A serviced battery is charged with a current equal to 10% of the nominal value of the battery capacity. For example, a 55 Ah battery must be charged with a current of 5.5 A. If the battery is serviced, charging is carried out with the covers turned out. Charging is carried out before the onset of abundant gas evolution and the achievement of a constant voltage and density of the electrolyte for 3 hours. The density of the electrolyte of a charged battery at 25°C should correspond to the data in Table. 10.5 for each climatic region.
Table 10.5. Electrolyte density at 25°С, g/cm3
| climatic region (average monthly air temperature in January,°С) | Season | Fully charged battery | by 25% | by 50% |
| Very cold (-50 to -30) | Winter | 1,30 | 1,26 | 1,22 |
| Summer | 1,28 | 1,24 | 1,20 | |
| Cold (from -30 to -15) | All year round | 1,28 | 1,24 | 1,20 |
| Moderate (-15 to -8) | Same | 1,28 | 1,24 | 1,20 |
| warm humid (from 0 to +4) | » | 1,23 | 1,19 | 1,05 |
| hot dry (+4 to +15) | » | 1,23 | 1,19 | 1,15 |
When charging the battery, periodically check the temperature of the electrolyte and do not allow it to rise above +40°C. If the temperature reaches +40°C, reduce the charging current by half or interrupt charging and cool the battery down to +25°C.
Charging is stopped when abundant gas evolution begins in all battery cells, and the voltage and density of the electrolyte during the last three measurements (carried out 1 hour after disconnecting the battery) will remain constant.
If at the end of charging the density of the electrolyte (determined taking into account the temperature correction) differs from that indicated in the table. 10.5, correct it. With increased density, remove some of the electrolyte and add distilled water. If the density of the electrolyte is below normal, then, having taken a part of it from the cell, add electrolyte of increased density (1.4 g/cm3).
After adjusting the density of the electrolyte, continue charging the battery for another 30 minutes to mix the electrolyte. Then disconnect the battery and after 30 minutes measure the electrolyte level in all cells. If the electrolyte level is below normal, add electrolyte, the density of which corresponds to the given climatic region (see table. 10.5). If the electrolyte level is above normal, remove excess electrolyte with a rubber bulb.
A maintenance-free battery is charged with a special charger that automatically reduces the amperage as the battery charges (at constant voltage). When the battery is fully charged, the device will automatically turn off. It is allowed to charge a maintenance-free battery with a current equal to 5% of the nominal value of the battery capacity (battery with a capacity of 55 Ah - current 2.75 A).