Determining State of Charge

TM 11-6140-203-14-1

contact with copper, which could occur if the nickel-

plating is scratched, corrosion will set in. For these

reasons, the battery must be carefully cleaned and

kept free of potassium carbonate deposits.

f. The potassium carbonate deposits, when dry, may

be removed with a nylon brush. Be sure that the filler

caps are secured before brushing. Any foreign matter

can contaminate the battery. If necessary, the tops of

the batteries may be flushed with water. Be sure that

the filler caps are tightly closed before flushing with

water. Batteries must be thoroughly dry before use.

g. Do not attempt to determine the state of charge

of a nickel-cadmium battery by a voltage check or by a

specific gravity check of the electrolyte. The electro-

lyte is not Changed by the chemical reaction which

takes place in the battery; the specific gravity is the

same whether the battery is charged or discharged. An

EL4GRO07

indication of the approximate state of charge is the

Figure 2-11. Typical Discharge Voltage Curve Under Moderate

amount of current the battery draws when it is con-

Load for Nickel-C!admium Battery.

nected to a constant potential charger. The higher the

state of charge, the less current the battery will draw.

2-20. Periodic Maintenance Cycling

2-19. Determining State of Charge

Figure 2-12 shows the maintenance sequence that

should be performed at direct support or general sup-

The actual state of charge of a nickel-cadmium battery

port categories. For aircraft nickel-cadmium batteries,

is difficult to determine. It cannot be determined by

maintenance shown in flow chart is required every 100

the specific gravity of the electrolyte since it does not

flight hours or every 120 days (whichever shall occur

change during battery charge and discharge. The

first). For nonaircraft nickel-cadmium batteries, main-

methods described in a and b below may be used for a

tenance shown in flow chart is required quarterly or

general determination of charge condition. However

every 100 cycles (whichever shall occur first). The pe-

the most accurate method is to fully recharge the bat-

riodic maintenance cycle should result in the rebalanc-

tery before use if it has been charged and is on standby

ing of all cells in the battery, the reactivation of in-

for any length of time.

active plate material, and the replacement of defective

a. Battery Current Drawn From Constant Potential

cells. In addition, the battery as returned to the user

Source. A common method of checking the state of

contains the proper electrolyte level and a full charge.

charge is to connect the battery across a constant po-

tential charging source and observe the charging cur-

rent drawn by the battery as indicated by an ammeter

2-21. Adjustment of Electrolyte Level

in series with the battery. The charging-source voltage

The proper height and method for adjusting the elec-

would normally be 1.5 volt times the number of cells;

trolyte level has caused considerable confusion in the

for example, 28.5 volts for a 19-cell, 24-volt battery at

past. This has been because of the wide variety of

70ºF. If the current drops to 1 or 2 amperes or less

charging methods, conditions of use, and available

within 5 minutes, the battery can be assumed to be in a

head space within each size and type of nickel-cad-

near state of full charge.

mium cell. Refer to TM 11-6140-203-14-2 or TM

b. Voltmeter Reading With Battery Under Load.

11-6140-203-14-3 for detailed procedures covering

Since the discharge voltage of a nickel-cadmium bat-

the battery being serviced.

tery is essentially constant until the battery is approx-

imately 90 percent discharged, voltmeter readings

a. Checking Electrolyte Level. Because a nickel-cad-

across the terminals while the battery is under load are

mium cell contains large amounts of electrolyte below

not too reliable. Figure 2-11 shows a typical voltage

the top of the plates, the fact that electrolyte is not

discharge curve under moderate load, and illustrates

visible in the partially discharged state is not neces-

the problem associated with voltmeter readings. A

sarily cause for alarm. When a battery has been

reading of approximately 24 volts may mean that the

charged on an aircraft bus, the electrolyte should be

battery is almost fully charged (point 1 on the curve)

visible above the cell plates immediately after the

or almost totally discharged (point 2 on curve).

charge is ended. If not, the electrolyte level in the cell

2-14