8 Frequently Asked Questions About Lithium Ion Battery Capacity

To check if your Li-ion battery is in good health or not, the main test is to detect its capacity. Apart from using a professional capacity tester, you can even use a good smart charger for the capacity testing easily. And there are eight frequently asked questions about Li-ion battery’s capacity and testing. Detailed answers are also shown as below.

What is the maximum capacity of a 18650 battery at present?

Currently, the highest capacity 18650 battery is 3600mA. Any 18650 battery that claims to have a capacity over 3600mAh right now might be not reliable. Some manufacturers are trying to develop new battery technology, in order to improve Li-ion batteries’ energy density. Maybe there is larger capacity 18650 battery in the coming future.

How long will my Li-ion battery power up the device? So the capacity decides the runtime?

You know the capacity of a battery is normally given in terms of mAh (Milli Ampere hour) rating. For example, let’s assume your battery is 2000mAh which is 2Ah (Ampere/hour). This means that if your draw 2A from this battery, it will last for 1 hour. And similarly, if your draw 1A from this battery, it will last for 2 hours.

Does the battery capacity affect its charging time?   

Yes, for example, a 18650 battery with a higher capacity, will take longer time to charge compared to one with lower capacity. For example, a 1800mAh battery can take about 1 to 1.5 hours to charge whereas a 3500mAh battery may take about 3 to 4 hours to be fully charged using the same adapter.

Does the minimum charge time for 0%-100% increase as charge cycles increase? Or does it actually decrease because the capacity reduces? 

It’s not a linear relationship though. The efficiency of the batteries’ drops with aging. So instead of getting back 95% of the energy pumped into the battery during charge for a relatively new battery. You may get less than 75% back after it degrades. This is why when using active battery testers, you should pay attention to the discharge capacity rather than charge capacity since that’s all the “load” cares about.

What factors affect the Li-ion battery’s capacity?

The capacity is being directly influenced by these factors:

* The charging/discharging rates affect the rated battery capacity. If the battery is being discharged very quickly (i.e., the discharge current is high), then the amount of energy that can be extracted from the battery is reduced and the battery capacity is lower.

* The temperature in which a device operates is the main factor impacting a battery’s power consumption. At extreme temperatures, the electrode and electrolyte no longer have the optimal shape to enable efficient lithium-ion exchanges.

* Multiple cycling and usage. The battery capacity will stay at or close to its rated capacity for a limited number of charge/discharge cycles. With usage, the battery will lose capacity due to the physical and chemical degradation.

What are SoH and SoC for a battery? Are they related to the capacity?

State of Health (SoH) of a battery describes the difference between a battery being studied and a fresh battery, and considers cell aging. It’s defined as the ratio of the maximum battery charge to its rated capacity.

State Of Charge (SoC) % of a battery describes the difference between a fully charged battery and the same battery in use. It’s associated with the remaining quantity of electricity available in the battery. It’s defined as the ratio of the remaining charge in the battery, divided by the max charge that can be delivered by the battery.

If I test a 18650 battery’ capacity on VC4SL charger, will it show similar capacity numbers regardless of what slot it’s in? And how about the accuracy of this grading function? 

A grading sequence on some xtar chargers consists of charging the battery, discharging it (generally to 2.5V-2.8V) while measuring capacity and then charging it again. The way of these chargers testing mAh is usually by using a current shunt and an internal clock. The slots on chargers have their own current shunt. And those PCB current shunts are usually slightly different in resistance, which causes differences in capacity ratings when the battery is tested in different slots. Even with calibration, there will be a little changes due to heat generation on the PCB, among other things.

Users probably get battery capacity readings that are within 10% or so of each other. Factors affecting the grading function, may include what temperature the room and batteries are, in combination with the small differences in shunt resistance.

Why should I store Li-ion battery around 50% of usable capacity (~3.7V) rather than 100% (4.2V) for long term storage?

As the Li-ion battery is stored empty however it could be drained too much, and lithium ends to age quicker if it stored while it’s full. It’s ideal to store Li-ion batteries at 30-70% state of charge, which means any where from 3.5v to 3.8v resting for common Li-ion batteries. Some smart chargers, such as vc4sl, vc8 plus, with STORE function that can increase or decrease a Li-ion battery to its rated voltage for long-time storage and maxing the battery lifespan.

Download the PDF document in Facebook Group: Lithium Ion Battery & Charger Talk (18650/21700/26650 & more)