•  Use a hydrometer to determine the specific gravity of  the lead-acid battery electrolyte, which is the weight of  the electrolyte compared to the weight of pure water.  Take care to ensure the electrolyte is returned to the  cell from which it was extracted. When a specific  gravity difference of 0.050 or more exists between  cells of a battery, the battery is approaching the end of  its useful life and replacement should be considered.  Electrolyte level may be adjusted by the addition of  distilled water. Do not add electrolyte. •  Battery state of charge is determined by the cumulative  effect of charging and discharging the battery. In  a normal electrical charging system, the aircraft  generator or alternator restores a battery to full charge  during a flight of 1 hour to 90 minutes. •  Proper mechanical integrity involves the absence  of any physical damage, as well as assurance that  hardware is correctly installed and the battery is  properly connected. Battery and battery compartment  venting system tubes, nipples, and attachments, when  required, provide a means of avoiding the potential  buildup of explosive gases, and should be checked  periodically to ensure that they are securely connected  and oriented in accordance with the maintenance  manual’s installation procedures. Always follow  procedures approved for the specific aircraft and  battery system to ensure that the battery system is  capable of delivering specified performance. The state of charge of a NiCd battery cannot be determined  by measuring the specific gravity of the potassium hydroxide  electrolyte. The electrolyte specific gravity does not change  with the state of charge. The only accurate way to determine  the state of charge of a NiCd battery is by a measured  discharge with a NiCd battery charger and following the  manufacturer’s instructions. After the battery has been  fully charged and allowed to stand for at least 2 hours, the  fluid level may be adjusted, if necessary, using distilled or  demineralized water. Because the fluid level varies with the  NiCd sealed batteries are similar in most respects to NiCd  vented batteries, but do not normally require the addition of  water. Fully discharging the battery (to zero volts) may cause  irreversible damage to one or more cells, leading to eventual  battery failure due to low capacity. NiCd vented batteries have a 1.2-volt nominal cell voltage.  Occasional addition of distilled water is required to replace  water loss due to overcharging in normal service. Cause  of failure is usually shorting or weakening of a cell. After  replacing the bad cell with a good cell, the battery’s life can  be extended for 5 or more years. Full discharge is not harmful  to this type of battery. The following information is provided to acquaint the user  with characteristics of the more common aircraft battery  and battery charger types.  [Figure 9-38]  Products may  vary from these descriptions due to different applications of  available technology. Consult the manufacturer for specific  performance data. Lead-acid batteries are usually charged by regulated DC  voltage sources. This allows maximum accumulation of  charge in the early part of recharging. The lead-acid battery is economical and has extensive  application but is heavier than an equivalent performance  battery of another type. The battery is capable of a high rate  of discharge and low-temperature performance. However,  maintaining a high rate of discharge for a period of time  usually warps the cell plates, shorting out the battery. Its  electrolyte has a moderate specific gravity, and state of charge  can be checked with a hydrometer. Lead-acid vented batteries have a two volt nominal cell  voltage. Batteries are constructed so that individual cells  cannot be removed. Occasional addition of water is required  to replace water loss due to overcharging in normal service.  Batteries that become fully discharged may not accept  recharge. Lead-acid sealed batteries are similar in most  respects to lead-acid vented batteries, but do not require the  addition of water. NOTE: Never connect a lead-acid battery to a charger, unless  properly serviced. 9-25 Figure 9-38.  Battery charger. NiCd Batteries Battery and Charger Characteristics Lead-Acid Batteries 