A sulphated battery is one which has been left standing in a discharged condition or undercharged to the point where abnormal lead sulphate has formed on the plates.
When this occurs the chemical reactions within the battery are impeded and loss of capacity result.
This document does not cover all the electricity theory and technology involved in the process of sulphation in battery operated system. For more information, please refer to specialized literature.
Most cases of sulphation are caused by:
UNDER CHARGING OR NEGLECT OF EQUALIZING CHARGE
When a cycled battery is charged repeatedly at low rates but not fully charged, the acid is not effectively driven out of the plates, particularly the lower parts, and sulphation results.
Repeated partial charges which do not effect thorough mixing of the electrolyte also result in sulphation. It is difficult in normal battery operation to determine just when sulphation begins, and only by giving periodic equalizing charges and comparing individual cell specific gravity and voltage reading can it be detected in its early stages and corrected or prevented.
Troublesome sulphation does not occur in less than 30 days.
STANDING IN A PARTIALLY OR COMPLETELY DISCHARGED CONDITION
Permitting a battery to stand in a partially discharged condition for long periods allows the sulphate deposited on the plates to harden and the pores to close.
Batteries should be charged as soon as practicable after discharge and not allowed to stand in a completely discharged condition for more than one month. During freezing weather the battery should be recharged immediately following discharge to prevent freezing.
If the level of the electrolyte is permitted to fall below the tops of the plates the exposed surfaces will harden and become sulphated.
If acid is added to a cell in which sulphation exists the condition will be aggravated.
HIGH SPECIFIC GRAVITY
In general, the higher the fully charged specific gravity of a cell the more likely is sulphation to occur and the more difficult to reduce. If in any battery there exists cells having specific gravity more than 0.015 above the average, the possibility of sulphation in these cells will be enhanced.
High temperatures accelerate sulphation, particularly of and idle, partially discharged battery.
All cells of a sulphated battery will give low specific gravity and voltage readings. They will not become fully charged after normal charging. An internal inspection will disclose negative plates having a slatelike feeling, sulphated negative-plate material being hard and gritty and having a sandy feeling when rubbed between thumb and forefinger.
The internal inspection should be made after a normal charge, since a discharged plate is always somewhat sulphated. A good fully charged negative plate is spongy and springy to the touch and gives a metallic sheen when stroked with the fingernail or knife. A sulphated positive plate is a lighter brown color than the normal plate.
Thorough and careful attention to the following steps often will restore a sulphated battery to good operation condition using a MMF charger and a Discharger/Analyzer.
- 1. Clean battery;
- 2. Bring electrolyte level to proper height by adding water;
- 3. Put battery on charge using the MMF charger.
The MMF charges the battery following the charge curve IUa as shown in the following picture.
With reference to the MMF User's Manual, set the charge parameters as follow:
- Initial Charge Current (I) : 4-5% of the battery capacity,
- Constant Voltage (V): 2,4V/cell
- Time of charge: from 12 to 24 hours
During the charge process keep under control the battery temperature that must not exceed 50°C.
If any cell gives low readings (0,20 V less than the average cell voltage of the battery), pull and repair the cell before continuing with the procedure.
- 4. The sulphated batteries have an high internal resistance; when a sulphated battery is put on charge the battery voltage reaches in few time the value of the Constant Voltage set and the charger current is very low.
Continuing to keep the battery on charge for long time with this very low current the lead sulphated is removed by the plates. The battery voltage would tend to decrease but the MMF charger increases the charger current to keep the voltage constant to the value set.
The charge process have to last a long time, from 12 to 24 hours.
If it's possible, keep under control the specific gravity of the electrolyte. If, during the charge the specific gravity doesn't increase for about 4 hours the charge process can be stopped.
After the end of the charge process, record the specific gravity of the electrolyte. The density of the electrolyte indicates the state of the charge.
- 5. Place battery on discharge at the 8 hours rate.
To discharge the battery It's advisable to use a Battery Discharger / Analyzer. Refer to the following site to have more details regarding the Discharger/Analyzer.
The unit can discharge the battery with a controlled discharge current while the overall battery voltage and the time of discharge are keeping under control.
If a unit is used to discharge the battery it's advisable to set the discharger with the following parameters:
- Discharge Current: 1/8 of the battery capacity
- Time of discharge: 8 hours
- Final Battery Voltage: 1,70V/cell
The discharger is fully automatics and stops the discharge process if the time of discharge or the minimum battery voltage set are reached. During the discharge, keep under control the individual cell voltages and overall battery voltage at the following time intervals: 15 minutes after test is started, then hourly until voltages on one cell reaches 1.80 and, from then, on at 15 minute intervals. Stop manually the discharge process if the voltage of one of the cell falls below 1,60V .
- 6. If the battery, with reference to the discharge parameters set, gives at least the 80% of the rated capacity no further treatment is required other than normal recharge and equalization of gravity.
- 7. If the battery does not deliver near-rated capacity, execute a new charge/discharge process.
Recharge the battery as described above using the MMF.
Again charge the battery for long time between 12 or 24 hours and/or until there is no further rise in specific gravity over a 4 hour period, readings being taken hourly. Keep under control the battery temperature.
8. Discharge again at the 8 hour rate and if the battery gives full capacity, recharge and put into service.
9. If this procedure does not result in full capacity, repeat once more as noted above. To have a stronger recharge cycle, set the constant voltage of the MMF to 2,5V/cell. Refer to the user's manual to have more details regarding the setting of the charge parameters.
10. If battery does not respond to this treatment, it is sulphated to the point where it is impractical to attempt further treatment and battery should be replaced.