Battery Knowledge

The rechargeable batteries's main parameters

The rechargeable batteries's main parameters include battery voltage,capacity,internal resistance(IR),Self discharge rate,discharge platform,charge end off voltage,discharge cut-off voltage,charge/discharge rate,energy density,memory effect and cycle lifes.

1. Capacity: battery capacity unit is Ah or WH,1Ah capacity means it could hold on 1 hour under 1Amp discharge current. AH VS WH,WH= Norminal capacy(AH)* Norminal Voltage. Normally the bigger size battery is with higher  capacity for a same typoe battery.

2、Norminal voltage: Each cell has a positive terminal, or cathode, and a negative terminal, or anode. The terminal marked positive is at a higher electrical potential energy than is the terminal marked negative. The terminal marked negative is the source of electrons that when connected to an external circuit will flow and deliver energy to an external device. When a battery is connected to an external circuit, electrolytes are able to move as ions within, allowing the chemical reactions to be completed at the separate terminals and so deliver energy to the external circuit. It is the movement of those ions within the battery which allows current to flow out of the battery to perform work. Although the term battery technically means a device with multiple cells, single cells are also popularly called batteries.

3. IR: The inner resistance (IR) of a battery is defined as the opposition to the flow of current within the battery. There are two basic components that impact the internal resistance of a battery; they are electronic resistance and ionic resistance. The electronic resistance plus the ionic resistance will be referred to as the total effective resistance. 

4. Self discharge rate: Self-discharge is a phenomenon in batteries in which internal chemical reactions reduce the stored charge of the battery without any connection between the electrodes. Self-discharge decreases the shelf-life of batteries and causes them to initially have less than a full charge when actually put to use.

How fast self-discharge in a battery occurs is dependent on the type of battery, state of charge, charging current, ambient temperature and other factors.
 
Typically, among rechargeable batteries, Li-ion batteries , Li-Polymer batteries  and LiFePO4 batteries absorb the least amount of self-discharge (around 2% to 3% discharge per month) than lead-acid at 4% to 6%, while nickel-based batteries are more seriously affected by the phenomenon (NiCad, 15% to 20%; NiMH, 30%,) with the exception of Low self-discharge NiMH batteries (2 to 3%.) Primary batteries, which aren't designed for recharging between manufacturing and use, have much lower self-discharge rates, with shelf lives of 2 to 3 years for zinc–carbon batteries, 5 years for alkaline, and 10 years for lithium.
 
5. Charge end off voltage: The battery voltage will be not increased any more when fully charged,this voltage calls charge end off voltage.
 
6. Discharge cut off voltage: In batteries, the cut-off (final) voltage is the prescribed lower-limit voltage at which battery discharge is considered complete. The cut-off voltage is usually chosen so that the maximum useful capacity of the battery is achieved. The cut-off voltage is different from one battery to the other and it is highly dependent on the type of battery and the kind of service in which the battery is used. When testing the capacity of a NiMH or NiCd battery a cut-off voltage of 1.0 V is normally used, whereas 0.9 V is normally used as the cut-off voltage of an alkaline cell. Devices that have too high cut-off voltages may stop operating while the battery still has significant capacity remaining.
 
7. Battery discharge platform: It indicates the stable discharge time of rechargeable battery. For Ni-MH battery and lithium battery, discharge plateau requirements are different.
For nickel metal hydride battery, battery voltage does not falls with a same slope during discharge after battery is fully charged. Generally, at the beginning of discharge, voltage drops quickly; after a period, battery voltage hardly changes or has little change. This discharge period, which takes up the vast majority of whole discharge time, is discharge voltage plateau. After battery almost finishes discharging, voltage goes down quickly until cutoff voltage.
Briefly speaking, when battery discharges under constant current, voltage experiences down, stable and down process, among which, stable stage is discharge plateau.
As far as lithium battery, discharge platform means capacity or discharge time from full charged voltage to rated voltage under specified discharge condition (for instance, discharge rate, temperature, etc). Longer discharge time or higher discharge capacity proclaims higher battery discharge platform.
Discharge platform of lithium battery is discharge time from 4.2V through CV charge to 3.6V under any discharge rate current. It is an important criterion to measure battery performance.
Discharge platform curve is smooth and has a long duration time at a certain voltage. For different discharge rate, discharge platform is different. Many factors should be considered during choosing discharge platform because sometimes higher discharge platform implies low capacity.Only high capacity with long duration time under a certain voltage and higher platform voltage proves whether battery is good or not.
 
8. C-Rate: Charge and discharge rates of a battery are governed by C-rates. The capacity of a battery is commonly rated at 1C, meaning that a fully charged battery rated at 1Ah should provide 1A for one hour. The same battery discharging at 0.5C should provide 500mA for two hours, and at 2C it delivers 2A for 30 minutes. Losses at fast discharges reduce the discharge time and these losses also affect charge times.
 
A C-rate of 1C is also known as a one-hour discharge; 0.5C or C/2 is a two-hour discharge and 0.2C or C/5 is a 5-hour discharge. Some high-performance batteries can be charged and discharged above 1C with moderate stress. 
 
9. Energy density: it defines battery capacity in weight (Wh/kg); energy density or volumetric energy density, reflects volume in liters (Wh/l). Products requiring long runtimes at moderate load are optimized for high specific energy; the ability to deliver high current loads can be ignored.
 
10. Memory effect: It only happens on Ni-Cd batteries. As in traditional technology a Ni-Cd battery's negative is agglomeration with thick Ni crystal, if Ni-Cd batteries are recharged before they have been fully discharged, Ni crystal easily gathers to form agglomeration, which makes the primary discharge platform come into being. The battery stores the platform, which will be considered as the end of discharge for the next cycle even though the capacity decides that the battery can be discharged to a lower platform. The battery will store this process in its memory so during the next discharge, the battery only remembers this reduced capacity. Similarly any further incomplete discharge in each use will aggravate the effect makes lower capacity. The effect there are two methods to remove the effect: firstly deep discharge at trickle current (i.e. 0.1C to 0V), secondly several cycles at high currents (e.g. 1C).
 
11. Battery cycle life: When a battery experiences a charge and discharge, we call a cycle or a period. In the stated charge and discharge principles and the capacity decline before a stated standard, the total cycles it can undergo called rechargeable battery cycle life.
 
 
China Battery Factory

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