4.5. Overview
The following table gives an overview of how batteries can be recharged after a 50 % discharge. In practice recommendations can vary from one manufacturer to another and also depend on how the battery is used. Always ask your supplier for instructions!
Type
Alloy
Approximate absorption time at 20°C after 50 % DoD
Float voltage at 20°C
Automotive
Antimony (1.6 %)
4 h at 2.50 V / cell (15.0 V) 6 h at 2.45 V / cell (14.7 V) 8 h at 2.40 V / cell (14.4 V) 10 h at 2,33 V / cell (14 V) 4 h at 2.50 V / cell (15.0 V) 8 h at 2.45 V / cell (14,7 V) 16 h at 2.40 V / cell (14.4 V) 1 week at 2.30 V / cell (13.8 V) 5 h at 2.50 V / cell (15.0 V) 7 h at 2.45 V / cell (14.7 V) 10 h at 2.40 V / cell (14.4 V) 12 h at 2.33 V / cell (14 V) 6 h at 2.50 V / cell (15.0 V) 8 h at 2.45 V / cell (14.7 V) 10 h at 2.40 V / cell (14.4 V)
2.33 V / cell (14 V) after a few days decrease to: 2.17 V / cell (13 V)
Spiral-cell
Pure lead
2.3 V / cell (13.8 V)
Semi-traction
Antimony (1.6 %)
2.33 V / cell (14 V) after a few days decrease to: 2.17 V / cell (13 V) 2.3 V / cell (13.8 V) after a few days decrease to: 2.17V / cell (13 V)
Traction (tubular-plate)
Antimony (5 %)
VRLA-gel Sonnenschein Dryfit A200 VRLA-gel Sonnenschein Dryfit A600
Calcium
4 h at 2.40 V / cell (14.4 V) voltage not to be exceeded!
2.3 V / cell (13.8 V)
Calcium
4 h at 2.34 V / cell (14.04 V) voltage not to be exceeded!
2.25 V / cell (13.5 V)
Notes:
1) In practice, when shore power is not available, batteries on a boat tend to be charged as fast as possible, with shortened absorption time or no absorption period at all (partial state of discharge operation). This is quite acceptable, as long as a charge to the full 100 % is applied regularly (see sect. 4.3). 2) When charging at a voltage exceeding the gassing voltage, either the current should be limited to at most 5 % of the Ah capacity of the battery, or the charge process should be carefully monitored and the voltage reduced if the current tends to increase to more than 5 % of the Ah capacity. 4) About service life and overcharging: Starter- or bow thruster batteries are often charged in parallel with the house battery (see sect. 5.2). The consequence is that these batteries will frequently be charged at a high voltage (15 V or even more) although they are already fully charged. If this is the case, VRLA batteries should not be used for this purpose because they will start venting and dry out. The exception is the spiral-cell VLRA battery, that can be charged at up to 15 V without venting. Flooded and spiral cell batteries will survive, but age faster. The main aging factor will be corrosion of the positive plate grid, and the corrosion rate doubles for every 50 mV of voltage increase per cell. This means that an Optima battery for example, which would last 10 years at its recommended float voltage of 13.8 V, would age 4 times faster at 15 V (((15 – 13.8) / 6) / 0.05 = 4), reducing service live to 2.5 years if it would constantly be charged at 15 V. Similar results are obtained for flooded batteries. While this calculation is theory and has not been tested in practice, it nevertheless shows that regular overcharging during short periods (in practice only during the absorption charge period of the house battery) of starter or bow thruster batteries does not decrease service live to an unacceptably low period. 3) When float charging batteries at 2,17 V per cell a regular refreshing charge will be needed.
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