2.5.5. Premature aging 1. The battery is discharged too deeply.
The deeper a battery is discharged, the faster it will age due to shedding (sect. 2.2.4.), and once a certain limit is exceeded (approx. 80% depth of discharge) the aging process advances disproportionately fast.
Additionally, if the battery is left discharged the plates will begin to sulphate (sect. 2.2.4.).
As was also explained in section 2.2.4, a battery ages even when kept charged and doing nothing, mainly due to oxidation of the positive plate grid.
The following table gives a rough idea of the number of charge/discharge cycles that batteries can withstand until the end of their service life, and how they could be destroyed by sulphation or due to plate corrosion.
Batteries are considered to have reached the end of their service life when the capacity they can hold has reduced to 80% of the rated capacity.
Number of cycles until end of service life
Resistance to 100 % discharging
Expected service life in float or shallow cycle use at 20°C ambient temperature
Type
DoD 80 %
DoD 60 %
Years
Start
Not suitable for cyclic use
5
Spiral-cell
400
650
Irreparably sulphated within a few days Irreparably sulphated within a few days Survives up to 1 month in short-circuited state Survives up to 1 month in discharged state
10
Semi-traction
200
350
5
VRLA AGM battery Traction (tubular-plate)
250
800
4 - 10
1500
2500
10 – 15
VRLA-gel Sonnenschein Dryfit A200 VRLA-gel Sonnenschein Dryfit A600
250
450
Survives up to 1 month in discharged state
4 – 5
600
900
Survives 1 month in discharged state
15 – 18
Although most batteries will recover from a full discharge, it is nevertheless very detrimental to their service life. Batteries should never be fully discharged, and certainly not left in discharged state.
It should also be noted here that the voltage of a battery that is in use is not a good measure for its level of discharge. Battery voltage is affected too much by other factors such as discharge current and temperature. Only once the battery is almost fully discharged (DoD 80% to 90%) will voltage drop rapidly. Recharging should have been started before this happens. Therefore a battery monitor (chapter 3) is highly recommended to manage large, expensive battery banks effectively.
2.5.6. Premature aging 2. Charging too rapidly and not fully charging.
Batteries can be quickly charged and will absorb a high charge current until the gassing voltage is reached. While charging with such high current might work well a few times, this will actually shorten the service life of most batteries substantially (the exception: spiral-cell and some other AGM batteries). This is due to accelerated loss of cohesion of the active material, which results in shedding. Generally it is recommended to keep the charging current down to at most C / 5, in other words a fifth or 20 % of the rated capacity. When a battery is charged with currents exceeding C / 5, its temperature can rise steeply. Temperature compensation of the charging voltage then becomes an absolute necessity (see sect. 2.5.9). My own experience is that charging a 50 % discharged 12 V 100 Ah flooded battery at 33 A (C / 3) results in a temperature increase of 10 to 15°C. The maximum temperature is reached at the end of the bulk phase. Bigger batteries will become even hotter (because the amount of heat generated increases with volume and the dissipation of heat increases with the available surface) as well as batteries with a high internal resistance, or batteries which have been discharged more deeply.
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