and No. 2 fuel oil, No. 1-D and No. 2-D diesel fuel oil, per American Society for Testing Materials (ASTM) D396-80 and D975-80, respectively. A complete profile of fuel standards is outlined in Engine Data Sheet 60.1 and available through our service department for your reference.
Clean fuel meeting Caterpillar’s fuel recommendations will provide your customer with outstanding engine service life and performance. ASTM D613 includes standards on a fuel’s cetane number, which is a measure of the ignition quality of fuel oil. Cetane numbers, bases on American Petroleum Institute (API) gravity and mid-boiling point of a fuel, are available on the ASTM D975 cetane index. Caterpillar engines require cetane numbers in the range of 35 to 40.
In areas where fuel quality is less certain, fuel analysis by lab sample may be helpful in determining the need for fuel additives and/or a water separator. We can provide fuel analysis services to further assure your customer satisfactory engine performance.
Jacket Water Heaters
A jacket water heater may be specified in order to maintain coolant temperature of 90 degrees F (32 degrees C) (CSI 201.J10), and must be installed if ambient temperature drops below 70 degrees F (21 degrees C), to ensure 10-second starting. Heaters, either 240- or 480-volt AC single-phase powered, should include adjustable thermostats to allow for manual temperature control.
We are ready to assist you in writing a specification that will assure your customer of trouble-free 10-second starting operation. If you have a bid in progress, or anticipate writing specifications for 10-second starting capability, please call us and allow us to assist you in the process.
We have the equipment, the expertise, complete preventive maintenance and service programs, and the dedicated professional staff to ensure you gen set specifications will provide anticipated performance.