I have a powerbank that I use to charge my cell phone.

10,400 ma is it's capacity.

Output 3.7 V

It has 2 outputs.

1.0 and 2.1 amps and I use the 2.1 amp

My cell battery is about one yr old. (Replacement for orig battery)

After charging the battery, it only lasts about 2 days before going dead.

During those 2 days, there was no cell phone usage.

Battery capacity is listed as 850 Mah.

Does it look like my battery capacity is seriously degraded ?

What is the make/model of your powerbank? How are you measuring the output voltage? Is the output measured with any devices connected?

What is the make/model of cell phone?

There are many factors that can determine battery life i.e age of battery, type of phone, distance from a cell tower. If using a smart phone the operating system version and the number of running apps.

As a FYI it helps if you provide as much information as possible in your first post.

I provided as much info as I could.

There is no way I can anticipate what will be needed for a first time problem.

Adreama is the manufacturer and no model no. is listed.

Cell phone is Alcatel 510A

No way to measure output voltage as pins are too small.

My post lists what is listed on power bank.

How long does the battery last if charged via its AC charging adapter? If it lasts about the same then it probably is not a powerbank problem. How long did the phone battery last when it was new?

I can not find your particular model on the Adreama's website but would guess the 3.7 VDC is when the phone's battery is low. It should measure ~5.0 VDC when fully charged.

As batteries age they do not last as long but it also depends on their environment. I would expect longer but

Phone Battery is a 3.7 volt battery.

When it needs recharging I will use wall charger and see how long it lasts.

Your post is confusing since you listed the phone battery voltage mixed in with the description of the powerbank. It goes back to my statement of not posting enough information in your threads. It took 5 posts to pull everything out.

It probably does not make much of a difference and it could be just the age of the phone battery.

The phone should not care that it is connected to the battery (assuming it has enough capacity left) or an AC adapter. The charging time if connected to the powerbank might be longer.

I listed battery capacity of cell phone separately from that of the power bank.

But none of the supplied info indicates the short life of the cell phone battery.

I made this post on an electronics forum.

That is their forte and I expect a good explanation.

There's a ton of things to consider in batteries. Time the battery was on the shelf. That's a primary concern to me. If the phone is older, the batteries may also be older, and thus they've sat on the shelf for maybe more than 2 years. I find it very typical that people have older cell phones, there's nothing wrong with them and they do not wish to upgrade or spend money, etc, however the batteries end up becoming the issue. Either you can no longer purchase them, or the ones you can purchase are as old as the phones.

The battery technology, likely LiON, but who knows, could be NiCd, or NiMH, also matters. Older Nickel based rechargeable batteries will discharge faster, that's a known fact in battery technology.

Modern batteries are designed to take about 500 charge cycles, thus they are ballpark'ed to last about 2 years.

Too bad they just won't make cell phones which run off of AA or AAA. There were some, and probably still are, just very rare.

After a day after charging, the battery is still showing 4 bars.

My problem may have been a fluke. Battery is a lithium ion.

There have been times when I had to take the power bank off before it was fully charged in order to use the phone.

It would show 4 bars while the charging indicator had showed it was still charging.

Try to keep your battery "topped off" at all times. The life-span of a Lithium-Ion battery is about 150-200 "charge cycles." If you let it fully discharge then charge it back up, you've just used an entire cycle.