A serviced battery is charged with a current of 10% of the nominal value of the battery capacity. For example, a battery with a capacity of 55 Ah should be charged with a current of 5.5 A. If the battery is serviced, charging is carried out with the caps turned out. Charging is carried out until abundant gas evolution begins and constant voltage and electrolyte density are achieved 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°C, g/cm³
| Climate zone (average monthly air temperature in January,°C) | Time of year | Fully charged battery | by 25% | by 50% |
| Very cold (from -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 (from -15 to -8) | Same | 1,28 | 1,24 | 1,20 |
| Warm humid (from 0 to +4) | Same | 1,23 | 1,19 | 1,05 |
| Hot dry (from +4 to +15) | Same | 1,23 | 1,19 | 1,15 |
When charging the battery, periodically check the electrolyte temperature 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 to +25°C.
Charging is stopped when abundant gas emission begins in all battery elements, 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 specified in Table 10.5, adjust it. If the density is increased, remove some of the electrolyte and add distilled water. If the electrolyte density is below the norm, remove some of it from the element and add high-density electrolyte (1.4 g/cm³).
After adjusting the electrolyte density, continue charging the battery for another 30 minutes to mix the electrolyte. Then disconnect the battery and measure the electrolyte level in all cells after 30 minutes. If the electrolyte level is below the norm, add electrolyte whose density 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 current as the battery charges (at constant voltage). When the battery is fully charged, such a device automatically switches off. It is permissible to charge a maintenance-free battery with a current of 5% of the nominal capacity of the battery (a battery with a capacity of 55 Ah - with a current of 2.75 A).