I have a Dell Inspiron "P66F", running Mint 18.2 Cinnamon 3.4.3, that sometimes charges very slowly for no obvious reason. Today for instance, it took way too long to charge from under 5% to 71%, stating, at 71%: "1hr, 40 min" remaining until fully charged.Then the charge went to 80% fairly quickly, but then gave the remaining charge time as "2hr,55min" !?!

I have noticed this type of behavior before, where the charge rate seems to vary widely for no obvious reason, and the alleged remaining time to full charge also varies widely and seemningly randomly, often unconnected to the indicated charge %.

I have a charger rated the same as the spec on the bottom of the machine (19.5V, 3.34 A), it is a Chinese no-name knock-off, but since the charge time is often fast, I don't think that is the problem.

Before I invest in a new battery, I want to rule-out any sort of charge algorithm issues.

Is it possible to reset the software, to assure something isn't amiss there?

P.S. : In just the 10 minutes or so it took to post the above, the indicated charge went from 82 % to 100% !

Have you looked at the original battery capacity? For example:

On above output, battery is 73.1% of original factory capacity?

Lots of ways to check this. Here is a good example: https://unix.stackexchange.com/quest...ery-is-healthy

I am just using the charge indicator that in accessible from the Mint Cinnamon desktop by selecting the battery icon on the right side of the pop-up bar.

Since this indicator does give specific info on the battery actually installed, in this case it says "SMP DELL 3CRH65B 65%"; I suppose it's a % of the original capacity of that battery, but I don't know for sure.

The power adapter that plugs into the laptop is not the actual battery charger. Of all the Dell laptops I have used the power adapter contained a memory chip that is read by the BIOS. If it can't read the memory chip the laptop will not charge and you will see a BIOS pop warning message. If yours is charging with a non Dell power adapter then I would guess they are no longer using that feature.

The actual charging circuit is internal to the laptop and there are several factors that determine the actual charge rate of the battery i.e temperature and battery voltage are critical values. If the battery is getting to hot it should slow down charge rate. It is possible for some reason something is starting to fail with the charging circuit and you might be getting some wonky readings. Its also possible there is sufficient dirt inside to prevent proper cooling.

lithium-ion batteries charge typically in two main stages, i.e. constant current and saturation. Approx the 1st 1/3 of the charging cycle is constant current and the remaining is saturation. As far as I know the time to full charge is directly read from the ACPI