Automobile Battery Characteristics

Voltage

The voltage of a lead acid battery is about 2.1V for one cell, 3 cells in a series is 6.3V (Nominal voltage is 6V), and 6 cells in a series is 12.6V (Nominal voltage is 12V).
The voltage above is at full charge status and without any loads or circuits, which is called open-circuit voltage. The EMF (electromotive force) and sulfuric acid concentration are in positive correlation which is showed below. (Temperature is 25℃)

Besides, Temperature can affect EMF, which is 0.2mV/℃ per cell when temperature is between 20℃ and 40℃. The change of voltage is related to current and internal resistance that is showed below.

According to relation above that larger discharging current makes greater voltage drop。1

If battery is discharged to 0V, it would not recover to previous performance even recharge it. According to 1 discharging current decides final discharge voltage. Please avoid discharging lower than final discharge voltage. By the way, Open-circuit voltage after discharged would reverse to the voltage of 1 , which depends on sulfuric acid concentration of 2through discharge to final discharging voltage.

If battery is discharged to 0V, it would not recover to previous performance even recharge it. According to1 discharging current decides final discharge voltage. Please avoid discharging lower than final discharge voltage. By the way, Open-circuit voltage after discharged would reverse to the voltage of1 , which depends on sulfuric acid concentration of2through discharge to final discharging voltage.

Capacity

Battery capacity is the amount of electric charge it can deliver at the rated voltage,which is measured in units as current (A) multiply hour (H). The following statement is showed that battery is discharging at the rated current under specific final discharge voltage:
Capacity=Discharge current (A) X Discharging time before final discharge voltage(H)
Higher current would cause lower discharging time that is why hour-rate (HR) stands for battery capacity unit. Motorcycle represents 10 hour-rate and automobile represents 5 hour-rate.13

● Temperature and capacity
Battery runs in electrochemical reactions. Higher temperature gets greater reaction. On the other hand, lower temperature gets slower reaction. Because capacity has the relation with temperature, the battery capacity showed in datasheets is refer to 25℃。4


● Discharging capacity and specific gravity of electrolyte
Specific gravity of electrolyte decreasing rate meets with declining discharge capacity during discharging, which finalized in 1.120/20℃. That the reason why measuring specific gravity of electrolyte can realize approximate discharging capacity (or remaining capacity)。5


● Capacity calculating
If the battery would be used out of automobile (engine) application, it is necessary to choose appropriate kind and capacity of the battery.The following conditions should take into the calculation.
1.Final discharge voltage (minimum voltage requirement)
2.Discharging current
3.Discharging time
4.Discharging temperature


(Example)
If temperature is 25℃, final discharge voltage is 10.5V, the battery has been discharged for 2 hours in current of 10A, how does the capacity of the battery need?
According to 6, we can find that coefficient ratio of capacity to discharge current = 10/0.45 = 22(Ah/5HR)


Furthermore, battery performance would decline gradually during cyclicre/discharging, and so do capacity. That is why the actual capacity requirement is 2~3 times of calculated result。


● Discharging depth and lifespan
Generally, automobile battery lifespan would apparently decrease though deep discharge. If there has deep discharge situation in need, please use cyclic battery。 7

Engine start performance

The performance of engine start is a benchmark that depends on how much voltage can a battery maintain when suppling sufficient current to the starter motor for engine start.
Discharge voltage has the relation among EMF, discharge current, and internal resistance. Smaller internal resistance makes greater discharge current.
Therefore, as the standard battery, internal resistance and capacity are in inverse proportion. As the result, if you want more powerful engine start performance, you should choose bigger capacity or lower internal resistance battery.
Relation between EMF and battery discharging voltage:

● Terminals and high-rate discharge characteristic
High-rate discharge characteristic stands for engine start performance which depends on the changes of temperature, discharge current, and recharging status. JIS tests the battery in -15℃,discharge current of 2for benchmark。8
● Temperature and high-rate discharge characteristic (5-second or 30-second voltage at discharge)
Battery capacity and high-rate discharge characteristic are affected by temperature, which are going down rapidly when lower than -10℃. Lower temperature makes higher internal resistance, and slower internal chemical reaction. High performance battery has lower internal resistance than standard one, which has engine start ability even in -30℃。9

● Terminals and high-rate discharge characteristic
High-rate discharge characteristic stands for engine start performance which depends on the changes of temperature, discharge current, and recharging status. JIS tests the battery in -15℃,discharge current of 2for benchmark。8
● Temperature and high-rate discharge characteristic (5-second or 30-second voltage at discharge)
Battery capacity and high-rate discharge characteristic are affected by temperature, which are going down rapidly when lower than -10℃. Lower temperature makes higher internal resistance, and slower internal chemical reaction. High performance battery has lower internal resistance than standard one, which has engine start ability even in -30℃。9

Recharge

The discharged battery should be recharged as soon as possible (Discharged battery may be crashed if stored for a long time). There has variety of charging methods to battery usage, which has different recharge characteristics。

● Constant-current recharge characteristics
Constant-current recharge is the way that charge at rated current from the start to the end. 5 hour-rate and 1/10 ratio current recharge is the general standard. Specific gravity of electrolyte is hardly increasing during initial phase of charging, but when voltage reached 14.1~14.4V (2.35~2.4V/Cell), specific gravity would rapidly increase because of electrolysis that cause gas emitting. Finally, specific gravity would stop increasing when reached the final phase of charging. Voltage gradually rises due to sulfuric acid diffusion. At the final phase, lead(II) sulfate has reduced, which cause water electrolysis that emits oxygen and hydrogen ast. Furthermore, charging voltage is also affected by current and temperature. 10

● Constant-voltage recharge characteristics
Constant-voltage (2.3~2.5V/cell) recharge method is charging current counts on voltage. The more difference of electric potential, the more current will supply. This method has advantages of no gas emits and efficient. Due to initial charging current is high that needs a charger with much more capacity, constant-current and constant-voltage recharge method is better way for charging.

● Quasi-constant-voltage recharge characteristics
It is a transformation of constant-voltage recharge whose voltage and current are both changed during charging. Initial phase has high current when battery’s voltage is low, final phase has low current when voltage goes up. It is widely adopted to common battery rechargers. 11

● Constant-current and constant-voltage recharge characteristics
Restrict maximum charging current and voltage. Initial charge phase uses constant-current, and then uses constant-voltage which is faster and more efficient. This method has been adopted to automobile charging system。 12

● Other recharge methods
1.Float and trickle charging
The method of sustain charging status of the battery.
a.The battery and the load are always in connection
(Float charging)
When DC power is supplying electricity to the load, it is supplying trickle current to the battery at the same time because of self-discharge. If the loading comes
more or suddenly blackout occurs, the battery will supply electricity to the load。 13 3
b.The battery and the load are separated that usually used in common
(Trickle charging)
DC power usually supplies trickle current to the battery in the use of engine start. When engine starts, the battery will supply electricity to engine. (This recharge method which just fulfilled self-discharged is called maintain charging, too.)
Float charging depends on specific gravity of electrolyte in 3. Charging current (which is 1/500~1/2000 of 5HR in common) depends on battery is new or old, and temperature.
Moreover, please use float charge type battery.
2.Equalizing Charge
When the battery is in float charge situation for a period, some cells' voltage might become lower. Equalizing charge can brings all cells to similar levels by increasing the voltage, which should be done every 3~6 months. When taking constant-voltage method in equalizing charge, recharge for 8 hours at 14.4V (2.4V/cell) in general if specific gravity is 1.280, and battery is 12V。
Besides, when recharging at 5HR with constant-current method, keep charging until full-charged for 4 hours. If battery is used for float charge, please refill the water before equalizing charge。 (Do not refill the water during float charge. If it is necessary, stop charging, refill the water, and then apply equalizing charge.)

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