A "lightly sulphated battery" has a layer of insulating lead sulphate on its plates. When you try to charge a sulphated battery, the voltage rises quickly, as per Ohm's law, and for an uncontrolled battery charger it rises all the way up to its maximum. It is often said that sulphated batteries "will not accept a charge". However, they can sometimes be "revived" by charging them with a low current over several days. Another way is to raise the voltage to 20-25V in order to get the sulphated battery to charge a little. This may work with a sulphated battery that has been protected on a workbench, but it is risky for the vehicle when the battery is still connected to the mains. Many manufacturers including BMW motorcycles, have prohibited the use of this type of battery charger without first disconnecting the battery in the vehicle. This may result in you losing the settings for your radio and sometimes even for the engine control unit, which makes this method of desulphation a lot more labour intensive.
Another more efficient method is applied by CTEK battery chargers. They charge at full current right up to maximum voltage whereupon the battery charger returns to zero. Charging starts up again immediately and keeps on in this way by sending pulses of energy to the battery. You can see this as the lamps in the upper row are flashing. If they are still flashing after 60 minutes, you will have to accept that the sulphated battery is spent, but if a lamp indicating CHARGING is lit permanently, you have succeeded in desulphating the battery. Bear in mind, however, that the sulphated battery is probably at the end of its service life and will need to be replaced sooner rather than later.
A CTEK battery charger uses 4-8 stages depending on the model:
Charge rate in relation to acid density and battery voltage:
|
SOC |
Acid density |
Battery voltage (approximate) |
|
100% |
1.28 |
12.9V |
|
75% |
1.23 |
12.5V |
|
50% |
1.18 |
12.15V |
|
25% |
1.14 |
11.9V |
|
0% |
1.10 |
11.4V |
The way in which the charge is assessed, State of Charge, is shown in the table. Note that the battery voltage only provides a rough indication for a wet battery. The amount of water is a factor and you could have a high voltage although the battery is not working particularly well.
A fully charged battery can handle cold conditions well and will not freeze until at around – 67C. This is worse if the battery is discharged as it can freeze at a few degrees above zero, which is another reason to keep the battery charged during the winter.