Laptop battery health questions

Hi there! I got myself a used Dell Latitude E7470 (i5-6300U, 8 GB RAM, 256 GB SSD, 1080p, magnesium chassis) for around 400 USD from a reputable online seller here in my country (1000+ positive transactions; guy's business was selling secondhand laptops from suppliers). Everything is in good working order and the laptop is still in really good shape even if it's more than 3 years old now.

As soon as I got it, I fired up HWiNFO and noticed that the battery health was at an amazing 97%. After two weeks of mixed usage though (battery power and plugged in), it deteriorated quickly to around 88%. I'm normally not a fan of "calibrating" lithium-ion/polymer batteries but I just said screw it and did so anyways by draining the battery to complete 0 in the BIOS and charging it to full uninterrupted. This brought my battery health down even more to around 84% and it's been at this for a few days now.

My first question is, why did the battery health report itself as 97% when I got it only for it to deteriorate quickly over the span of two weeks? My theory is the battery readings got screwed or something and that my repeated usage of the laptop brought back the readings' accuracy over time (kind of like how AccuBattery works in Android where it shows more accurate battery health after multiple cycles of charging and discharging). What do you guys think?

My second question is, after setting the battery to stop charging once it hits 80% in the BIOS, would be safe for me then to use it while plugged in for extended periods of time versus the normal behavior of the laptop stopping charging at 100% and leaving it plugged in while I use it? From my understanding, once it the battery hits my desired threshold, the system disconnects the battery and runs on AC power instead, but I'm not sure if this is the case with my system (well, it probably is since I noticed my battery % drop from 80 to 79 while I was using it while plugged in).

Apologies if my post was a bit unorganized. Thanks in advance to those who will try to help me out

 

I'm not sure about the deterioration from 97% to 84% - maybe the usage case of the laptop previously was always plugged in, so the battery BMS never had any opportunity to set the approprate discharge and charge "flags". It is unlikely that the seller deliberately modified the battery firmware; I'm not aware of any method to do this, and if you could modify the battery IC chip data, that would make laptop battery modding significantly easier. Your discharge/charge cycles will have given a more accurate reading.


My ThinkPad also has the battery charge threshold. That is entirely safe to use. 80% is an excellent max charge state for Li-ion longevity. In my case, 80% equals a per-cell voltage of ~3.97v, which means I should get well over 1000-2000 charge cycles, compared to 300-500 cycles at 4.2v. There is no adverse effects on the AC adapter, power circuitry within your laptop, or your battery. Nonetheless, it is a good habit to do a full charge/discharge cycle every 30 battery cycles, this avoids having a sudden shutdown when the BMS becomes out of sync.

 

Thanks for the comprehensive reply! By full charge/discharge cycle, do you mean just running it until the OS shuts down and charging it to full or truly discharging it through the BIOS?

 

Either is fine. The full cycle is purely to avoid disappointment or surprises when your laptop suddenly shuts down. It has no effect whatsoever on battery health, because Li-ion batteries have no memory effect.

I wouldn't bother worrying too much about battery health. You can replace a battery for about 50 dollars anyway. I fear the day when all manufacturers begin to glue batteries into place, like some have begun to do (MS, crApple etc)

 

Is there any proper way to discharge a battery? Is it okay to leave it on charge while gaming for long periods?