A “Failure to Start” is one of the top reasons for failures in emergency power systems. The start system for a diesel-fueled emergency generator consists of batteries, a battery charger, the engine starter motor, and an engine-mounted battery-charging alternator. When there is a “failure to start”, the engine batteries are often found to have failed. But why do batteries fail? Can you mitigate the risk of failure? This article will identify typical failure modes, and describe a simple approach for designing additional reliability for the emergency generator’s start system.
“The single most frequent service call for generator failure is related to battery failure.” – Darren Dembski, 4/1/2007 via EC&M
Why do batteries fail?
Assuming batteries are of good quality, and properly selected for the engine-starting loads, failures are usually a result of:
- Excessive operating temperature: Batteries are sensitive to their surrounding temperature and will have a shorter life in excessively hot environments. The typical generator set installation will have the batteries located on a battery tray, only inches away from the engine. Heat radiated by the engine can easily raise the temperature surrounding the batteries to more than 100ºF. These same batteries can then see much lower ambient temperatures while the engine sits idle.
- Lack of maintenance: Loose or corroded battery connectors, and a failure to maintain proper electrolyte levels are common maintenance issues, all leading to premature failure.
- Improper battery charging: Batteries used with standby generator systems often suffer from having an incorrect charge rate applied to them, whether due to a lack of temperature compensation (see #1 above), or because the charger is not configured to correctly charge the types of batteries in use. Being overcharged or undercharged has a detrimental effect on the longevity of batteries (learn more about the importance of selecting the correct battery charger for a given set of batteries).
Beware of the “Sudden Death Syndrome”
Lead-acid batteries are well known to suffer from “sudden death syndrome”. This refers to the way in which they fail without warning. Usually prompted by either an open cell or a shorted cell, the results are immediate and total failure. Nickel-cadmium (Ni-Cd) batteries are often preferred by designers for their better performance in lower temperatures. Nickel-cadmium batteries also exhibit a more gradual decrease in performance. The typical failure associated with nickel-cadmium batteries is related to a loss of internal charge efficiency due to the deterioration of internal cell composition. This failure mode leads to progressively worse charging performance, and ultimately an inability to hold a charge.
Redundancy and Ultracapacitors!
Given the critical function performed by the battery system, designers often look to mitigate the sudden failure of batteries by introducing redundancy into the system. A redundant battery system employs a second set of batteries, intended to step-in whenever the primary batteries are unavailable. These installations rely on parallel banks of batteries, with a “best battery selector” automatically choosing between the best available source. Employing dual battery banks is a step in the right direction, but this approach relies on the same battery technology, and a similar risk profile as before. How about building redundancy by use of a power source derived from a different technology? Are you familiar with ultracapacitors?
Ultracapacitors are energy storage devices that provide bursts of power for applications requiring high power functions. Unlike batteries, which store energy via chemical reaction, ultracapacitors store energy by electrostatically (physically) separating positive and negative charges. This technology has been in use for many years to start diesel engines in the on-highway truck market. When compared to batteries, ultracapacitors provide a faster delivery of energy to the starter motor, and a longer service life, with a wider temperature operating range, and no lead nor acid.
So, how can you integrate this technology into emergency generator designs?
It’s actually very simple. The traditional battery system (battery charger and traditional batteries) remains in place, but the reliability of the existing system is increased by the addition of the following three components:
- The Ultracapacitor Module – this is the energy storage device, designed to deliver a high current to the engine’s electric motor starter.
- The Ultracapacitor Charger – This charger is specifically designed to provide a tailored charge profile to the Ultracapacitor Module, and can bring it to a 100% charge state in approximately 20 minutes (compared to 12-hours with lead-acid batteries under a typical NFPA-110 profile).
The Automatic Redundant Battery Selector (ARBS) – this device is connected to the outputs of the batteries and the Ultracapacitor Module. The ARBS monitors each source and will automatically select the best source of energy for a given engine start cycle. As an example, if the batteries are below their end voltage (low voltage level), the ARBS will switch to the ultracapacitor to provide a strong energy source to the engine’s start motor. The ARBS incorporates blocking diodes to prevent one energy source from back-feeding into the other. It is important to note that this ARBS differs from a traditional “best battery selector” in that it purposely delays the switch-over to the ultracapacitor. This delay allows batteries in good condition to go through their normal voltage drop and recovery on engine start cycles, before the switch is made to the ultracapacitors. This function ensures that the batteries remain designated as the primary power source, and the ultracapacitor is only engaged when the batteries have failed to provide the necessary starting current.
Below is a schematic of a typical system, compliant with the requirements of NFPA-110:
In this schematic, the traditional batteries offer engine-starting energy, and also the lower power required by the engine electronics. Should the batteries reach a sustained low voltage condition, the ultracapacitor is ready to step in to support the engine starting loads. By the use of an ultracapacitor as the redundant energy source, the designer of the emergency generator system has minimized the risks associated with battery technologies, and therefore provided a much more reliable engine start capability to the owner of the equipment.
Have you worked with ultracapacitors in engine-start applications? Do you have any thoughts on this approach to increase the reliability of emergency generators? Please post below to let me know about your experience, or any questions on this subject.