Genset Starting Education Module #4
Genset Start Battery Strengths, Weaknesses & Use Strategies
William F Kaewert | SENS – Stored Energy Systems LLC
Lead-acid battery strengths, weaknesses and strategy of use
Two different types of batteries, lead-acid and Ni-Cd, are commonly used to start gensets. These two technologies have different characteristics that specifiers and users need to understand in order to successfully use each type.
The lead-acid battery is readily available, low in cost and likely to fail suddenly at end-of-life. The most successful approach to using lead-acid starting batteries is to regard them as consumables needing regular replacement. Regardless of replacement frequency, correct charging and battery maintenance remain essential because new batteries can fail as frequently as a few months after installation if not properly charged and maintained.
Summary of strengths and weaknesses of lead-acid start batteries
| Strengths | Weaknesses |
|---|---|
Low initial cost |
Can fail suddenly |
Good energy density |
Plates degrade • Positive plate corrosion • Negative plate sulfation |
Standardized, small size footprints |
Susceptible to extreme temperatures |
Standard container sizes |
Limited life of 3-5 years under ideal conditions |
Widely available at short notice |
Shelf life1 of 3-6 months maximum |
Integrated as OEM equipment |
Lower system reliability than Ni-Cd |
Relatively easy to charge to full capacity |
|
Easily recycled |
Ni-Cd battery strengths, weaknesses and strategy of use
Ni-Cd technology is inherently more robust than lead-acid technology, does not fail suddenly like lead acid and is very long-lived. The higher up-front cost and long life expectation of Ni-Cd batteries mean that they would never be considered consumables. Unlike lead-acid batteries, though, Ni-Cd cells are not kept in stock at local battery distributors, so in the event of a cell failure, replacement could be days or weeks away. When a cell fails, users must choose to either jump out the bad cell or deploy a temporary lead acid battery until the replacement cell arrives. Ni-Cd batteries require a well-defined commissioning charge to achieve the manufacturer’s specified performance when installed. These parameters are typically provided in the battery maker’s installation and operation instruction manual.
Defined as time sitting idle in a warehouse after last charge. Shelf life is shortened by high ambient temperature.
Some variants of Ni-Cd2 are much more difficult to fully charge than lead-acid types because recharge efficiency is worse than in lead-acid batteries.3 The user manuals for some Ni-Cd types specify that two rate charging4 is mandatory to achieving manufacturer’s specified performance. Battery chargers used with Ni-Cd batteries therefore should be specified to include automatic boost charging after discharge.
As with lead-acid batteries, correct charging and battery maintenance are essential.
Summary of strengths and weaknesses of Ni-Cd start batteries
| Strenghs | Weaknesses |
|---|---|
No sudden failure mode |
High cost |
Survives extreme temperatures |
Much larger footprint than lead-acid |
Resistant to mechanical and electrical abuse |
Poor availability (none are made in the US), meaning a long wait to replace failed cells |
Gradual end of life |
Pocket plate type Ni-Cd is more difficult to charge than lead-acid types |
Resistant to overcharge & undercharge abuse |
Requires proper commissioning charge to perform properly |
Longer (than lead-acid) shelf life |
67% more cells needed for same voltage as lead acid |
Summary of key points
1. Lead-acid battery:
• Strengths include that it is readily available, is made in standard sizes, and is low cost. • Weaknesses include sudden failure at end of life, loss of life when hot, loss of performance when cold, and short shelf life.
• The most successful strategy to achieving reliable engine starting with lead-acid starting batteries is to regularly replace them well ahead of the date of expected failure.
2. Nickel-cadmium battery:
• Strengths include inherently robust design and construction, no sudden failure mechanism, long service life and relatively long shelf life.
• Weaknesses include higher initial cost and larger footprint than lead-acid, and lower availability of replacements.
• The most successful strategy to achieving reliable operation with nickel-cadmium is committing to use Ni-Cd batteries in the first place, then continuing to maintain them.
Pocket plate Ni-Cd technology is more difficult to charge than newer Sintered PBE or fiber plate Ni-Cd technology. Recharge efficiency of pocket plate Ni-Cd batteries is around 70%, meaning that the charger must deliver at least 140% of the ampere-hours (AH) withdrawn from the battery before the battery is fully recharged. The recharge efficiency of very shallowly discharged batteries is even lower because replacing the last AH of capacity is always more difficult than replacing the first AH when the battery is discharged. See SENS Genset Starting Education Module #6: Battery Charging Basics for additional information.
Two-rate charging is the most effective way to reduce charging time. Ideally, two-rate charging temporarily increases charging voltage above the normal float voltage setting during initial battery recharge. This is called “boost charging”. Excess voltage applied to the battery compensates for voltage lost to the battery’s internal resistance. Operating at the higher boost voltage allows the battery to accept the charger’s maximum current longer than it would at float voltage. A correctly engineered charger automatically reduces the charger’s voltage to the correct float voltage value when the battery reaches full charge. See SENS Genset Starting Education Module #6: Battery Charging Basics for additional information.