Why is battery health rapidly deteriorating and how can it be stopped?

Hi there

I got this relatively old gadget Vostro 3558 about a month ago. Dell Power Manager showed its state as "Excellent", with battery health at 90%. The previous owner does not seem t have used it much. I doubled the RAM to 8GB. I actually use the notebook only about 2 hours a day on battery & off the mains. At the beginning, I charged it to 100%, turned it off until later in the day when I needed it and not connected to the mains.

After about only a week, I noted battery health had dropped to 79%, which I thought in such a short time and only 2 hours/day of use spectacular!

I decided to see if there is any way to at least do what I read is recommended to prolong battery life, charging to 80% and hoped it would help. I did not see what more to do.

In the meantime, like yesterday I have noticed in the last week or so, that in spite of having set it to stop charging at 80%, that it AT TIMES inexplicably goes up to 100% as I noted later when I turned it on, though the day before it stopped as expected at 80%. I crosschecked and the DPM showed the setting still at 80%! WHY did it charge up to 100%? I had not changed the settings!

I need to say 2 or 3 times when I have planned to use it longer, I have let it charge to 100% BUT NOT the days like the day before yesterday when I charged it, as I had changed it back to 80% as I was not intending to use it longer for the next couple of days. I am sure about that.

Today also, not only DPM shows battery health has dropped to "Good" but as you see, the other program shows 61%.

Is this rapid battery health deterioration rate normal, like from 90 - 61% within a month?

This is all in about 1 month of use of about 2 hours a day, other than the first days when I was setting up and generally testing it. In the 2 hours, I use minimum possible screen brightness and it has ALWAYS been in battery saver mode in 95% of the time I have had and used it, when off the mains. After charging, I disconnected it from the mains and most of the day it is at mostly room temperature.

• As I was writing this, it was charging and I was watching where it'd stop ....Result: The last time I looked at it, it was at about 71%. With that setting, I was expecting it to stop at 80%. I was alerted by an alarm, showing it was fully charged at 100% and that I disconnect the mains!!! So my previous observation was not wrong. It must also have jumped to 100% from some figure above 71% when I last looked at it as, if it did not stop at 80%, that rate of 20% to 100% was much higher than the time it took to move from around 30% - 71%.
• This reminds me of another observation: when using the notebook on battery, it discharges normally from 100 or 80% to around 40%, when I have seen this but it seems, once below this, it seems to JUMP to a level below 10 where the notebook warns me I should connect the notebook to the mains because of low charge. I would expect it to discharge normally below 40% as I plan to use it to about 30 or even 20%, but it does not go that far down.

What is going on and what more can I do to reduce this extremely rapid death of the battery? Why does this sudden jump in charge up to 100% when it has not normally stopped at 80% or the jump from just under 40% to a level below 10% (within very few minutes) mean?

Also what are the best battery settings for me, as said currently using the notebook about 2 hours a day on Dell Power Manager to be able to use this battery for as long as possible, with battery health not dropping at what appears to be a desasterous rate?

Thank you in advance