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8 Accumulating Trend Analysis Over a Period of Years
Trending of periodic Mhos measurements can yield valuable insights for estimating the remaining life of the battery.
Suppose that a 4.0 Ah battery has been in service for three years and that the LifeTester measurements after the first year
was 90 Mhos, 85 Mhos after the second year, and 80 Mhos after the third. Since the battery is still at 80 Mhos and is
dropping an average of 5 Mhos per year, it is likely that this battery will not need replacing before the end of the fourth
year. On the other hand, suppose that the readings were 90 Mhos after the first year, 82 Mhos after the second year, and
70 Mhos after the third, the rate of decay increasing each year. This indicates that although the battery is still serviceable,
it is degrading more and more rapidly. It is unlikely that this battery will be serviceable for another year. Under these
circumstances, either the battery should be replaced early or the service interval should be shortened to catch this battery
before a system failure.
9 Using Mhos to Estimate Battery Life
For standby batteries, peak capacity is normally reached after about three months on a float charging system. For cyclic
use batteries, full capacity may not be reached until after ten to thirty cycles. If a new battery has been in storage for an
extended period, say over six months without being charges, plate oxidation from self-discharge will occur, causing a
decrease in the Mhos reading. Plate oxidation also occurs in standby batteries during a power failure, particularly if a
battery remains in a highly discharged state for an extended period of time. Plate oxidation is unhealthy and can destroy a
battery's capacity. Once a battery is weakened by plate oxidation, it is difficult to recover full capacity without special
charging or conditioning methods. In some cases a battery will recover and pass a test after being recharged. However, a
second test should be performed a few days later to accurately assess the overall life of the battery. The average life cycle
of a sealed lead-acid battery in standby use on a float charge is three to five years. A battery with a Mhos reading of less
that 70% of its original capacity is no longer considered serviceable and should be replaced.
Continued on next page...
HPBLT Installation Document P/N 52763:A 03/10/06
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