Hobby charger termination question

[supergravy]

I am currently using a Hyperion EOS 606i hobby charger for all of my lithium charging needs. While this is proving to be a great charger, the documentation is very basic and apparently there are undocumented features.

The thing I want to better understand is charge termination and an undocumented "trickle-like" feature of my charger. This thing definitely follows the constant current / constant voltage algorithm - no problems. The thing that concerns me is that when I choose a charge current greater then 1A it terminates different then usual. It will do it's thing until the main charge is feeding 100mA or so and then beep that it is done. The different part is that it will then continue to apply small decreasing current all the way down to 5mA or so before truly shutting down. If my initial charge rate is less then 1A, it shuts completely down at 100mA as documented.

This appears to actually be a nice feature and something similar to what the Duratrax ICE charger does. The difference is that the ICE does it with all charge rates AND it always shuts things totally down at 30mA.

Here is my concern. This additional charge time from 100mA down to 5mA or so can take a long time. While it never allows the battery to go over 4.2V, it does mean the battery is hooked up and taking current a long time. The last bit from 30mA and below goes forever - I can see why the ICE stops at 30mA as it seems like a good compromise.

Is there any harm in this extended charge? It does seem to put every last bit into the battery. If I take a battery off the charger when the main charge (100mA or better) ends it will quickly drop down to 4.17V. If instead I let the thing go full cycle to the very end, I find my batteries hold 4.2V even with hours of rest.

Appreciate any help from you charging guru's out there!

 

[Mr Happy]

One thing I can say is that higher resting voltages in a lithium ion cell cause faster degradation of the internal chemistry, leading to a shorter lifetime. This effect is much faster when voltages reach the 4.2 V region.

Therefore you can weigh up how important that little bit of extra charge is, from 4.17 V to 4.20 V, compared to the slightly fewer cycles you will get from the cell if you charge it all the way up to 4.20 V every time. Remember that the longer a cell remains at 4.20 V without being discharged, the more internal degradation is sustained.

I can't really quantify how big the difference is, but I do believe the damaging effect of higher voltages increases exponentially. So for instance, the harmful effect of going from 4.2 V to 4.3 V is far greater than the effect of going from 4.1 V to 4.2 V.

 

[SilverFox]

Hello Supergravy,

It sounds like the charger is designed around charging battery packs. When charging packs, you can also balance the pack during the charge. The charger is beeping when the current drops to 100 mA, but in order to accommodate the balancer, it continues to balance the battery pack by allowing some charge current to flow.

The balancer has limited capabilities and it appears that charge rates over 1 amp are above its capability to balance.

To get a full charge you just have to put up with the extra time that it takes to get the last little bit of charge into the cell. 4.17 volts is about 97% full, and many people are good to go with that level of charge.

As Mr Happy mentioned, the time spent at full charge can be detrimental to cycle life. In "non-emergency" situations, I will limit the charge to 4.1 volts. This gives me about 90% of the total capacity, and greatly increases the expected cycle life from the cell. Of course the down side is the reduced run time due to only having 90% of the total charge.

There was a study done by the cell phone people. They found that charging to 4.2 volts gave a cycle life of around 500 charge/discharge cycles, however, if the charge was limited to 4.1 volts, that cycle life went up to around 1500 charge/discharge cycles.

Tom