Charging quirks

[ultrarunner2015]

Hey guys;
Whether it be Li, NiMh, or NiCd, I have noticed that if you wait for the battery to fully charge - as indicated by the charger - then restart the charge cycle by power-cycling the charger or removing then replacing the batteries - that the batteries will take an additional charge.
While this additional charge is quite small, I am curious as to whether or not my batteries are receiving the full charge from one normal cycle or if I may be overcharging them by restarting the charge process.
What I am trying to do is 'top off' the batteries. However, I don't do this often. It was just something I have noticed over the years with different batteries and chargers.

One example is a Bluetooth dongle that was included with my HiFiMAN Deva Pro headphones. The first time I charged the dongle, it only took about 1/2 hour before the indicator showed fully charged. Then when I used the device it seemed that I did not get the full advertised capacity of the battery.
The battery had run down to the point where the device shut itself off. Then I connected it to a USB port to recharge.
The 2nd charge cycle took much longer - perhaps 2 hours before the indicator showed full.
I then decided I wanted to know whether this device was allowing a 'trickle charge' of the Lithium battery.
I unplugged it from the USB port and inserted a dongle that measures voltage and current.
When I plugged the Bluetooth dongle into the measurement dongle, the current was 190mA, and the LED indicated that the battery was taking a charge. The current began dropping by about 10mA per minute until reaching 100mA, then the LED turned green and the current dropped to 0.0mA as measured by the dongle.
So I am curious to know whether this 'topping off' is normal and whether I can actually get a little more time from each charge, or is it dangerous?

Thanks for your advice.
Ultrarunner

 

[Lynx_Arc]

I'm not totally familiar with lithium ion charging electronics but I do know that good chargers will detect a rise in voltage that happens when the cell is fully charged. This rise doesn't happen immediately when you start the charge cycle over depending on the circuitry one charger may measure the voltage and not charge it another may start charging it and wait for the rise to happen and shut off when the peak voltage hits a higher than completed voltage.

Also if your device is powered by USB then the USB source can differ in voltage output and charging rate and these combined can influence the charging voltage under a charging load on the source. For example a charger that is rated at 1A vs 500ma (0.5A) if the device can charge at greater than 500ma some 500ma supplies may find they are less able to keep voltage up on a load that wants twice as much power thus the 1A source can charge it not only faster but maybe a little more.

Chargers that pulse can over time trickle charge if the pulse is high enough current too. Batteries that are old and have gone through a lot of cycles plus charging cables and connections can further make things differ. Some cables cannot handle higher currents as well as others and under a heavier load they can actually have increased charging time as they cannot sustain the power output the source can manage. I have a phone that supports fast charging which normal phones when it was made were 5V 500ma or 5V 1A charging which is 2.5 or 5 watts charging rate, my phone supports 9v 2A or 12v 1.5A for 18watts charging rate and some cables I've tried on it won't fast charge others will as they cannot support the higher power transfer.

I also have a JBL Charge 3 that comes with a 2.3A charger and it takes 2-3 times longer to charge using a 500ma or 1A charger and when I use the higher power charger the indicator lights light up 1-2 more. I figure the lights are measuring voltage on the battery and the higher power charger can sustain higher voltages when charging. It is likely the circuitry pulses now and then to check battery condition when charging also which may average both the charging and battery voltages and the higher charge rate may increase that average.

There are a lot more variables out there hopefully someone else who knows a lot more than I do will chime in and either correct me or add substantial better information.